The basics of solid state devices. Aug 1, 1995 12:00 PM, DeDad, John A. 0 Comments ShareThis3 A device frequently used in solid-state equipment such as lighting dimmers and variable speed controls is the silicon controlled rectifier (SCR). As shown in Fig. 1, it's a semiconductor device having three electrodes: an anode, a cathode, and a gate. An SCR's anode and cathode are similar to those of an ordinary semiconductor diode. (See Back To Basics - Part 1, April 1995 issue for detailed discussion of a diode.) How does an SCR differ from a diode? Well, for one thing, it has the aforementioned gate electrode, which is its control point. (More about this later.) For another, it will not pass significant current, even when forward biased, unless the anode voltage equals or exceeds the forward breakover voltage. Once this breakover voltage is reached an SCR will switch ON and become highly conductive. SCR characteristic curve When an SCR is reverse biased and its gate diode is not connected, its voltage-current characteristic curve is as shown in Fig. 2. (See Back To Basics-Part 1, April 1995 issue for discussion of forward and reverse bias.) In this mode, an SCR operates like a regular zener or avalanche diode. (See Back To Basics-Part 2, May 1995 issue.) In other words, there is a small amount of current flow until avalanche is reached, after which the current increases dramatically. And, as is the case with a zener diode, this current can cause damage if thermal runaway begins. When an SCR is forward biased, there's a small current, the forward blocking current. This current will stay relatively
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The basics of solid state devices.Aug 1, 1995 12:00 PM, DeDad, John A.
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A device frequently used in solid-state equipment such as lighting dimmers and variable speed controls is the silicon controlled rectifier (SCR). As shown in Fig. 1, it's a semiconductor device having three electrodes: an anode, a cathode, and a gate. An SCR's anode and cathode are similar to those of an ordinary semiconductor diode. (See Back To Basics - Part 1, April 1995 issue for detailed discussion of a diode.)
How does an SCR differ from a diode? Well, for one thing, it has the aforementioned gate electrode, which is its control point. (More about this later.) For another, it will not pass significant current, even when forward biased, unless the anode voltage equals or exceeds the forward breakover voltage. Once this breakover voltage is reached an SCR will switch ON and become highly conductive.
SCR characteristic curve
When an SCR is reverse biased and its gate diode is not connected, its voltage-current characteristic curve is as shown in Fig. 2. (See Back To Basics-Part 1, April 1995 issue for discussion of forward and reverse bias.) In this mode, an SCR operates like a regular zener or avalanche diode. (See Back To Basics-Part 2, May 1995 issue.) In other words, there is a small amount of current flow until avalanche is reached, after which the current increases dramatically. And, as is the case with a zener diode, this current can cause damage if thermal runaway begins.
When an SCR is forward biased, there's a small current, the forward blocking current. This current will stay relatively constant, at least until the forward blocking voltage is reached. At this point, which is called the forward avalanche region, the current will increase rapidly. Here, an SCR's resistance is very small. In fact, an SCR acts the same as a closed switch here, with the current limited only by any external load resistance. As such, a short in an SCR's load circuit will destroy the SCR if inadequate overload protection is provided.
Gate control
As mentioned earlier, an SCR works just like a mechanical switch: it's either ON or OFF. When the applied voltage on an SCR is below its forward breakover voltage ([V.sub.BRF]), the SCR fires (is ON). It will stay ON as long as the current stays above the holding current value; it will turn OFF when the voltage across it drops to a value too low to maintain the holding current.
How does an SCR's gate electrode come into play here? Well, when the gate is forward biased and current begins to flow in the gate-cathode junction, [V.sub.BRF] is reduced. The higher the forward bias, the less [V.sub.BRF] needed to get the SCR to conduct. This is shown in Fig. 3.
Once an SCR is turned ON by its gate current, this current loses control of the SCR's forward current. Even with its gate current completely removed, an SCR will stay ON until its anode voltage is removed. It also will stay ON until the anode voltage is reduced enough so that the current is not sufficient to maintain a proper holding current level.
SCR applications
Basically, an SCR is used as a DC switch because of its many advantages over mechanical DC switching. These include arcless switching, low forward voltage drop, rapid switching time, and no moving parts. An SCR can be used for AC switching, although two SCRs are needed.
Varying power to a load is perhaps an SCR's most prominent application. This is because of its ability to turn ON at different points in its conducting cycle; thus, its usefulness in varying the amount of power delivered to a load. This type of variable control is called phase control. Don't confuse the term "phase" as used here with that pertaining to power distribution systems. Here, "phase" refers to the time relationship between two events, in this case, between trigger pulse and the point in the conducting cycle at which the pulse occurs.
Testing an SCR
You can "rough" test SCRs using an ohmmeter and a test circuit, as shown in Fig. 4, and the following steps. If an SCR does not respond as indicated for each of these steps, it's defective and should be replaced.
Step 1. Set the ohmmeter on the "R x 100" scale. Connect the ohmmeter's negative lead to the SCR's cathode and its positive lead to the SCR's anode. The ohmmeter should read infinity. (Resistance will actually be over 250,000 ohms.)
Step 2. Close the switch. This will short circuit the gate to the anode. The ohmmeter should read almost zero ohms. (Resistance will actually be about 10 to 50 ohms; this range of readings will not register on the "R x 100" scale.) Open the switch and the ohmmeter should still read zero ohms.
Step 3. Reconnect the ohmmeters leads, positive lead to the SCR's cathode and its negative lead to the SCR's anode. The ohmmeter should read infinity. (Resistance will actually be over 250,000 ohms.)
Step 4. Close the switch. This will short circuit the gate to the anode. The resistance reading should remain high because the SCR is reverse-biased and, therefore, can't conduct.
Step 5. Open the switch. The resistance should remain high because the SCR is reversed-biased and has no gate current.
1996 NEC issued with zone concept.
Aug 1, 1995 12:00 PM
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The hearing
The Council held a hearing on the question on July 19, 1995. Speaking in favor of sustaining the floor action and including the zone concept included Don Zipse and Craig Wellman, who had submitted formal complaints to the Council. Under NFPA rules, a complaint is "any request submitted in writing to the Standards Council for a reversal or modification of any action taken by any Technical Committee, Technical Correlating Committee, the Association, or the Standards Council, at any time in the document development process."
They, along with others, made the point that the concept was well supported by widespread international experience. Due to world-wide acceptance, it would promote international harmonization of standards. It could increase safety in the high-hazard Zone 0 areas, where power wiring is prohibited.
Bill Wusinich, representing the IBEW, had also submitted a formal complaint, in his case opposing the floor amendment. He was joined by brother union member Jim Naughton and also by Joe Ross of NEMA and Lon Ballard of Crouse Hinds. They argued that there is significant confusion over the concept that could lead to improper installations. They questioned if the proposal was completely thought through. For example, Article 501 is about five times the length of new Article 505 on the same subject. Joe Pipkin, who works for OSHA and sits on the Correlating Committee, expressed the opinion that OSHA wouldn't go along with the new approach.
Although the panel had said the new approach shouldn't be intermixed with traditional classification procedures, the actual text doesn't match that intention. For example, Sec. 501-11 provides specific rules for mixers that travel in and out of open mixing vats, which is normally a Zone 0 location (hazardous vapors routinely present in ignitable concentrations); would designers be able to describe this as Class 1 Div. 1 and then classify immediately adjacent areas as Class I Zone 1 simply based on which classification is more convenient, or even jointly classify the same area?
The concept has been discussed for over 25 years, beginning with a 1969 proposal to include it in the 1971 NEC. The arguments at that time are striking in that they are so identical to contemporary views on the same topic. Those in favor mentioned international trade and analogized to the subdivision of hazardous locations in the 1947 NEC (the advent of Div. 1 and Div. 2) so the tightest requirements would be targeted at areas with the greatest hazard. Those opposed questioned whether the effort was really needed, since the present system had proved its safety.
Don Zipse pleaded with the Council to proceed, noting that Article 780 ("Smart House") went into the Code before any facilities using those concepts had ever been built. If that could go ahead, why not a concept that had been used for a quarter of a century in extensive areas of the
world? Surely if there were problems, and he acknowledged that there were, they could be resolved over the normal course of the standards making process, using Tentative Interim Amendments (TIAs) if necessary.
The decision
The Council voted to uphold the action at the Annual Meeting, and to correlate that action with parallel action on related comments. The result is that a number of public comments (14-13, -30, -45, -171a, -174, -174a, -177, -178, -179, -182), which had been reported as "reject" are now in place as accepted at the panel meeting.
In its decision, the Council noted the long history of this proposal in the NEC revision process and the widespread use of the concept throughout much of the world. The Council noted that at the end of the process, it had received the support of both the Association membership and the Code Making Panel. Therefore, the Council declared that technical consensus had been achieved.
The Council noted that the Correlating Committee had not cited any technical objections to the concept, but that instead the basis for its objections lay in correlation problems with other actions. The Council's actions on the related proposals resolved some of those issues, and the others will be addressed in future actions.
Mr. Wellman also raised the issue of how the Council judges Technical Committee (particularly CMP 14) balance in his complaint. He felt that the user classification should be allotted greater representation (up to 50%). The user members of CMP 14 had consistently voted unanimously in favor of the new Zone concept, and they had been thwarted by other interests at times.
The Council voted to deny this portion of the complaint and to reaffirm support for the present procedures, with no interest group having more than one third of a committee. The Council concluded that "the need for a balance of interests on committees continues to be served by this rule."
The issues in this appeal are so important, and the opposing sides so intransigent, it is a virtual certainty that the Council decision will be appealed to the NFPA Board of Directors. If that happens, the 1996 NEC will still be printed as released by the Council, but a disclaimer will be printed in the front of the book notifying users of the pending appeal.
Troubleshooting signal attenuation in a CCTV system.Aug 1, 1995 12:00 PM, Lewis, Warren H.
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This case history shows how decibel knowledge, a handheld oscilloscope, and intuitive thinking can be used to solve a troublesome problem.
Now that we have a somewhat complete understanding of the decibel and how it applies to various types of signals, we can apply this knowledge, along with the many modern test instruments available today, in troubleshooting any electronic equipment operation problems that may come our way.
As an example of how this may work, let's review a case history involving closed circuit television, coaxial cable, and shoddy workmanship.
Background site information
We were called in by a client who complained about a very poor picture generated by a security camera on a closed circuit television (CCTV) video monitor on a simple point-to-point link. The picture had degraded from good to unusable over a short period of time and really got bad in the few days following a recent lightning and rainstorm. The general opinion was that lightning had somehow caused the problem.
Upon inspection, we found that the CCTV system consisted of National Television System Committee (NTSC) standard video generated and displayed in black-and-white. The main monitoring point of the CCTV system was a loading dock and a weather-proofed housed camera located on a 10-ft high post mounted atop the building's roof was used to monitor that location. The camera was connected to a video monitor in a security shack some 200 ft away via a 75 ohm coaxial cable, which was routed down into the shack by means of a vent pipe type of entry. Power to the camera was provided via a low-voltage DC link on another coaxial cable, which was installed right alongside the coaxial cable used to transport the video signal. Both the monitor and camera low-voltage DC supplies were simply plugged into a wall outlet convenient to the operator at the guard's shack.
Symptom
The picture on the video monitor looked as though someone had turned the contrast control all the way in one direction so that there was no contrast at all; the picture looked washed-out and was barely visible on the screen, which was a nearly uniform light gray.
