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8/14/2019 Training Edsa

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EDSA T2K Design MasterArc Heat Exposure

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Page AX.12002 EDSA Micro Corp. / PQ Logic Corp.

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IMPORTANT NOTICE

It is assumed that the user is familiar with the Standard for Electrical Safety Requirements for EmployeeWorkplace (NFPA 70E latest revision), that provides information on the protective performance of variousfabrics. It is also assumed that user has read the references and documents listed in the EDSA Arc Heat ExposureManual, ANSI/IEC Fault analysis concepts and the performance of a given system under fault conditions. It is alsoassumed that the user has read and understood all the related Protective Device Coordination ANSI/IECStandards and understands the theory and concepts. The formulas used in Arc Heat are based on experimentdata and uses theoretical and empirical equations to provide an approximation to the heat falling on a surface.Since temperature, humidify, barometric pressure, arc length, and the location of the fault within a switchgear

cubical have a bearing on the arc, an actual arc is likely to produce heat that differ from the results given by theprogram. The users should use the results from the program as a guide. The interpretation and use of thecalculation results encompassed by this program are the sole responsibility of the user.

The Arc Heat Exposure simulation programs are comprehensive and easy to use. Additional analysis capabilitieswill be made available as they are developed. Any comments, suggestions, or errors encountered in either theresults or documentation should be immediately brought to EDSA's attention.


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1.0 Heat Exposure Due to Arcing Faults

In the calculation of maximum short-circuit current magnitudes for equipment evaluation, arcing short-circuitimpedance or arc resistance is considered zero. When the fault does contain an arc, the heat released candamage equipment and cause personal injury. It is the latter concern that the heat exposure program was

developed. The heat exposure due to an arc can harm or burn bare skin or protective clothing. The Standardfor Electrical Safety Requirements for Employee Workplaces, (NFPA 70E-2000)provides informationon the protective performance of various fabrics to limit heat exposure to second-degree burns.

Other than burns, there are other exposure risks to arcing faults. These are:

a. Electrical shorts due to touching energized conductors.b. Arc blasts, due to expanding gases, that can cause flying debris, knock a person off balance, and cause

ear damage.c. Arc plasma can result in temporary or permanent blindness.d. Arc plasma or heat can result in a fire.e. Metal vaporization that can condense on cooler materials

The above list of points does not express the amount of energy in an arc. However, if you compare the arc blastto dynamite exploding, the heat produced can ignite clothing more than 10 feet away. Clearly any exposure to

an arcing fault can be hazardous.

Minimum arcing currents are often used for protective device settings since the protection engineer want toinsure clearing of these faults. A much higher arcing current is of concern for arc flash hazards and generally thethree-phase arcing fault current is used.

1.1 Arc Resistance

Short-circuit arc resistance is a highly variable quantity that changes non-linearly with the arc current during a

cycle and on a cycle-by-cycle basis. As the current increases, so does the ionized area, and consequently theresistance becomes lower. The voltage across the arc varies non-linearly with the length and current flowing in it.

Arcing short-circuit current magnitudes on low-voltage systems (


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gases allowing easy current flow. An arc occurring on open conductors is elongated due to heat convection,thereby lengthening the arc allowing cooling of ionized gas, so the arc may extinguish itself. The results of testsshow that arcing short-circuit currents are very erratic in nature and do not provide a constant resistance duringany one cycle. Over several cycles the arc re-ignites, due to un-cooled ionized gases, almost extinguishes, andthen fully re-ignites again. There is not an exact equation available to determine arc resistance. The

bibliographies references by Alm, Brown and Strom [2, 3, 4] provide approximations to the arc resistance.

1.2 Exposure

The amount of heat from an arc depends on the voltage across the arc, the current, single phase or multi-phasearc, confinement of the arc, and the distance the subject is away from the arc plasma. Most of the data collectedfor heat exposure has been staged, since the modeling of the arc is very complex [5, 6, 7, 8, 12]. The power inthe arc (VARC* IARC) is radiated out as incident energy falling onto a surface. Again, test results are often used tocompare the amount of energy produced in the arc and radiating to a surface a distance away. As expected, theradiated energy depends if the arc is unrestricted in free air or semi-confined or directed as it would be in aswitchgear cubicle with a panel removed or the door open. The latter directs the radiating energy toward theopen area, greatly increasing the incident energy falling onto a surface. The arc produces quickly expandinggases. These gases heat the surfaces they contact. Thus, the energy of an arc can burn due to both radiant and

convection heat transfer.

Low voltage switchgear type of equipment can have bare buses and a line-to-ground or a line-to-line fault andcan quickly become a three-phase arcing fault with the corresponding increase in arcing energy. Arcing faultsbeginning as line-to-ground faults in cables and on insulating buses, must burn though the second insulatingmaterial before a multi-phase fault can result. This can be several cycles to 10's of cycles depending on theenergy in the fault.