Testing procedures carried out
Our first task was to go up to the camera at the roof and see what the video signal looked like as it exited the camera. This test was much aided by the fact that our handheld, 50 MHz bandwidth, solid state, digitizing oscilloscope with LCD display had an internal battery pack and did not require any AC power for operation.
First test. First, the coaxial cable was disconnected at the camera and a BNC style "TEE" fitting was installed. This fitting was equipped with a 75 ohm terminator resistor on one leg. Then, we connected our handheld oscilloscope into the remaining open end of the TEE. The result, as
shown in Fig. 1, was a healthy NTSC composite video signal. Conclusion: the camera was clearly putting out a good signal, which was about 1.8V peak-to-peak across the 75 ohm termination. (There is also a DC component with the AC video signal.)
We then placed our handheld oscilloscope into its meter-mode and the above signal at the camera into the 75 ohm load was taken as a zero dB reference and stored into memory. This is shown in Fig. 2, where + 000.1 dBV DC is taken as being close enough to zero to do the job. Now, the "good" signal right out of the camera was available to be used again and again as a comparison with signals we would measure at different locations. We then would be able to see how much loss of signal occurred along the path, all of which was supposed to be a consistent 75 ohm.
The TEE was removed and the 75 ohm coaxial cable was reconnected.
Second test. The next test was made at the video monitor end of the cable and right at the point where the cable was connected to the monitor. Again, the TEE was used, but this time no 75 ohm termination resistor was used with it since the TEE was attached to both the monitor and the cable. Thus, there was a fairly good 75 ohm load on everything. The result of this test was that almost no video signal could be seen on our handheld oscilloscope's screen.
We then changed the oscilloscope's vertical scale from 500 mV/cm to 100 mV/cm and another measurement was taken, which is shown in Fig. 3. As you can see, the video signal is simply attenuated but does not appear to be distorted in any way that is easy to see. Conclusion: the signal loss was occurring along the 75 ohm cable path, or was it?
Video monitors have been seen to "load down" a signal due to an internal failure on its input circuit; as such, we didn't want to rule this possibility out. A quick test with the TEE and the 75 ohm termination resistor in place of the video monitor quickly ruled out this possibility; the signal was essentially unchanged from that shown in Fig. 3. Now, we really could conclude that the signal's loss was occurring along the 75 ohm cable.
Third test. We next placed our handheld oscilloscope into its meter-mode, while maintaining the connection to the TEE at the junction of the cable and video monitor. This allowed us to take a relative dB measurement reading, as shown in Fig. 4, using the original zero-level as the reference. (Remember, we did this at the camera end to establish a comparison reference.) Here we see that a - 13.7 dBV DC loss exists. This loss represents a voltage loss ratio of 4.84:1, or a signal loss of nearly 5V for every volt put into the cable!
How much signal attenuation should you expect on a 200-ft long, 75 ohm coaxial cable? A quick look at the coaxial cable manufacturer's Master Catalog gave us the approximate answer: around 2 dB of loss at 10 MHz for 200 ft of RG-59/U type cable as used in CATV applications. The whole attenuation chart is shown in the accompanying table below.
What we were seeing in this path was more than 11 dB loss over and above that stated in the manufacturer's literature. Also, the baseband video we were looking at shouldn't have a lot of
really high-frequency in it; thus, the cable probably shouldn't attenuate as much as 2 dB (per manufacturer's literature) for 200 ft in any case.
Oh yes, since the manufacturer's information was provided only in dB form, what would we have done if we didn't understand dB and weren't working in terms of dB on our handheld oscilloscope? You guessed it. We would have had no idea what was "normal" and what was not on a coaxial cable run of the type being investigated. All we would have had was some guesswork, which is not a very good way to go in most cases.
Further analysis
What was happening on the cable? The BNC connector at the video monitor end was inspected and it looked OK, except that it seemed to be a little wet after it was handled and the cable was flexed.
Following this the same examination of the BNC at the camera end also failed to show any problems. We also made sure that the connections were well protected from the environment by the camera's enclosure.
Was the moisture a clue? Was it significant, or not? Past experience with coaxial cables with water inside of them showed that this condition caused severe signal attenuation.
Back to the rooftop we went to make a closer examination of the 75 ohm cable and its route back to the video monitor. First, we looked at the vent pipe, the rooftop penetration through which the cable was passed. We found that it was not equipped with a weatherhead and that the cable was simply stuck down into it from its open top. Sealing was done with some kind of putty or caulking material and it looked as though it was really dried-out. Thus, water (from the storm, remember?) could follow the cable's sheath down into the building around the bad seal.
But how did this condition let the water into the cable? We pulled the sealing material out of the vent pipe and then hauled the coaxial cable up out of it. About 10 feet down, we found a connection made up of two BNC fittings and a male-male adapter. The end of the cable going into the bottom BNC fitting from the building was mostly pulled out of the connector and the braid/sheath was fully exposed to any water flowing down the cable from above. In fact, the arrangement was a pretty good funnel for the water to flow into the cable between the outer sheath/shield and the inner dielectric material. Corrosion was also rampant in the damaged connector set since it had not been sealed from moisture in any way. Obviously, this was not good for reliable signal transport.
Where did this splice come from? After a little discussion with the personnel, we learned that the camera came from the factory with about 10 ft of cable. Rather than throw this cable away, it was simply kept in place and used by connecting it to the end of the cable being routed from the video monitor. There was no explanation as to why such a poor rooftop penetration was made; nobody would own up to it while we were there.
Solution
The whole existing 75 ohm coaxial cable run was replaced with a continuous length one. Where this cable came from and what its quality was, we didn't know and couldn't find out; it might have been surplus stock from somewhere (World War II?). After installation of the "new" cable, the signal at the video monitor end was again checked with the test TEE and the monitor in place. This signal is shown in Fig. 5. Here we see that there is still some attenuation, but nowhere near as much as before.
Again, using our handheld oscilloscope in its meter-mode, we made a relative dB measurement reading using the original zero-level as the reference. The "new" cable's signal loss, as shown in Fig. 6, is about -4.5 dB. Compared to the previous dB measurement readings [ILLUSTRATION FOR FIGURES 2 AND 4 OMITTED], this amount of signal loss is acceptable in this application, as was evidenced by the good picture on the video monitor.
ARTIFAX
EC&M article:
"An Introduction To The Decibel," July 1995. Cast: Article cost $9.95. Order No. 2258. Orders are taken via facsimile machines only. To order by fax dial 800-234-5709. (Have a credit card and your fax number ready when you dial by fax.)
Warren H. Lewis is President of Lewis Consulting Services, Inc., San Juan Capistrano, Calif. and Honorary Chairman of EC&M's Harmonics and Power Quality Steering Committee.
Providing four megs of power to protect computers.Aug 1, 1995 12:00 PM, Bender, Gayland J.
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Fast track design and construction, thorough testing, and adjustments as needed provide power assurance to client's data operations.
A strong concern for power reliability at a data site is satisfied by an electrical design that includes installed on-site power, dual primary feeders, and a full UPS with N+1 redundancy of components (no single point of failure causing system shutdown). The on-site power consists of two engine-generator (EG) sets, with each generator serving as backup to one of the dual primary feeders. These same generators also can operate in parallel with utility service.
How the system works
The peaking/emergency electrical system is designed for five modes of operation. A review of the system one-line diagram shown on page 24 will help you understand these modes.
Normal off-line. In this mode, the generators are not operating. The 13.8kV feeder breakers are closed, energizing the client's step-down transformers from the respective utility feeders. The 13.8kV bus tie breaker is open. The generator systems must be in an automatic switching posture during this mode.
Peaking mode. In this mode, the generators are operating in parallel with the utility's 13.8kV system to provide peak power. The 13.8kV feeder breakers as well as the 480V generator breakers are closed. Alternately, one 13.8kV feeder breaker is open, the other closed, and the 13.8kV bus tie breaker is closed. The generator's controls are operating to provide fixed power output (usually equal to the prime rating) as well as adjusting the power factor (remote controllable, usually set to near unity).
Feeder transferred mode. In this mode, the generators may or may not be operating. The power system configuration calls for one of the 13.8kV feeder breakers to be open, the other closed, and the 13.8kV bus tie breaker closed, thus supplying both the client's transformers from one utility primary feeder.
Emergency stand-by power mode. In this mode, the generators are operating asynchronous to the utility's 13.8kV system, providing power to the client. The 13.8kV feeder breakers and bus tie breaker are open. The 480V generator breakers are closed. This mode is initiated automatically as a result of loss of normal power to the client (loss of both of the utility feeders). The generator controls operate the power output and the power factor. In this mode, the generators are not operating in parallel and the client's tie breaker between Busses MSB-1 and 2 is open.
Manual mode. During this mode, the 13.8kV switchgear may be placed in any configuration desired, and the generators may be run manually, through local control switches. All automatic control aspects of the switchgear system and the engine generator sets are suspended.
Benefits of flexible operation
The option of parallel operation of each of the two generator sets with each of the two primary feeders was a main design objective and meets several operational conditions. First, parallel operation allows both generators to supply power to a single utility feeder in peaking mode. If, while operating in the peaking mode with power flowing into the utility grid, utility power is lost on one of the primary feeders, the engine generator controls switch to an emergency stand-by mode. The affected 13.8kV feeder breaker opens. If an outlet exists through the other utility 13.8kV feeder, the 13.8kV bus tie breaker is synchronized closed, paralleling the two generator sets on the healthy utility feeder. Each primary feeder is sized to handle the entire 4000kVA load. This methodology allows maximum power delivery to the utility grid during a time when the utility may be in great need of power generation.
An emergency 13.8kV loop feeder is provided between the client's two 2000kW transformers in the event one of the primary feeders between the EG set switchgear and client's respective transformer is lost. This loop feeder can be quickly connected between the transformer with the healthy feeder and the other transformer after the dead feeder is disconnected. Both transformers contain loop-feed bushings on the primary with the emergency feeder "parked" in each
transformer primary section. The use of this emergency loop feeder allows connection of the client's entire load to the two EG sets in the event of total loss of utility power. When the generators are paralleled, procedures allow for synchronization and equal load sharing of the client's load.
While these abnormal conditions may be very unlikely, the criticality of the data site's load and the utility's need for generated power during peak loads easily justifies the added expenses of providing for parallel operation of the generators.
Electrical design parameters
Several important considerations impacted the electrical design of the peaking/emergency electrical system.
Load factors. The facility was able to shed data processing load down to a required continuous operation limit of 2000kW. As such, 2000kW was the lowest module of power requiring continuous service to the data site. This estimated demand lead to the selection of two 1825kW prime (2000kW standby) rated EGs.
On-site power generation. During discussions with the local utility at the initial design stages, a dispersed generation program (on-site power) was identified. This program provides for the utility company to construct, own, and operate prime rated EG capacity located on a client's property for the dual purpose of providing standby emergency power to the client as well as prime peaking power to the utility. This program is part of an overall demand side management objective of the local utility and one that is carried out with concurrence of the State Utility Commission. A fixed dollar per prime kW is invested by the utility with the remaining costs contributed by the utility's customer. All operational and maintenance costs of the EG sets, including fuel, are borne by the local utility, which has ownership rights to the equipment. The customer has purchase rights after an agreed-upon time, normally 20 to 25 years.
System voltage. The serving utility had a standing agreement with a local dealer of EG sets for complete system construction, assembly, and delivery to site of such units. The dealer had successful experience with these installations.