1.3 AC Heat Exposure Program

The EDSA heat exposure program uses empirical equation based test results and IEEE-1584 [12] to provide an

estimate of the energy falling on a surface removed from a fault. As more data become available, this test datawill be used to refine the program empirical equations. The arcing current used in this program is greater thanthose often associated with arcing currents used to set relays. In setting relays, the minimum arcing is used sothat the relays can be set to insure that they operate. While for heat exposure, the maximum current is ofconcern. In the above references, it was found that there is a driving voltage needed to sustain an arc. As anarc becomes longer, the arc voltage increases and becomes greater than the voltage needed to maintain itself.This voltage is approximately 150-V to 180-V rms depending on the fault X/R ratio [9,10]. The circuit use in Fig.1 is a simplified model for arc current calculations. The power dissipated in the arc radiates to the surrounding


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surfaces. The further away from the arc the surface is, the less the energy is received per unit area. One use ofthe program is to identify what grade of clothing is required by the operator working with energized equipment.The program allows either a manual input of the source voltage and short-circuit bolted fault current or entry viathe EDSA short-circuit program. Using the information in the reference papers, empirical equations from IEEE1584 are used to determine the arc voltage and the radiated heat.

There are several uses for this program. For example, it could be used to provide a protective sign on a piece ofelectrical equipment stating the type of protective clothing required when working around energized equipment.Warning of Arc Flash Hazard is a requirement given in 2002 National Electrical Code (NEC), Article 110.16.

Personal Protective Equipment (PPE) requirement are given in NFPA 70E-2000. Alternatively, the converse,knowing the thermal capability of the protective clothing being used, the program could be used to indicate if itsatisfactory. In this regard, the protective level of the clothing is entered into the program and the program givesa pass or fail result.

Fig. 1 - Circuit for Arc Model

Vs

Source

Zs

Arc Volts

Zcable


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Since most equipment is enclosed and a workman would have a door open or a panel off, the box will direct thearc radiant and blast energy in one direction. Therefore, the higher energies from Switchgear box or MCC box(if it applies) are recommended to be use switchgear and switchboards.

Reference #10 test results are for two conditions: an open arc and an arc-in- box. The open-arc had the arcing

electrodes extending in air approximately 2 feet from a wall. The arc-in box had the barrier on all sides exceptopen in the front. This would be similar to an open door in a switchgear cubicle. The latter directs the energy sothat it increased 2 to 3 times on a surface. ArcHeat provides both the open-arc and switchgear and MCC arc(arc-in-box) energy to a surface.

If all the heat in the arc is considered radiation, then it will reduce by the distance from the arc squared. Basedon the measured data more than just the radiate energy is reaching surface. Therefore, some heat must be dueto the hot gases touching the surface. The difference between the calculated radiant energy and total measuredenergy must be due convection of heat by the explosive gases reaching the surface. From the test results, theincident energy is dispersed by a different exponent when being in a box. The exponent is not squared, but alow factor. These factors are included in the EDSA program.

1.4 Personal Protective Equipment

Personal protective equipment covers many items such as gloves, tools, face protection, glasses as well as theclothing worn. The main arc flash considerations are burns to the body that could cause death. Therefore, thehead and chest areas are critical. While burns on the persons limbs are serious, they are not likely to causedeath. For example, when working on electrical equipment, gloves are voltage rated to protect from electricalshock while fire retardant overalls have a thermal rating. The fact that gloves are worn, some thermal protectionis provided.

Table 1 and 2 provide guidance to the thermal capabilities of some clothing articles. Table 2 is from NFPA 70E.NFPA 70E-2000 has divided the personal protective clothing (PPE) requirements into four (4) risk categories,Table 2. The fifth risk category is planned in future NFPA 70E updates. These hazard risk categories are listed

below. Table 3 gives the voltage capabilities of gloves up to 40-kV.


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Table 2 NFPA-70E Flash Hazard Risk Categories

Flash HazardRisk Category

Range of Calculatedincident energy

Min. PPERating

Clothing Required

0 0-1.2 cal/cm2 N/A 4.5-14.0 oz/yd2untreated cotton

1 1.2+ to 5 cal/cm2 5 cal/cm2 FR shirt and pants

2 5+ to 8 cal/cm2 8 cal/cm2 Cotton underclothing plus FR shirt and pants

3 8+ to 25 cal/cm2 25 cal/cm2 Cotton underclothing plusFR shirt, pants, overalls or equivalent

4 25+ to 40 cal/cm2 40 cal/cm2 Cotton underclothing plus FR shirt, pants, plusdouble layer switching coat and pants or equiv.

5 40+ to 100 cal/cm2 100 cal/cm2 Cotton underclothing plus FR shirt, pants, plusmulti-layer switching suit or equivalent

FR = Fire resistance fabric


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Table 3 Glove Classes

Glove Class Maximum Voltage

00 2.5-kV

0 5.0-kV1 10-kV

2 20-kV

3 30-kV

4 40-kV

1.5 DC Heat Exposure Program

The EDSA heat exposure program uses empirical equations based on ac test results to provide an estimate of theenergy falling on a surface removed from a fault. Tests on dc arcs are very limited. As data become available,this test data will be used to refine the Arc Heat programs empirical equations. Control tests have not yet beenmade for DC systems. The AC formulas were adjusted for the DC results based on engineering judgment of the

rms heat produced and voltage at which the arc will extinguish. Since, the DC does not have zero currentcrossings, the heat produced was adjusted up and the arc-extinguish-voltage lowered. Therefore the cal/cm^2 inthe DC program will be greater than those of the AC line-to-ground fault.The dc program differs from the acprogram in the following ways.

1. It is always single phase.2. The voltage needed to sustain an arc is lower. Ac voltage is more likely to re-initiate an arc due to the

peak voltage being greater than the rms voltage. However, a dc arc does not extinguish every half cycle.3. For the same arc rms fault current, the dc program is design to give higher energy exposure. This was

done to cover unknowns due to limited data available.

DC Arc Heat Exposure will be available in EDSA Technical 2000 version 4.00 which will available Summer 2003.

1.6 Key Concepts

a. Open Arc

This term is used to describe a none-enclosed Arc, in which the energy is radiated equally in all directions. Anarcing fault on an overhead line would be an example of an open arc topology.


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b. Directed Arc

This term, also known as arc in a box, describes an Arc that occurs in a partially enclosed area such as a MCCor a Switchgear cubicle. In this case the energy radiated includes that which is reflected from the enclosurewalls. A fault in a switchgear cubicle with the door open would an example of a directed arc.

c. Radiant Energy

This term refers to the energy in the form of light, which is released by an Arc during a fault.

d. Blast Energy

This term describes the energy released by an Arc, in the form of convection. When the Arc occurs, the gaseousmass surrounding the area is violently displaced and heated. The energy contained in this rapid moving mass, asit collides with surrounding objects, is called the Blast Energy of the Arc.

e. Distance from Subject

Distance between the live equipment and a persons chest area. Refer to the table below for IEEE Standard 1584default distances. Every bus could have a different working distance. The default values of medium voltageswitchgear are the distance from the live parts to a workman rolling in a breaker. If the worker has the coversoff the rear of the switchgear and working in that area, a smaller distance may be desired.

Three-Phase Bolted Short Circuit, and Line-to-Ground currents are automatically transferred to the field fromEDSA Short Circuit answer file. User can increase, decrease the calculated values or enter his/her own desirednumbers.

Class of Equipment Typical Working Distance

15-kV switchgear 910 mm (35.8 inches)

5-kV switchgear 910 mm (35.8 inches)

LV switchgear 610 mm (24.0 inches)

LV MCC and panelboards 455 mm (17.9 inches)

Cable 455 mm (17.9 inches)


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f. Equipment categories

208-1000V >1000-5000V >5000-15000 kV >15000V

Equipment Arc Gap Equipment Arc Gap Equipment Arc Gap Equipment Arc Gap

Cable 13 mm Cable 13 mm Cable 13 mm Cable -

Open 32 mm Open 102 mm Open 153 mm Open -MCC 25 mm Box 102 mm Box 153 mm Box -

Switchgear 32 mm

Box: Medium voltage switchgear. Default conductor distances between live parts are typical.Arc is directed.

Open: Arc in open air which allows radiation of energy in all directions. Default conductor distances betweenlive parts are typical.

Cable: Arc energy is radiated in all directions. Default conductor distances are typical.Swgr: Low voltage switchgear. Default conductor distances between live parts are typical. Arc is directed.MCC: Motor control center low voltage switchgear. Default conductor distances between live parts are typical.

Arc is directed.

Choosing an equipment type changes the Arc Gap Spacing, which can have an effect on the energy in the arc. Ifthe user knows the distance between live parts it should be entered. No tests have been made at voltages over15000-V. Selecting box, cable or open will not change the arc energy. This selection may make adifference when tests are made at higher voltages.

g. Grounded and Ungrounded Systems

Tests have shown that the arc energy does change with the type of grounding. Ungrounded systems will yieldhigher Arc Energies values than grounded systems. Hi-impedance grounding is considered ungrounded.

h. Fault Duration

The duration of the fault should be obtained from the protective device study and for the device protecting theequipment. The operating time is based on the arcing current (which is found in the text output). For systemsprotected with extremely inverse relays or fuses, an arcing current less than the maximum available currentshould also be checked since a longer fault clearing time may result in higher energy exposure. IEEE-1584suggests fault current of 85% of maximum. The easiest way to approximate this is to make a second run withthe current reduced to 85% and using the new operating time. If a bus does not have a protective device, anupstream device such as a transformer fuse would be the clearing device. The fuse operating time would bebased on the current flowing in it for a fault on the secondary. The Cfterm shown is the IEEE-1584 default value.By increasing this number, the user can allow a safety factor.