The original intent was to order the generators with 13.8kV output to match the incoming primary feeder voltage for parallel operation. The EG manufacturer, however, didn't offer a 13.8kV set. Because they were available and could be promptly delivered, and because the sets had proven reliability, two 2596-hp turbo-charged diesels were specified through the utility, each connected to a 480V generator. This selection required that the generator output voltage be stepped up to 13.8kV for direct parallel connection to the utility grid. Thus, two 2000kVA step-up transformers were needed to bring the 480V generator power up to the 13.8kV level:These mineral oil-filled pad-mounted transformers were installed simultaneously with the 15kV switchgear. Each transformer is connected in series with it's respective generator set and power breaker in the switchgear.
The 13.SkV primary power is then connected to two facility-owned, 2000kVA, 13.8 kV/480V, pad-mounted transformers located adjacent to the data site.
Meeting site conditions
The two EG sets are each housed in separate weatherproof insulated enclosures, which were constructed by a local specialty switchgear shop. The EG sets were shipped directly to the local shop and fully assembled with controls inside the enclosures. The enclosures contain electric heating for the rugged Minnesota winters and ample ventilation for the hot and humid summers. Each EG set contains a 3000-gal bladder tank located in the sub-base, with double wall containment in the event of a leak. External sound attenuation hoods and oversized internal radiators are included to reduce the running noise while keeping the engines cool in mid-summer. It was determined from previous experience that more ground-level noise is generated by high tip speed of the radiator fan blades than from the muffled exhaust; hence, sound attenuation hoods were provided. All assemblies were delivered to the site on flatbed trailers, hoisted in place with a large crane, and installed within days.
In the mean time, the 15kV class switchgear, which includes utility grade relays and meters, programmable logic controllers, drawout circuit breakers, monitors, gauges, etc., was constructed by a local specialty switchgear shop. It was installed inside a separate weatherproof enclosure, interconnected, and tested before leaving the shop. The entire assembly was delivered to the site and set on a prepoured concrete pad within a matter of hours. To expedite construction, the concrete pads, primary cables, and manholes were constructed just prior to delivery of the switchgear and step-up transformers.
Equipment scheduling
The purchase order for the 13.8kV, twin peaking, 4000kW, generator-transformer system was placed with the local engine generator dealer on March 15. This dealer subcontracted the electrical apparatus (transformers, switchgear, etc.) with various other dealers. The schedule called for switchgear and transformers to be delivered to the site by June 25 and switchgear in service by July 16 of the same year. The EG sets also were to be delivered to the site by July 16. The full system was to be in service by August 27. This schedule provided the client with permanent dual-primary power by mid-July, and gave it additional standby EG-set power by the end of August.
Fortunately, the state's Pollution Control Agency (PCA) (see sidebar on page 30) granted preapproval for concrete pad construction in May, with final permit approval granted June 1.
This was a very aggressive schedule requiring a high degree of coordination among many parties. A rainy spring nearly ruined the underground primary feeder installation and concrete pads construction schedule. However, the team approach to construction paid off to everyone's benefit as the schedule was met and came within budget.
The local utility was especially cooperative and provided specialists from several of its divisions to help assure timely delivery of equipment by working with the engine generator vendor, by assisting with the installation of equipment, and by performing some testing services.
Testing the components
Testing of the switchgear was initiated even before it left the assembly shop. Once in place and connected to the utility primary feeders, load transfer testing was done on each feeder individually. With the client's scheduled relocation date near, the switchgear was permanently connected to the client's step-down transformers before the generators were on site. This way, the client's regular load could be powered up, the data processing equipment connected for a trial run, and internal adjustment procedures carried out.
Shortly after permanent power was established, the EG sets were transferred to the site and hoisted in place. The generators were load bank tested before final connection to the switchgear. Following testing of the EG sets, the next step was to cut over the generator feeders to the switchgear and test the entire system simulating an actual power outage. By this time, the client's data site was up and running. Therefore, any planned outage had to occur between the hours of 12:30 a.m. and 6:00 a.m., one Sunday a month, when the client was performing internal data processing maintenance. And, there had to be advanced notification.
The first full system test and cutover occurred on an early Sunday morning in mid August. A detailed time-based schedule of events and testing sequence was prepared, with alternate backup routines established in the event the actual systems test resulted in equipment failure or damage. Temporary site lighting was set up and backup personnel placed on call. As each feeder was cutover, power transfer sequence was tested first using a dummy 120V source of power, then each primary feeder was connected to the system.
In mid October, the generators were run in parallel with the utility for 10 hrs at full load output, including multiple start/stop sequences. Since this operation did not require an outage, the generators were run during normal hours.
On October 31, the generators were again tested early in the morning. A voltage potential of 59V was detected on Phase C to ground at the No. 2 generator output and the test halted. Subsequent investigation suspected the source of the problem to be a C-phase ground detection lamp with incorrect voltage rating, creating a low impedance path to ground. This caused the bulb to burn out, clearing the low impedance path before any protective devices operated. The problem was corrected by using a higher wattage ground detection resistor and matched ground detection lamps having equal voltage and wattage. At this point, the system still had not been tested with the primary feeders actually shut off. Loss of utility power had been simulated by opening the incoming power breaker.
Finally, on the early morning of December 19, the two primary feeders were sequentially interrupted at their respective riser poles, resulting first with total load transfer to the remaining primary feeder and then independent load transfer to respective generators upon startup and stabilization. No. 1 generator started and picked up its respective load of about 600kW within 24
sec. No. 2 generator failed to start right away and was manually restarted after a quick fuel-mixture adjustment was made. It also then picked up its respective load.
Unfortunately, No.2 generator exhibited a load imbalance: 500kW on Phase A, 300kW on Phase B, and 450kW on Phase C. Since the actual load was a balanced 3-phase load, something was wrong within the on-site generation power transfer system. After about 30 min of generator run time, the individual loads were automatically retransferred back to the utility and the generators initiated orderly shutdown. Follow-up investigation revealed that the B-phase fuse connecting the generator to the switchgear bus had become disconnected from its holding clip.
The team learned from this project that, even with thorough systems testing prior to leaving the factory or assembly shop, and with extensive testing on site prior to actual load pickup, small unanticipated problems can still occur when the real load is transferred during an actual loss of utility power. Therefore, you should specify and demand full systems operational testing under all scenarios, including pulling the plug on the incoming utility feeders. Monthly manual exercising of the plant under no load, but with maintenance personnel on site and alert, will also allow timely identification of nuisance problems without risking the loss of power to a critical load.
It was during this same cold December morning testing that the need for sound attenuation hoods on the generator enclosures was fully identified. Previous generator testing in late summer and fall occurred with leaves on the trees and other landscape vegetation helping to absorb the sound. But with all vegetation gone and the air still and cold, sound is transmitted long distances. It's not uncommon for generators to have a loading less than their full ratings, as occurred with this project as well. Under lightly loaded conditions, the engine exhaust noise is lower than at full load. However, the radiator fan blades rotate the same speed regardless of load. It's the noise of the fan blades (high tip speed) that created the need for the sound attenuation hoods. This is the same condition that causes an airplane's propeller to sound so much louder on engine run up and takeoff on a cold, still day.
Monitoring helps client keep track of status of equipment
The facility receives limited operational information directly from the switchgear for each of the two power modules for the following conditions.
* Run in peak mode.
* Run in standby mode.
* Fire detection alarm.
* System abnormal summary alarm.
* Programmable controller failure.
This information is imported directly to the facility monitoring system along with electrical load data, which includes voltage, kW load, kVA load, and power factor for each power module. This allows operating personnel to know what's going on with the power and assist the utility in monitoring local site conditions.
A review of the facility's incoming power monitoring logs showed utility loss on one of the primary feeders 14 times between June of one year and April of the following year. Twelve of these outages occurred during the months of June through August. The other primary feeder experienced eight outages during the same time period, with five of these during the June-through-August summer months.
Recent utility power outage
On a recent Sunday, at 7:42 a.m., another outage occurred. One of the local utility's primary feeder cables faulted inside the utility main circuit breaker cubicle, causing the respective substation feeder breaker to trip. Upon reclosure of the substation breaker, the nearby utility pole cutouts opened due to the faulted cable. As a precaution, the remaining feeder's breaker was opened upon hearing some crackling sounds coming from its cubical.
Unfortunately, the utility had the engine-generator controls on lockout mode, which prevented automatic start of each of the generator sets. The customer started preparing for an orderly shutdown of the datasite because UPS battery power was rapidly being used up and because computer rooms were getting hot due to loss of cooling.
The utility was immediately contacted upon loss of power and they quickly switched the generator sets into automatic operation. Within 15 min of initial loss of power, both sets were running, delivering full power to the datasite, with the faulted feeder disconnected. The faulted feeder was repaired and the circuit breaker line-side bussing replaced within a week. Full utility operation commenced five days later.
TERMS TO KNOW
Demand side management: A process for reducing the demand on the power generation facility (usually the local utility) by the power user (utility customer). Various strategies can be used such as synchronizing the operation of large motors so that they do not operate concurrently.
Peak shaving: Reducing electrical power usage by a facility during a period when the serving utility is experiencing a heavy demand for its power, and/or, by providing on-site power to help the utility meet its power requirements.
Prime power: This is the rating for continuous operation of an engine-generator set (often, in lieu of purchased power from a utility) and represents the highest electrical power output available for unlimited hours per year, less time for maintenance.
Standby power: This is the rating of an engine-generator set when used as a secondary source of electrical power. This rating is based on the set operating 24 hours per day for the duration of the
outage of the primary power source. Because there is only limited operating service of the set, the rating of the electrical output is higher than for the rating of the set when operating in prime power mode. Operation at the standby rating results in greater mechanical wear rates and greater stress on the mechanical and electrical components.
PROJECT BACKGROUND
Its enlightening to see a complex project come together at breakneck speed, one that includes installation of on-site power with microprocessor control of the power, for the mutual benefit of both the client and the serving utility. The client, a Fortune 100 company, decided early in December 1992 to build a major computer datasite and office support facility that would accommodate nearly 1200 employees, consolidating its local work force. The company was growing very rapidly and needed to obtain the facility quickly. Therefore the client decided to lease an existing building in lieu of building a new one.
A vacant 340,000 sq ft manufacturing and storage building was found in a nearby community and remodel plans were immediately initiated. The schedule required a new 80,000 sq ft computer site, part of the overall project, to be completely operational within 8 months, with design professionals and contractors quickly selected for the team. Our firm was chosen to carry out the electrical and mechanical engineering, other than the engineering associated with the computer room systems. Fast track construction techniques were used to provide "hypertrack" construction.
Schematic design plans were started in early January 1993 with long lead equipment identified first. Tentative orders were placed with escape cancellation clauses in the event the lease of the building wasn't resolved in time to meet the client's schedule. Since the computer datasite was the driving force behind the project, the total project power needs were assembled and reliability of local utility power was analyzed. It was determined that two primary feeders, each from a different substation, were desired. However, the costs and construction time needed to obtain power from two separate substations was prohibitive. Therefore power from two separate feeders served by a single nearby substation was agreed upon. Analysis of existing utility power in the immediate area revealed that there had been multiple outages within the past 5 years, one caused by an auto collision with a utility pole at the corner of the project site.
A voluminous contract covering the engine-generators sets and the utility service being offered was worked out between the lawyers representing the utility company and our client. The whole project had a very tight time frame and the contract was resolved at a critical time that just avoided a time delay while the project permit was being reviewed by the state's PCA. The state legislators had recently enacted a requirement for all new pollution contributors, such as engine-generator sets, boilers, etc., to have the owner, or his or her agent, submit a highly detailed plan for review by the local PCA. Approval of the plan was required before any construction could begin, with stiff monetary penalties levied for early construction without a PCA permit. Therefore the utility constitution was extra cautious not to start any construction until the permit was approved, even though the project deadline was fast approaching.
Both the client and the serving utility benefited from this joint project. With new power generation plants costing upward of $2000 per kW to build, and a number of years for regulatory approval and construction, procuring 4MW of peaking power for $250 per kW is a bargain and a savings to the utility. Likewise, the client has 4MW of available standby power on site with an up-front contribution of about $188 per kW, but without the costs and headaches of maintaining, operating, or replacing the system. Even 100% of the preventative maintenance and fuel costs are borne by the utility. This allows the client to concentrate on it's core business with the peace of mind that highly reliable power will always be available to maintain its datasite operations.
Credits:
Architect: Ankeny, Kell, Richter & Walsh Electrical Contractor: Electric Repair & Construction Co. Engine-Generator Vendor: Ziegler Power Systems Co. Utility Company: Northern States Power Co. Computer Room System Engineering: Hypertect, Inc.
SUGGESTED READINGS
EC&M Articles
"Mobile Generators Power Up Newark Airport," February '95 issue. "When Standby Systems Are Emergency Systems," May '94 issue. Cost: $9.95 for articles. Order No. 2248. Orders are taken via facsimile machines only. To order by fax dial 800-234-5709. (Have a credit card and your fax number ready when you call.)
Gayland J. Bender, P.E. is Chief Electrical Engineer for Lundquist, Wilmar, Potvin & Bender, Inc., Consulting Engineers, St. Paul, Minn.
Multiple generators provide power for peak shaving/emergency systems.Aug 1, 1995 12:00 PM, Lawrie, Robert J.
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Modular 4160V generators slash $1 million off annual electric bills and assure emergency power. Versatile monitoring, control, and PLC systems integrate for total building control.
To provide peak-shaving/emergency power for the new International Concourse E at Atlanta's Hartsfield airport, eight 1250kVM 1100kW self-contained engine-generator modules are installed in a dual parallel 4160V scheme. For peak shaving duty, the generators can supply 8.8MW of prime power in parallel with the utility. In the emergency power mode, they can supply 10MW of emergency power to all critical loads and numerous selected loads as desired.
The single-line diagram shown on page 36 reveals how a very high reliability level of power is attained. The accompanying photos and related data provide details of the selected equipment and installation methods. In addition, the photos and diagram serve as a guided photo tour of the site. On the diagram, numbered arrows point to key system components; the position of each arrow indicates the angle of view as shown in the appropriate photo. Please refer to the diagram and photos as you read on.
Dual design assures dependability
After several feasibility studies, the engineering firm of Stevens & Wilkinson (S&W) of Georgia, Inc., Atlanta, selected 4160V distribution mainly because of the system's heavy loads and demand as well as long runs to and within the very large building. A lower voltage level would result in excessive voltage drop and losses.
An unusual but highly effective scheme for power distribution and the emergency power system combines redundant features of building power distribution with the generator power supply. The key is the use of dual sources at all voltage levels, dual feeds, tie switches, etc. The distribution system design utilizes a dual network, redundant supply, and primary distribution that works in coordination with a secondary-selective scheme.
It's important to note that the Concourse receives power in a dual utility/generator supply arrangement. Power comes from either of two entirely separate utility sources, or from either of two separate engine-generator sources, each of which supply medium-voltage (MV) power via multiple feeders to two separate main 4160V switchgear assemblies. From the two main MV switchgear line-ups, dual feeders supply 4160V to seven double-ended 4160/480/277V substations, each with two 2500kVA liquid-filled transformers. The substations are furnished with tie circuit breakers and automatic transfer switches as needed.
Multiple generators provide redundancy for emergency power as well as for diversification when they are on peak shaving. The eight generators are normally at rest, except when called on for peak-shaving duty. (A utility power failure automatically places the generators in an emergency power mode.)
In the event that a utility power supply fails, potential transformers (PTs) sense the utility supply loss and send a signal to generator controls, starting all generators. At the same time, all nonessential loads are shed. When two generators on Systems A or B have stabilized, circuit breakers are closed to reenergize the substations. If the critical loads demand more power, additional generators synchronize and come online as needed.
Integrated monitoring and control
Installed monitoring and control systems are numerous and integrate to provide automatic control, monitoring, and alarms. Included are four major systems with numerous subsystems that all work well together.
* Generator control system (action initiation).
* Power monitoring system (action initiation).
* Building management system (monitors and provides automatic control of HVAC and other systems).
* Ground-fault protection and monitoring, radio and phone communications, and coordinated relays that protect and control power distribution when on normal, emergency, or peak-shaving operation.
Also incorporated in the system are lighting controls and energy management functions.
The heart of all monitoring and control for the entire power system is based in a central control room, which is furnished with a number of 486DX computers equipped with programs that operate on "Window-type" programs. These programs bring up single-line diagrams of various portions of the power system and allow the operator to call up real-time readings of volts, amps, demand, kW, etc. at any component. Stored in the computer memory are minimum and maximum values and other data.
Ancillary equipment such as color printers, modems, and radio and communications equipment are included. Similar computer arrangements are installed in strategic locations throughout the 1.3 million sq ft building as well as in other parts of the huge airport complex. Printouts of data or diagrams provide a record of all activity on any system as desired, such as minimum or maximum demand and the date on which it occurred. Results of scheduled testing of the generators and emergency-power system are recorded for regular reference.
The generator control system is complex. When normal utility power fails, a PT sends a 24V signal to the generator controls, which initiate the following actions.
* Open utility circuit breakers.
* Start and parallel all generators.
* Transmit a DC signal to the central and remote computer control. If any type of malfunction occurs, an alarm sounds and appropriate action (automatic or manual) takes place as required.
On peak shaving, a similar control system called an I/O system is activated. This system includes reverse current, overcurrent, and over- and under-voltage relays. These relays send an appropriate analog signal to the I/O board, where the signal is converted for application to a computer for monitoring or action. The system will initiate an alarm if a generator has a problem or can open or close circuit breakers at either the 4160V or 480V level.
TERMS TO KNOW
Peak Shaving: The reduction of electrical power usage by a facility during a period when the serving utility is experiencing a heavy demand for power. To help the utility to meet its power demand requirements, on site power at the customer's site can be utilized.
Prime Power: The rating of an engine generator set based on its continuous operation. It represents the highest electrical power output available for unlimited hours per year, less the time required for normal maintenance.
Detecting moisture in dry type transformers.Aug 1, 1999 12:00 PM, Campbell, Dean
Failure of dry type transformers can occur during operation when moisture is present in the windings. Experience has shown this to be a particular problem in transformers ranging in size from 500kVA and larger; and with a primary voltage greater than 600V. As such, you should test these transformers before energizing to verify that windings are dry. This is especially important for units that have been stored and/or deenergized in locations with high humidity, dampness, or wide temperature fluctuations. If this testing is not done, catastrophic failure may occur.
Testing for moisture presence
The test method used in determining whether or not windings have taken on moisture is relatively simple. All you have to do is test for resistance between individual windings and between each winding and ground. (On existing transformers that are connected, make sure to disconnect primary and secondary leads from everything, including feeders, secondary bus, lightning arresters, etc. before beginning this test.) This should be done for both the primary and secondary windings.
The test voltage should be a maximum of 1kV above the rated winding voltage, unless other voltage limits are recommended by the transformer manufacturer. The minimum resistance readings taken should be as recommended by the manufacturer before energizing the transformer. If manufacturer recommendations are unavailable, the minimum resistance readings at 68 [degrees] F should at least be 20,000 ohms per volt for the rated voltage of the transformer coils being tested. If any of the tests result in a reading less than these recommendations, the transformer should be dried out and retested before being energized and put into service.
For example, on the primary side of a 13.8kV/480V transformer, the test voltage should be a maximum of 14,800V and the minimum acceptable resistance readings should be at least 13,800V x 20,000 ohms, or 276 megohms. The recommended resistance readings are very dependent on the ambient temperature during testing. Correction factor tables are available for other ambient temperatures. One source for these tables is the InterNational Electrical Testing Association's (NETA's) MTS-1993 standard, Maintenance Testing Specification for Electrical Power Distribution Equipment and Systems. (Call 1-303-697-8441.)
A resistance reading less than 20,000 ohms per volt would suggest moisture in the windings, unless the reading is extremely low (less than 100 ohms per volt), which could suggest a short circuit. If a short circuit is suspected, further testing should be done.
The above testing can be done with a megohmmeter or a high potential (hi-pot) tester. A hi-pot, with a knowledgeable individual running the tests, is the safest due to the controlled rate of increase in voltage.
For reference, the voltage (applied volts) divided by the microamps (leakage current read from hi-pot tester) is approximately equal to megohms.
Leakage current should remain constant over time at any constant voltage or the test should be immediately discontinued to avoid a failure.
If test results are OK, you should energize the unit as soon as possible.
If the results of the tests are marginally less than these readings, it may be possible to energize without drying out the transformer, but an electrical engineer or trained testing technician should be consulted to make such a decision.
Drying out a transformer
Drying out a transformer should be done as per the manufacturer's recommendations. If no recommendations are available, you can use either of the following methods.
Method 1. Place a 60W to 100W incandescent lamp under the front and back of each coil and leave them on for a minimum of two weeks if possible. Then, retest as per above and decide whether or not more drying out is required.
Method 2. Disconnect the primary and secondary leads from everything, including feeders, bussing, lighting arresters, etc. Then, short all the load ends of the secondary windings together but not to ground. If you have to use a shorting jumper, the calculation below will help you determine its size. If you can, just bolt all the secondary leads together.
Connect the primary leads to a voltage source as determined by the calculations below. If possible, the drying out period should be one week at a minimum. The transformer should then be retested as noted above. The resultant resistance readings will determine whether or not more drying out is required.
This method is probably more reliable than Method I in uniformly drying out the entire transformer.
Voltage source calculation
Step 1. Determine maximum primary dryout voltage with secondary shorted.
The maximum primary dryout voltage with secondary shorted ([V.sub.MPDV]) is found by using the following equation.
The connected primary dryout voltage ([V.sub.CPDV]) must be equal to or less than [V.sub.MPDV]. Therefore, depending on the available voltages at the respective site, a choice is made.
Step 3. Determine primary amps with connected primary dryout voltage applied.
First, calculate the primary amps with the normal application voltage applied ([I.sub.P]) using the following equation.
[I.sub.P] = VA / ([V.sub.P] 1.732) (Eq. 2)
Next, calculate the transformer's primary with [V.sub.CPDV] applied ([I.sub.CPDA]) using the following equation.
[I.sub.CPDA] = ([V.sub.CPDV] X [I.sub.P]) / [V.sub.MPDV] (Eq. 3)
This value is then used to determine appropriate circuit overcurrent protection and feeder sizing.
Step 4. Determine the magnitude of current flowing in the secondary jumper.
The current in the secondary jumper ([I.sub.SJ]) must be equal to or greater than the secondary amps with [V.sub.CPDV] applied. To determine [I.sub.SJ], we use the following equation.
[I.sub.SJ] = ([V.sub.P] X [I.sub.CPDA]) / [V.sub.S] (Eq. 4)
where [V.sub.S] = secondary voltage.
This looks complicated but really isn't. Let's do a sample calculation to see how easy it is.