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2.0 Running the Arc Heat Exposure Analysis

Step 1.Open the fileEDM5.axd.

Step 2.

Run connectivity error checks in order toensure that the file is viable. Correct errorsas required.


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Step 3.

Invoke the AC Arc Heat exposureprogram by selecting this icon.

Step 4.Read CAREFULLY.

Step 5.Select Yes. This will run ashort circuit study and ensurethat the most up-to-dateresults are available.

Step 6.Select Next to continue.


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Step 7.Run the Short Circuit analysis and exitthe Short Circuit program to return.

Step 8.

Select the calculation units.

Step 9.Select the bus to be analyzed,and add a description.

Step 10.Specify the suggested distance or leavethe default as per IEEE std. 1584

NOTE:These settings allow the user to

modify the calculated current valueswithin a tolerance of +/- 10%.

NOTE:The User Defined settings can beused when a single line diagram isnot available. They allow the user tomanually enter the required values.

Step 11.Select Next.


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2.1 Testing Selected Clothing

Step 1.

Select the equipmentconfiguration from this list.

Step 2.Enter the fault duration orleave the IEEE std.1584defaults as shown here.

Step 3.

From the CalculationOptions select TestSelected Clothing.Press Next.

Step 4.From the library, select theclothing material to betested. Select 2.

Step 5.Select Test, and verifywhether your selectionpasses or fails. In thiscase it fails.


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2.2 Calculating Clothing Requirements

Step 1.Select the equipmentconfiguration from this list.

Step 2.Enter the fault duration or

leave the IEEE std.1584defaults as shown here.

Step 3.From the CalculationOptions select CalculateClothing Requirements.Press Next.

Step 4.Select Calculate, and verify thesuggested selection. In this case

is Category 3. If the ClothingRequired field displays CategoryX (Cat X), the energy available inthe system exceeds the limits ofany NFPA-70E listed clothing.


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2.3 Generating Output Reports

Step 1.Select the requiredoutput report sections.

Step 2.Select either Report to generate a text report of a single bus (the one selected in theanalysis), or Report All for a report on all the busses in the network. The reports will begenerated in class.


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2.4 Generating Output Plots & Labels


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2.5 Tables & Databases


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3.0 References

1. Kaufmann, R. H. and J.C. Page, "Arcing Fault Protection for Low Voltage Power Distribution Systems - TheNature of the Problem", AIEE Transaction, PAS vol 79, June 1960, pp 160-165.(Note: the value in table 1should be multiplied by 2 due to correction with CT probe ratio.)

2. Alm, Emil, " Physical Properties of Arcs in Circuit Breakers", Transactions of the Royal Institute ofTechnology, Stockholm, Sweden, No. 25, 1949.

3. Brown, T. E., "Extinction of A-C Arcs in Turbulent Gases", AIEE Transaction Vol 51, March 1932, pp 185-191.

4. Strom, A. P., "Long 60-Cycle Arcs in Air", AIEE Transaction, March 1946, Vol 65, pp 113-118,(See discussionPP 504-506 by J. H. Hagenguth).

5. Wagner C. F., and Fountain, L.L., "Arcing Fault Currents in Low-Voltage A-C Circuits." AIEE Transactions.1948, vol 67, pp 166-174.

6. R. Lee, The other electrical hazard: Electrical arc blast burns. IEEE Trans. Ind. Appl. Vol. 18-1A, May/June

1982, pp 246-251.

7. R.A. Jones et al, Staged tests increases awareness of arc-flash hazards in electrical equipment. Conf. Rec.IEEE PCIC Sept 1996, pp 298-281

8. J.R. Dunki-Jacobs, The impact of arcing ground faults on low-voltage power system design, GE publicationGET-6098

9. Lawrence Fisher, Resistance of Low-Voltage AC Arcs, IEEE Trans. Ind. Appl. Vol. IGA-6, Nov./Dec 1970,pp 607-616.

10. Richard Doughty et al, Predicting Incident Energy to Better Manage the Electric Arc Hazard on 600-V Power

Distribution Systems. IEEE Trans. Ind. Appl. Vol. 36-1, Jan/Feb 2000, pp 257-269.

11. O.R. Schurig, Voltage Drop and Impedance at Short-Circuit in Low Voltage Circuits, AIEE trans, Vol 60,1941, pp 479-486.

12. IEEE Std 1584-2002, IEEE Guide for Performing Arc-Flash Hazard Calculations

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