Sample calculation
Suppose we have a 1500kVA, 13.8kV-480/277V transformer with an impedance of 8% that needs drying out. What is the maximum primary dryout voltage (with secondary shorted) needed? What is the magnitude of current the secondary jumper will have to conduct?
Step 1. Determine maximum primary dryout voltage with secondary shorted.
Since [V.sub.MPDV] is 1104V, the next lower readily available voltage is 480V. Therefore, [V.sub.CPDV] is 480V.
Step 3. Determine primary amps with connected primary dryout voltage applied.
First, we have to find the transformer's primary current with normal application voltage applied ([I.sub.P]) by using Equation 2.
[I.sub.P] = VA / ([V.sub.P] x 1.732)
= 1,500,000 / (13,800 x 1.732)
= 62.8A
We then insert this value into Equation 3 and solve for [I.sub.CPDA].
[I.sub.CPDA] = ([V.sub.CPDV] X [I.sub.P]) / [V.sub.MPDV]
= (480 x 62.8) / 1104 = 27.3A
Therefore, we should connect the primary windings to a 480V, 35A, 3P breaker (or fuse) for dry out.
Step 4. Determine the magnitude of current flowing in the secondary jumper.
We insert [I.sub.CPDA] as determined from Step 3 into Equation 4 and calculate the amount of current that will flow in the shorting jumper ([I.sub.SJ]) as follows.
[I.sub.SJ] ([V.sub.P] X [I.sub.CPDA]) / [V.sub.S]
= (13,800 X 27.3) + 480 = 785A
Therefore, we need a jumper of 785A minimum capacity to short the secondary windings of the transformer to each other (but not to ground).
We strongly recommended that you check with the transformer manufacturer for methods, voltage levels, resistance readings, dryout procedures; etc.
Implement a disaster recovery plan for telecom systems.Aug 1, 1995 12:00 PM, Knisley, Joseph R.
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Since telecom/network wiring is becoming so prevalent in facilities, what can be done to prevent complete shutdown in case of a disaster?
The loss of elements that support the transport of information (voice, data, image, and other signals) from one location to another can devastate operations at a facility. Therefore, you should develop a disaster recovery plan in case such a catastrophe occurs. Even though this plan may be well thought out, bringing back a downed network will not be a simple task. What cables are still intact? Can rerouting accomplish anything? Can critical segments be remapped? These and many other questions must be answered in great detail and as quickly as possible after a disaster occurs.
Suggested design alternatives
Before we discuss restoration procedures, let's look at some system design concepts that could be used in conjunction with setting up our disaster plan.
Alternate telephone service facility. Since the telephone service facility (where outside telephone network cables come into a building) is subject to damage from a variety of causes, an alternate entrance facility capable of handling part of the external telecommunications network is recommended. This service facility should be in a different part of the building. Thus, if the communication link to the telephone central office(s) is lost at the first entrance facility, some circuits will still be available at the alternate location.
Additionally, if a roof space is available, you may want to modify your existing system to accommodate satellite and microwave systems, which could be used to bypass the underground conduit service entirely.
Seismic design. If your area has even a small potential for seismic activity, the conduit and pathway systems should be designed to survive a credible event. Seismic and vibration restraints (springs, bracing, and aircraft cable) should be added to the structural support elements to keep cable systems and equipment in place.
You should segment entrance conduit runs to eliminate the possibility of a long conduit acting as a battering ram during seismic activity. This can be accomplished by using flexible couplings at
some conduit joints to provide freedom of movement. Also, wall and slab penetrations should be designed to permit conduit movement independent of the building.
Use trapeze hangers and two-plane bracing to support overhead ladder racks or cable trays. House telecom/local area network equipment in seismic-rated cabinets.
Parallel backbones and telecom closets. Another design alternative is the use of two telecom closets per floor along with two smaller-pair-count parallel backbone cables rather than one large pair count cable. The chance of a single backbone cable being damaged is greater than the chance that two backbone cables would be damaged at the same time. Each of these cables should have separate routes or shafts so that if one riser shaft is damaged, the other can be used to pull in new backbone cable.
Spare capacity. Spare capacity for future growth normally is not designed into a telecom infrastructure. However, since cable costs are such a small part of the total installation costs, this is an excellent way of providing spare media should temporary connections be needed.
Fire stops. Make sure that all wall and floor penetrations for cables and conduits have fire stopping materials installed. This will help prevent the spread of fire and smoke. While floor slab openings are generally fire-stopped when sleeves are installed in a new facility, this practice is often omitted when additional penetrations are made in an existing facility.
Record keeping. Document and keep up-to-date your cable plant. A good cable plant administration system makes it easy to identify both damaged and undamaged cables, thereby eliminating the difficult task of identifying cables under a time constraint. Also, record keeping makes it easy to assign spare capacity as replacement for damaged circuits.
Security. Access to network components should be controlled. This may range from locks and remote monitoring to guards and access codes. Where feasible, cable should be routed through secure parts of a building; outside cables should be buried to limit access. Manholes, handholes, pull boxes, and pedestals located outside should have strong, tamper-proof locking mechanisms.
Testing and restoring a network
Unless complete plant destruction occurs, there will be parts of a cable plant that are still functional. A visual inspection of wiring closets and wall plates can help you determine the condition of cable ends. With this information, you can decide what to test. This also will help in determining what sections of the cable plant to repair first. Also, you should compare the cable plant records with notes made during this inspection. This information can be used in making estimates about damage and preparing for cable evaluation.
Obviously, your first priority is to test the cables and separate the usable from the damaged. This can be done with hand-held testing equipment. Several types of compact reasonably priced test tools are available for copper cable evaluation.
An intelligent loopback plug and a signal injector are two devices used to identify conductors in a multiple cable run.
The signal injector does just what its name implies: inject tones onto telephone and/or copper data lines. Usually 3 distinct tones are generated so that several test devices can be used on the same line without creating confusion. An inductive tracer, or probe, is the used to identify the tone in the specific wire, usually from a distance of within 12 in. of the wire without piercing the wire's insulation. The main benefit is that the tone can be traced through dry wall, wood, or other nonmetallic surfaces.
This type of wire tracing test equipment is capable of checking for line polarity, continuity and ringing current in telephone lines and is also suited for twisted pair cable, multi-conductor cable, speaker wire, coaxial cable, alarm cable, and local area network (LAN) cable.
Handheld cable and network analyzers can do performance tests (Category 3, 4, or 5) to ensure that the network cable passes traffic properly. Most handheld testers support battery-powered printers; thus, testing can be done even if building power is not available.
In many locations, fiberoptic network restoration must also be considered. The equipment most often used to do an end-to-end test is a loss test set (test light source and a power meter). It's available as a piece of integrated equipment or as two separate components. The first step in fiber restoration is to locate the damage. Measure the system power level with a power meter and if the level is below that specified for the fiber, use an optical time domain reflectometer (OTDR). An OTDR transmits pulsed light signals down the fiber, providing component loss and reflectance information.
Although an OTDR can be used for troubleshooting, it has inherent limitations. It has dead zones or blind spots following reflective events. Event dead zones refer to the minimum distance from the start of the fiber where a reflective event can be distinguished.
Another handheld tool, the visual fault locator, is useful within OTDR dead zones. The fault locater is a visible light source semiconductor diode with a wavelength of 650 nanometers; it emits a red beam down the fiber. For breaks and significant fault points, the light is visible through a 3-mm coated jacket.
Using EIA/TIA standards
EIA/TIA 568, Commercial Building Telecommunications Wiring Standard, and 569, Commercial Building Standard for Telecommunications Pathways and Spaces, are used as the basis for voice/data design so that a structured cabling system can be developed. EIA/TIA-606,The Administration Standard for the Telecommunications Infrastructure of Commercial Buildings, is recommended as the basis for documentation. This standard tells us how to label and document the elements of the 568 and the 569 standards, such as the media (copper twisted pair, fiber, etc.), the pathways, and the spaces in the building containing the equipment.
TERMS TO KNOW
Attenuation: Deterioration of the strength of signals as they pass through a transmission medium (e.g. through cables, outlets, connecting hardware, patch panels, etc.).
Backbone: A term referring to certain cabling segments used to provide connectivity over long distances within buildings as well as between buildings in a campus. It also refers to certain network architectures used to connect multiple sub networks to one another.
SUGGESTED READING
Standards:
EIA/TIA-568, Commercial Building Telecommunications Wiring Standard.
EIA/TIA-569, Commercial Building Standard for Telecommunications Pathways and Spaces.
TIA/EIA-606, Administration Standard for Telecommunications Infrastructure of Commercial Buildings.
* "The EIA/TIA 568 Cabling Standard," October 1993 issue. "What To Know About EIA/TIA 606," December 1993 issue. "What To Know About EIA/TIA 569," February 1994 issue.
Cost: A set of these articles cost $14.95. Order No. 2208. Orders are taken via facsimile machines only. To order by fax dial 800-234-5709. (Have a credit card and your fax number ready when you dial by fax.)
* EC&M's Voice/Data Engineering/Installation Guide, May 1993 issue.
Cost: Guide cost $14.95. Order No. 2218. Orders are taken via facsimile machines only. To order by fax dial 800-234-5709. (Have a credit card and your fax number ready when you dial by fax.)
* EC&M's Voice/Data Engineering/Installation Guide, May 1994 issue. Cost: Guide cost $14.95. Order No. 2228. Orders are taken via facsimile machines only. To order by fax dial 800-234-5709. (Have a credit card and your fax number ready when you dial by fax.)
* EC&M's Voice/Data Engineering/Installation Guide, May 1995 issue.
Cost: Guide cost $14.95. Order No. 2238. Orders are taken via facsimile machines only. To order by fax dial 800-234-5709. (Have a credit card and your fax number ready when you dial by fax.)
Implement a disaster recovery plan for telecom systems.Aug 1, 1995 12:00 PM, Knisley, Joseph R.
0 Comments ShareThisNew
Since telecom/network wiring is becoming so prevalent in facilities, what can be done to prevent complete shutdown in case of a disaster?
The loss of elements that support the transport of information (voice, data, image, and other signals) from one location to another can devastate operations at a facility. Therefore, you should develop a disaster recovery plan in case such a catastrophe occurs. Even though this plan may be well thought out, bringing back a downed network will not be a simple task. What cables are still intact? Can rerouting accomplish anything? Can critical segments be remapped? These and many other questions must be answered in great detail and as quickly as possible after a disaster occurs.
Suggested design alternatives
Before we discuss restoration procedures, let's look at some system design concepts that could be used in conjunction with setting up our disaster plan.
Alternate telephone service facility. Since the telephone service facility (where outside telephone network cables come into a building) is subject to damage from a variety of causes, an alternate entrance facility capable of handling part of the external telecommunications network is recommended. This service facility should be in a different part of the building. Thus, if the communication link to the telephone central office(s) is lost at the first entrance facility, some circuits will still be available at the alternate location.
Additionally, if a roof space is available, you may want to modify your existing system to accommodate satellite and microwave systems, which could be used to bypass the underground conduit service entirely.
Seismic design. If your area has even a small potential for seismic activity, the conduit and pathway systems should be designed to survive a credible event. Seismic and vibration restraints (springs, bracing, and aircraft cable) should be added to the structural support elements to keep cable systems and equipment in place.
You should segment entrance conduit runs to eliminate the possibility of a long conduit acting as a battering ram during seismic activity. This can be accomplished by using flexible couplings at some conduit joints to provide freedom of movement. Also, wall and slab penetrations should be designed to permit conduit movement independent of the building.
Use trapeze hangers and two-plane bracing to support overhead ladder racks or cable trays. House telecom/local area network equipment in seismic-rated cabinets.
Parallel backbones and telecom closets. Another design alternative is the use of two telecom closets per floor along with two smaller-pair-count parallel backbone cables rather than one large pair count cable. The chance of a single backbone cable being damaged is greater than the chance that two backbone cables would be damaged at the same time. Each of these cables should have separate routes or shafts so that if one riser shaft is damaged, the other can be used to pull in new backbone cable.
Spare capacity. Spare capacity for future growth normally is not designed into a telecom infrastructure. However, since cable costs are such a small part of the total installation costs, this is an excellent way of providing spare media should temporary connections be needed.
Fire stops. Make sure that all wall and floor penetrations for cables and conduits have fire stopping materials installed. This will help prevent the spread of fire and smoke. While floor slab openings are generally fire-stopped when sleeves are installed in a new facility, this practice is often omitted when additional penetrations are made in an existing facility.
Record keeping. Document and keep up-to-date your cable plant. A good cable plant administration system makes it easy to identify both damaged and undamaged cables, thereby eliminating the difficult task of identifying cables under a time constraint. Also, record keeping makes it easy to assign spare capacity as replacement for damaged circuits.
Security. Access to network components should be controlled. This may range from locks and remote monitoring to guards and access codes. Where feasible, cable should be routed through secure parts of a building; outside cables should be buried to limit access. Manholes, handholes, pull boxes, and pedestals located outside should have strong, tamper-proof locking mechanisms.
Testing and restoring a network
Unless complete plant destruction occurs, there will be parts of a cable plant that are still functional. A visual inspection of wiring closets and wall plates can help you determine the condition of cable ends. With this information, you can decide what to test. This also will help in determining what sections of the cable plant to repair first. Also, you should compare the cable plant records with notes made during this inspection. This information can be used in making estimates about damage and preparing for cable evaluation.
Obviously, your first priority is to test the cables and separate the usable from the damaged. This can be done with hand-held testing equipment. Several types of compact reasonably priced test tools are available for copper cable evaluation.
An intelligent loopback plug and a signal injector are two devices used to identify conductors in a multiple cable run.
The signal injector does just what its name implies: inject tones onto telephone and/or copper data lines. Usually 3 distinct tones are generated so that several test devices can be used on the same line without creating confusion. An inductive tracer, or probe, is the used to identify the tone in the specific wire, usually from a distance of within 12 in. of the wire without piercing the wire's insulation. The main benefit is that the tone can be traced through dry wall, wood, or other nonmetallic surfaces.
This type of wire tracing test equipment is capable of checking for line polarity, continuity and ringing current in telephone lines and is also suited for twisted pair cable, multi-conductor cable, speaker wire, coaxial cable, alarm cable, and local area network (LAN) cable.
Handheld cable and network analyzers can do performance tests (Category 3, 4, or 5) to ensure that the network cable passes traffic properly. Most handheld testers support battery-powered printers; thus, testing can be done even if building power is not available.
In many locations, fiberoptic network restoration must also be considered. The equipment most often used to do an end-to-end test is a loss test set (test light source and a power meter). It's available as a piece of integrated equipment or as two separate components. The first step in fiber restoration is to locate the damage. Measure the system power level with a power meter and if the level is below that specified for the fiber, use an optical time domain reflectometer (OTDR). An OTDR transmits pulsed light signals down the fiber, providing component loss and reflectance information.
Although an OTDR can be used for troubleshooting, it has inherent limitations. It has dead zones or blind spots following reflective events. Event dead zones refer to the minimum distance from the start of the fiber where a reflective event can be distinguished.
Another handheld tool, the visual fault locator, is useful within OTDR dead zones. The fault locater is a visible light source semiconductor diode with a wavelength of 650 nanometers; it emits a red beam down the fiber. For breaks and significant fault points, the light is visible through a 3-mm coated jacket.
Using EIA/TIA standards
EIA/TIA 568, Commercial Building Telecommunications Wiring Standard, and 569, Commercial Building Standard for Telecommunications Pathways and Spaces, are used as the basis for voice/data design so that a structured cabling system can be developed. EIA/TIA-606,The Administration Standard for the Telecommunications Infrastructure of Commercial Buildings, is recommended as the basis for documentation. This standard tells us how to label and document the elements of the 568 and the 569 standards, such as the media (copper twisted pair, fiber, etc.), the pathways, and the spaces in the building containing the equipment.
TERMS TO KNOW
Attenuation: Deterioration of the strength of signals as they pass through a transmission medium (e.g. through cables, outlets, connecting hardware, patch panels, etc.).
Backbone: A term referring to certain cabling segments used to provide connectivity over long distances within buildings as well as between buildings in a campus. It also refers to certain network architectures used to connect multiple sub networks to one another.
SUGGESTED READING
Standards:
EIA/TIA-568, Commercial Building Telecommunications Wiring Standard.
EIA/TIA-569, Commercial Building Standard for Telecommunications Pathways and Spaces.
TIA/EIA-606, Administration Standard for Telecommunications Infrastructure of Commercial Buildings.
* "The EIA/TIA 568 Cabling Standard," October 1993 issue. "What To Know About EIA/TIA 606," December 1993 issue. "What To Know About EIA/TIA 569," February 1994 issue.
Cost: A set of these articles cost $14.95. Order No. 2208. Orders are taken via facsimile machines only. To order by fax dial 800-234-5709. (Have a credit card and your fax number ready when you dial by fax.)
* EC&M's Voice/Data Engineering/Installation Guide, May 1993 issue.
Cost: Guide cost $14.95. Order No. 2218. Orders are taken via facsimile machines only. To order by fax dial 800-234-5709. (Have a credit card and your fax number ready when you dial by fax.)
* EC&M's Voice/Data Engineering/Installation Guide, May 1994 issue. Cost: Guide cost $14.95. Order No. 2228. Orders are taken via facsimile machines only. To order by fax dial 800-234-5709. (Have a credit card and your fax number ready when you dial by fax.)
* EC&M's Voice/Data Engineering/Installation Guide, May 1995 issue.
Cost: Guide cost $14.95. Order No. 2238. Orders are taken via facsimile machines only. To order by fax dial 800-234-5709. (Have a credit card and your fax number ready when you dial by fax.)
New developments in patch panel technology.
Aug 1, 1995 12:00 PM, McElroy, Mark W.
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Three types of patch panels (wireless, electronic, and intelligent)have benefits and drawbacks, depending on the application.
When it comes to making a move, add, or change in a voice/data cable plant, sending a technician to perform physical changes at a patch panel in a wiring closet is one of the most time- and cost-intensive operations in the overall support of a communications system. In fact, one industry research group recently estimated that the cost to maintain local area networks (LANs) consumes 84% of a network manager's budget, 44% of which is spent on physical management and troubleshooting.
There are new types of patch panels currently available that can help reduce the time and cost associated with these activities. These "unconventional" patch panels each have their benefits and drawbacks, depending on the application at hand.
Some definitions
Before getting into the subject of what's out there in the way of electronic patch panels, let's first attempt to define the term. An electronic patch panel is a device that enables cross-connecting electronically; that is, without the necessity of making by hand any physical changes using patch cords or other cross-connect components. This definition also implies the ability to make such changes from a centralized station or terminal such that the patch panels controlled in this manner can be remotely monitored and/or configured by a central operator. The promise of this technology could be very appealing, since it would effectively eliminate the need to dispatch technicians to wiring closets every time a cross-connect of some kind (i.e., a move, add, or change) is required.
The ideal electronic patch panel should also be very flexible and very intelligent. It should not only enable the making of distributed cross-connections from a central point, but should also support mixed media cross-connections as well. For example, you may, in some cases, want to cross-connect horizontal unshielded twisted pair cables (UTP) to fiber in the backbone or, at a minimum, make UTP-to-UTP connections. In any event, the ability to support opti-electronic transitions in media would seem to be of some value in certain cases.
In practice, an intelligent, electronic patch panel would give a network manager the ability to make cross-connections from any circuit in horizontal wiring to any circuit in the backbone subsystem without ever having to leave his or her desk. Moreover, the ideal system would also enable cross-connections between two or more circuits in the horizontal subsystem, up to and including daisy chained-circuits when required.
An example
Let's consider a simple example. Let's say we're dealing with a situation where a well-designed structured cabling system is in place. And let's say that this system provides for a standard faceplate having four RJ-45 jacks at each workstation. At one such station, you therefore would have four hard-wired 4-pair connections back to the wiring closet. Now, let's assume that each user's connection to the wiring closet is terminated onto what we have described as electronic patch panels. Let's also assume that the cabling system in our example also features the usual mix of copper and fiber in the backbone (copper for voice, and fiber for everything else).
Now, if we look at a typical cross-connect situation for an average user in this scenario, we might expect to find that one RJ-45 connection at the wiring-closet end is cross-connected to a circuit that supports "data," while another is cross-connected to a circuit that supports "voice." The third and fourth jacks might not be in use, but could be at some future time, if and when applications appear.
Let's assume that a need comes up for the third jack at a workstation, thereby requiring the cable manager to establish a new cross-connection to support a new application, a dial tone line to a desktop for a fax machine, for example. Rather than dispatching a technician with a set of tools and, hopefully, accurate documentation, the cable manager in this scenario (assuming electronic patch panels are in place) simply turns to the cable management/patch panel control system and makes the change on his or her screen within seconds of having received the request. Once completed, the change is instantly recorded, thereby updating the cable documentation in the same stroke.
Our example not only illustrates the power of electronics when applied to an otherwise physical task, but also illustrates the overlap of this technology with another one: cable management systems. But while cable management systems are a relatively mature technology at this point, the same can not be said for electronic patch panels. There are, however, some interesting developments out there that suggest it's time to start paying attention to this emerging technology. Let's take a look at what products are currently available to you.
Product classifications
A high level review of what's out there in terms of what we'll generally refer to as "unconventional" patch panel technology yields products in at least three different categories. In spite of their differences (which we'll describe), all products in these categories have one thing in common: they are designed to minimize, if not totally eliminate, the need to physically make moves, adds, and changes as traditionally done by technicians using patch cords and cross-connect wires in wiring closets.
The three categories of products, according to our view of the world, are as follows.
* Wireless patch panels
* Electronic patch panels
* Intelligent patch panels
Wireless patch panels. Products in this category fit the description of "unconventional" patch panels, but are far from electronic or intelligent. They are, in fact, passive devices. This is an important distinction compared to the other two categories, which themselves may be wireless, but are not passive.
Wireless patch panels are also wireless only to the extent that they avoid the use of external patch cords to achieve cross connections. Instead, they rely on internal cross-connections using hard-wired modules that connect "input conductors," or jacks, to "output conductors." This results in a straight-through cross-connection of media such that each conductor's identification is maintained all the way through the cross-connected circuit (e.g., the Tip of pair I is maintained; Ring of pair i is maintained; Tip of pair 2 is maintained; etc.). Wireless patch panels designed for copper connections are usually based on 2-, 4-, or 8-wire configurations (i.e., for UTP cable plants).
With wireless patch panels, the assumption is that "normal" prevails for most jacks most of the time, and patch cords should only be used to make changes or to deal with the exceptions. Patch cords, therefore, can be used with wireless systems, but only when deviations from the norm are required. Insertion of a patch cord into a wireless patch panel breaks the internal cross-connection, thereby freeing the input channel for reassignment to another output channel at the user's discretion.
All of the physical administration of a wireless patch panel must still be performed on a remote basis by technicians inside wiring closets. Wireless systems, according to our classification here, are therefore not addressable by remote devices and can not be controlled from a central management station of any kind. Wireless systems are also built using passive components and are not electrified in any way. (In terms of appearance, wireless patch panels closely resemble conventional patch panels and are comprised of RJ-type jacks on their front panels.)
Electronic patch panels. Unlike wireless patch panels, electronic patch panels are active in the sense that they are electrified. [ILLUSTRATION FOR FIGURE 1 OMITTED]. They, therefore, are capable of sensing, capturing, and storing certain operating or status conditions, which can then be reported to the network manager for proper handling. Electronic patch panels, again according to our classification, can not, however, be managed from a remote, centralized station, and still require physical handling by technicians inside wiring closets.
Examples of functionality common to electronic patch panels include "sensing" of patch cord insertion or removal from a jack, after which insertion/removal "events" are captured and reported to a network manager. Other examples include the ability to turn LED lights on or off around specific jacks to help guide technicians perform cross-connect work when changes are required. In general, a well-rounded electronic patch panel system will provide a variety of status and change-oriented reports, and may also be used to sense other systems of importance in managing the physical environment in wiring closets (e.g., cooling fans, thermometers, alarm systems, etc.).
Intelligent patch panels. This category of "unconventional" patch panel, as shown in Fig. 2, includes devices with all of the attributes of wireless and electronic systems. As its name implies,
the intelligent patch panel is fully manageable from a remote, centralized station. Products that fall into this category support a variety of automated functions including the following.
* Centralized online control: The ability to make cross-connection changes and assignments from a remote, centralized station, thereby eliminating the need to send technicians to wiring closets. This also implies the total elimination of patch cords, since all cross-connections are made through internal electronics.
* System monitoring and reporting: The ability to activate certain sensor functions, as in the case of electronic patch panels, along with powerful reporting capabilities for network management.
* Automated recordkeeping: Fully integrated cable management system such that moves, adds, or changes are controlled by the system as well as recorded for full "as-built" documentation reporting capabilities. Systems of this type make heavy use of user-friendly graphical user interfaces (GUIs) as well.
* Disaster recovery: The ability to withstand power failures using backup power supplies, robust memory systems, and secondary/back-up path selection.
* Security: The ability to password-protect configurations and associated databases is also common to these and all other intelligent systems.
Equipment analogous to intelligent patch panels
In many respects, the concept and functions of intelligent patch panels are very analogous to other forms of switches and intelligent hubs. In the voice arena, every PBX or telephone system out there minimally performs internal cross-connections between stations (extensions) and trunks (phone lines). Even the process of establishing station-to-station connections or multistation or conference calls resembles the same kind of flexible connectivity implied by our definition of intelligent patch panels.
In the world of high speed data networks, intelligent hubs (sometimes referred to as wiring concentrators) also provide dynamic internal cross-connections when used in conjunction with remote, centralized management software. Using systems of this type, a network manager can electronically group several network users together in one "logical" network on one day, and on the next, totally redefine the group such that a different mix of users results. This can all be done without having to dispatch technicians to wiring closets.
Performance considerations
In spite of the general appeal to the products discussed here, none of them offer a panacea from a functional standpoint, and all have their limitations. First of all, any of the products currently in place on the market are media-bound. In other words, they are all either copper-only or fiber-only in their makeup. This may represent a drawback in cases where users require multimedia connectivity such as needing to extend copper circuits over long distances where fiber may be the preferred choice.
All of the systems currently on the market also appear to be rather application-bound. In other words, the intended applications seem to be restricted to data-only requirements. This appears to be less the case with the wireless patch panels and more the case with the others. The granularity of treatment required for voice where, for example, 1-pair cross-connections are often required, would appear to be either impractical or cost-prohibitive based on how these systems are built and configured.
And lastly, even when it comes to data network performance, this technology is still somewhat behind the rest of the industry. If we assume that the expectation in the industry is that 100-megabit per second (Mbps), or Category 5 UTP, performance levels are required, products in each of the three categories discussed above roughly break out as shown in Table 1.
What's interesting about this performance analysis is that only the "unconventional" patch panel that still relies on "conventional" cross-connection methods rates at the industry-standard level of Category 5 performance. This is not surprising since Category 5 systems are very sensitive to deviations in crosstalk and attenuation. Unconventional methods of establishing and maintaining circuits appear to have a ways to go before these alternative technologies can stand up to the task of supporting continuous data streams at 100 Mbps. At this point in time, only one manufacturer of "intelligent" patch panels indicates that its products have been tested successfully at 100 Mbps, but have not yet been certified as such. This could prove to be problematic in the short run, since all of the industry standards for certification are based on the use of conventional connecting hardware, not unconventional electronic switches.
In any case, the nature of the products described here as electronic patch panels is such that all electronic functionality is essentially nonintrusive. In other words, the key functions of sensing and reporting really do not interfere with or participate in the cross-connections themselves. And certainly the presence of LED lights and so forth are external to the connectivity scheme inherent to these devices. Thus, it's no surprise that performance for these products are in line with the mainstream of traditional patch panels, and are higher than products in the other two categories.
Cost considerations
In looking at cost, we chose one representative product from each of the three categories defined above. Our attempt to reduce the comparison to equal terms is based on the notion of a typical cost per port. Our findings are shown in Table 2.
Well, as they say, "you get what you pay for." The lesson here is that with wireless and electronic patch panels, you're still paying for the performance of physical moves, adds, and changes by people (dispatching technicians to wiring closets, their salaries, etc.). With fully intelligent patch panels, you're not. Thus, the real costs of wireless and electronic systems also include the traditional cost of the human resources required to work with them. But given the still low levels of performance for intelligent systems coupled with their severe application constraints (data only), we would conclude that these systems have a long way to go before they begin to outweigh the flexibility offered by traditional connecting hardware schemes.
Nevertheless, it's definitely time to start watching this technology. In this business, everything can change in the course of a week!
The case of the soon-to-be overloaded neutral conductor.Aug 1, 1995 12:00 PM, Moravek, James
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Odd triplen harmonic currents and neutral overloading go hand-in-hand.
A facility manager of a data center was informed by the Information Systems Department that additional file servers would be installed and would be powered by an existing panelboard. Therefore, an analysis of the panelboard, its feeder, and loading need to be done.
To find out if a problem would occur with the proposed load addition, the facility manager asked the maintenance electrician to review the annual testing and maintenance reports on the feeder of the existing panelboard. The feeder in question consisted of three 500kcmil phase conductors, a 1/0 AWG grounded (neutral) conductor, and a 1/0 AWG isolated grounding conductor. All conductors had 75 [degrees] C insulation. The data center's feeders were installed with the practices of time, which included reducing the neutral conductor in the feeder. The annual testing and maintenance report indicated loads of 99A, 130A, and 77A respectively for the phase conductors and 130A for the neutral conductor. The load on the feeder after the addition of the new file servers was estimated to be 132A, 182A, and 101A respectively for the phase conductors and 197A for the neutral conductor.
The electrician noted that an overload problem would occur, not with the phase conductors but with the neutral conductor. The phase conductors would be loaded to approximately 60% of their ampacity (B phase as worst case) while the neutral would be overloaded to 131% of its ampacity.
Detailing the problem
The problem was more than an imbalance on the phase conductors. Using the following equation, the anticipated (normally expected) neutral current ([I.sub.N]) is 46A for the present load and 71A for the estimated revised load.
The measured neutral current of 130A is obviously more than the 46A calculated.
The cause of this problem was harmonics. The load on the existing panelboard was line-to-neutral connected nonlinear loads. This type of load consists primarily of odd triplen harmonic
currents (e.g. 3rd, 9th, 15th, 21th, etc.). These currents add in the neutral conductor and can cause measured neutral currents as much as twice the phase conductor currents.
How can neutral currents be twice as great as phase conductor currents? The rules change on power systems having odd triplen harmonic currents. Let's investigate this further.
Nonsinusoidal currents
The neutral current consists of the imbalance of the phase conductor currents ([I.sub.A], [I.sub.B], and [I.sub.C]). If these currents consist only of the fundamental (60 Hz) current, there is a 120 [degrees] phase shift between each phase and the summation of these currents at every instant in time is zero.
Graphically, this is shown in Fig. 1 below. Here, three phase currents are superimposed on a graph. By stopping time at the noted instant, the amplitudes of each of the currents when added together will equal zero. As a result, the neutral current is zero.
If the phase conductor currents contain both fundamental and odd triplen harmonic currents, the result is very different. Odd triplen currents are zero sequence currents in that they are in phase and will add in the neutral conductor. The harmonic current with the largest profile in the odd triplen harmonic currents is usually the third harmonic. Fig. 2 shows us the sinusoidal waveforms of each of the phase currents, along with that of the third harmonic for all three phases. While the fundamental (60 Hz) currents of the phases cancel each other, the third harmonic (180 Hz) currents of each of the phasesadd together. Fig. 3 shows how these currents flow in a 3-phase, 4-wire schematic.
When the third harmonic is present, a distorted voltage waveform will result and will provide a characteristic signature of the nonlinear load. Fig. 4 shows how the combination of sinusoidal voltage waveforms and third harmonic waveforms create harmonically distorted waveforms.
Possible solutions
In relation to our potentially overloaded neutral conductor, three options were developed to address this problem.
* Replace the feeder with one that has full size neutral.
* Use a neutral filter to reduce the anticipated neutral current within the rating of the conductor.
* Relabel the isolated grounding conductor and reconnect it as a parallel grounded conductor.
The first option was not considered due to the problems and time involved in replacing the feeder in this particular application. Financially, the data center could not afford the extended outage required to replace the feeder.
The second option was seriously considered. The neutral filter uses a zig-zag transformer design that reduces odd triplen harmonic currents by a factor of 7.5 or greater. It uses the phase shift characteristics of a transformer to cause cancellation of the odd triplen harmonics. This would reduce the anticipated neutral current to a value less than the current rating of the conductor. The filter was reasonably priced and could be installed with a little downtime of the feeder. The disadvantage of the filter was that it had to be installed adjacent to the affected panelboard. This took up floor space and meant additional cooling load to the center's cooling system.
Though the third option was a late entry, it proved to be the winner. The isolated grounding conductor (same size as the neutral conductor) was never used as intended. Other feeders to the data center did not have grounding conductors; instead, the metal raceway system was used as a ground. After discussions with the electrical inspector and the facility manager, the decision was made to relabel the isolated grounding conductor, disconnect it, and reconnect it as a parallel grounded conductor. This doubled the capacity of the neutral. The consensus was that this situation was no worse than other feeders in the system. And, the work could be accomplished with minimum downtime on the feeder. The anticipated neutral current would now be approximately 65% of the capacity of the conductor.
James Moravek, P.E. is Vice President of Hammel Green & Abrahamson, an architectural and engineering firm in Minneapolis, Minn.
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Contractor safety plan offers multiple rewards.Aug 1, 1995 12:00 PM, Lawrie, Robert J.
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Comprehensive electrical safely program reduces accidents, builds morale, increases productivity, and boosts the bottom line.
Electrical safety is of paramount importance at Nordling Dean Electric Co., Inc. Chatham, N.J. According to Jerry Murphy, president of the firm, electrical safety plays a major role in company operations because of the many benefits imparted to all concerned.
First and foremost is the health and welfare of employees, particularly on-site electricians who face possible accidents every day. Murphy says that when employees know that we care enough to plan, install, and enforce a total safety program, they appreciate it and reward our efforts by returning a sincere effort to comply with the program. It's a win-win situation.
Accidents cause injury to personnel, impact the family, and result in lost time as well as damage to property. When accidents happen, morale drops, efficiency suffers, insurance rates go up, customers are unhappy, and everybody loses.
On the other hand, a well-planned safety program benefits everyone.
Multi-facet safely program
The safety program was planned and organized by Carl Dumont, vice president of construction. As the years passed, the program has expanded, developed further, and diversified to cover essentially all aspects of safety. Some of the major parts of the safety program are as follows.
Corporate statement of policy. The firm has published their Safety Manual, a public document that includes the following corporate statement of policy.
Safety of employees and the public is a top priority that will not be compromised. Every effort will be made to prevent accidents by timely recognition and correction of unsafe conditions and unsafe practices. The firm will comply with all laws concerning safety and health enforced by local, state or federal authorities.
All-level safely coordination team. A safety coordination committee consists of four key members from all levels in the company. All members have multiple duties; however, each has specific duties. For example, Abe Bawarshi, vice president of engineering, coordinates and updates the entire program with special emphasis on the Safety Manual. Leon Baptiste and Charlie McCormick, project managers, monitor and review safety at job sites, especially work procedures, to assure maximum safety. Roger Dumont, warehouse manager, keeps tabs on all protective equipment, test instruments, and safety items. He regularly sends out rubber gloves, blankets, and instruments to be tested for reliability. Bill Davi, journeyman electrician, teaches apprentice safety classes.
Job start-up and site inspections. As soon as a job is scheduled, the superintendent and foreman meet to discuss accident prevention. Job-site conditions are examined and any danger spots are pointed out. Foremen make regular inspections of the site looking for any accident-prone locations.
Supervisor/foreman meetings. At these meetings, which are held at least quarterly, supervisors and foremen discuss safety data, experiences, accidents, and/or problems in detail. Their objective is to increase safety and reduce accidents. These meetings accomplish much the same as the job start-up and site inspection procedures do.
Tool box meetings. At each job site, a ten-min. meeting is held by the project supervisor or job foreman each week. Accidents or near accidents are reviewed and actions to prevent recurrence are discussed. Everyone is encouraged to contribute ideas that would enhance work safety.
Accident investigation. Accidents will happen, however. When they do, the Safety Coordination Team makes a full investigation, obtains all facts, details, and possible causes, and then takes
suitable action to prevent recurrence. The team uses a thorough accident investigation procedure to assure a meticulous inquiry. The procedure investigates sequence of actions, condition of protective equipment, if they were used properly, etc. The job foreman is responsible for assignment of men who are trained or competent for each kind of work. A comprehensive questioning guide is used and an accident report must be filled out and sent to top management.
Seminars/training. On a regular schedule, Leon Baptiste, power quality engineer, also serves as a safety director. One of his responsibilities is to attend OSHA and safety seminars to be certain that work procedures and the safety manual incorporate the latest data concerning safety. Training also includes sessions for all company personnel held at least twice a year. Training of journeymen is an ongoing process in the field, as they observe and use protective items, instruments, the safety manual, safety checklists, etc.
Safety manual. The company's 110-page safety manual is a mainstay of the safety plan. It covers all common safety situations and provides guidance for essentially any safety circumstance that might exist. For example, chapters in the manual discuss OSHA regulations, accident investigation procedures, first aid, training, fire prevention and control, lockout and tagout procedures, ground-fault protection on construction sites, just to name a few.
Safe procedure checklists. For common or repetitive work, certain hazardous work, and to meet OSHA rules, "Safe Procedure Checklists" must be read and used to guide work; completed checklist must be submitted to central accident record-keeping when the work is completed.
Working in confined space is especially hazardous; permits are required and checklists for the work must be used. The same is true for work procedures such as work performed with hazardous materials.
Protective equipment, tools, test instruments. All protective items are listed on a main index, signed out to job sites, and tested upon return and/or on a schedule. Typical equipment includes hard hats, hearing protection, glasses, face shields, respiratory equipment, first-aid kits, stretchers, ladders, scaffolds, power tools, test instruments, rubber blankets, gloves, and high-voltage testers.
OSHA training. The firm keeps up to date with the latest OSHA standards by sending safety directors to OSHA seminars and carrying out all regulations as required. Information concerning OSHA rules is included in chapters in the Safety Manual. Typical OSHA rules and activities include displaying OSHA posters in company buildings and strict use of signs, labels, color codes, and posters, etc. to warn of hazards. Additional regulations relate to maintaining detailed records (using the OSHA log and summary of occupational injuries and illnesses), maintaining a list of all hazardous chemicals present at the company or at work sites, training relative to hazardous chemical materials, and the use of safety data sheets.
Management review. Top level officers in the firm review all reports pertinent to the safety program and are well versed in all aspects of the safety program on the job. They are aware of the benefits that accrue as mentioned earlier. As a result, motivation program has been instituted.
Incentive plan. The firm started a program that motivates and rewards the foreman that has the best safety record each year. The award is given at the company annual holiday party. This award program reaffirms the company's dedication to an effective safety program and helps to motivate foremen to do their best.
Efforts to ensure continuity of power must not create potential hazards through improper grounding techniques.
Remote pumping stations, each with their own local disconnecting means and grounding electrodes, frequently cause difficulties in applying appropriate grounding rules. Often a property owner and the utility establish a remote service point where the power will be metered, and the owner takes over from there, providing the wiring to the remote stations. In some cases there may be from four to a dozen installations stretching out up to a mile or more in total distance. In the specific case involved here, the system is 480V corner grounded. Some utilities require this type of service in cases where there are no ground detectors in place.
At the point of supply to the customer wiring system, there will be a meter and a fused disconnect (or circuit breaker). This is the service disconnect for the installation, and it also allows the utility to troubleshoot and for overload protection for the transformer. Where the utility's primary distribution includes a grounded conductor (the usual case), there may be a 4-wire drop to the customer's pole. One option some utilities use is to connect the messenger wire to a driven ground at their distribution transformer only. The owner then usually distributes power from the meter location with three separate, insulated overhead conductors.
We received a report that when motors are connected to the load side of this distribution, there are instances of fault currents passing through local electrodes with magnitudes sufficiently high enough to literally bake dry the soil at the electrode. The result is an extremely high electrode resistance, and the frames of local motors and switchgear can be operating with a dangerous touch voltage.
When this happens, some owners treat the downstream equipment as though it were connected to an ungrounded system in order to increase reliability of service on these untended remote installations. This means that although there is a grounding electrode at each equipment location, there is no connection between the grounded phase conductor and this electrode.
According to the report we received, the owner, in cooperation with the utility, decided to lift the grounded phase off the center lug of the meter socket, and insulate it. In addition, he proposed to connect the load end of the messenger wire to a driven ground at his service pole. The meter
enclosure and disconnect will be grounded through a short piece of solid copper, bonded to the pole ground. The proposed solution is shown in the drawing.
The EC&M Panel's response
We strongly disagree with the proposed solution. The system is a chimera, an incongruous combination of grounded and ungrounded distribution practices that fails to offer the minimum safety requirements of either.
Lifting the grounded phase from the service grounding terminals does not make the system ungrounded. A fault in the service equipment will still return to the transformer over the pole ground wire and the quadruplex messenger. Note, however, that the pole ground may not have the required size and would not be "routed with the phase conductors" as required in Sec. 250-23(b).
The owner's grounding system
However, our real concerns arise from what the description implies about the grounding system. Grounding electrodes are part of a ground reference arrangement that stabilizes the voltage to ground and dissipates surges. In performing this function, they are never intended to carry significant amounts of current for extended periods of time. At utilization voltages, they must never be relied upon for the return of fault current.
Fault current from an insulation failure can only bake an electrode if it is denied the properly constituted conductive path back to the system source, as required for these systems. In this case, that path would be over the grounded phase conductor by way of the main bonding jumper at each structure disconnecting means. According to the description, the customers are omitting these bonding jumpers in order to treat the system as though it were ungrounded. However, the system is grounded at the transformer. And where distribution systems are grounded at any point, Sec. 250-23(b) and Sec. 250-53(b) require that the grounded circuit conductor be brought to the service disconnecting means and bonded to the enclosure and any equipment grounding conductors.
Sec. 250-24(a) requires similar bonding for the downstream pumps, because the pumping stations would be considered "structures" in the application of that rule.
In both cases (service and remote station), you must make a bond to the grounding electrode conductor, but the primary fault current path is over the grounded circuit conductor, which should be identified according to the rules in Art. 200.
If you don't make this bonding connection, in the event of an insulation failure that energizes conductive materials that are connected to earth through an electrode, and with no other return path, current will flow through the electrode in accordance with Ohm's law. If the ground rod resistance is 25 ohms, then about 19A of current will begin flowing (480/25 = 19) through the electrode. This won't trip the feeder overcurrent protective device.
Since power is equal to [I.sup.2]R, this will produce about 9kW of heat. Most electrode resistance is concentrated near the electrode, and therefore this is where the heat will be concentrated. If continued, this will indeed bake the ground to the point that the electrode is worthless (and, incidentally, in violation of Sec. 250-84 at locations with a single electrode). As the electrode resistance deteriorates, the touch voltage on local conductive surfaces will approach 480V (depending on local voltage gradients), an extreme hazard.
A true, ungrounded, alternative
If the owner truly wants the continuity advantages of an ungrounded system, then by Code he or she may have it. But the owner must wire per the Code, without using identified phase conductors and with overcurrent protective devices in all three phase conductors. And the owner must make whatever arrangements the utility requires to connect it. On installations like this, where exposed overhead conductors run for great distances, lightning arresters should be provided at each pump location. These surges are often more destructive on ungrounded systems because there is no circuit path to ground.
With a ground fault on an ungrounded system, the system simply becomes corner grounded at the point of the fault. The touch voltages on the connected enclosures do not increase significantly above ground. There is time to arrange an orderly shutdown and correct the problem. Normally, if a second fault occurs on a different phase, the resulting short circuit through the intervening equipment grounding paths simply opens one or more overcurrent protective devices. In this case, however, ground detectors take on additional practical significance.
However unlikely, if such a ground fault is not corrected at one location, and another fault occurs on a different phase at a remote location, current will attempt to flow between the electrodes connected at each. This will produce a similar result as that described for the improperly connected grounded system. Overcurrent devices may well not operate. The phase-to-phase voltage between the faulted phases will drop by varying amounts depending on electrode resistance. This may decrease the life of the motors connected to the distribution, as well as increasing shock hazards over time. The enormous amounts of energy wasted while such a fault is in progress will be metered and charged to the customer.
The NEC does not require ground detection on ungrounded systems, but a fine print note in Sec. 250-5(b) refers to its usefulness. It might be possible to rig a radio operated monitoring device that would alert a central station when a phase ground occurs. We think that the NEC applies and should be enforced on all of the wiring downstream of the service point.
EDITOR'S NOTE:
These answers are given by our panel of experts. I am chairing this panel, and the other panel members include Bill Summers, James Stallcup, and Dan Leaf. The opinion expressed is that of the panel. If a panelist disagrees with the majority opinion, his explanation is printed following the answer. Although authoritative, the answers printed here are not, and cannot be relied on as formal interpretations of the National Electrical Code.
