Transistorized ground-faultinterrupter reduces shock hazardEven the home outlet can deliver a lethal electric shock.Yet for a decade there have been simple sensitive devicesfor ridding us of such dangers

Charles F. Dalziel University of California

The ground-fault interrupter is the most successful de- The French-Austrian developments were followed invice for eliminating the hazard from low-voltage elec- the U.S.A. in 1962 by the development of two- and three-tric shocks in the home, on the farm, and in industry. wire transistorized ground-fault interrupters having a con-It is the newest of four recognized means-isolation, tinuous rating up to 100 or 200 amperes and a ground-insulation, grounding, and shock interruption-for current trip value of 5 mA. This performance parameterreducing the danger from electric shock and it is, by means that the circuit breaker will trip in the event of afar, the most radical. 5-mA line-to-ground fault current. In the case of an

accident, this would be the current through the victim.The ground-fault interrupter, or GFI, is a device The 5-mA level is prescribed both by Underwriters'

that interrupts an electric circuit when the fault current Laboratories, Inc. (U.S.A.), and by the Canadian Stan-exceeds a predetermined value less than that required dards Association.to operate the overcurrent devices of the circuit. (In The operating time of these devices is so fast, and theEurope this device is called an earth-leakage circuit corresponding shock energy so small, that the modernbreaker.) Such apparatus have been used to protect GFIs virtually eliminate electrocutions, burndowns, andhigh-voltage power lines since the 1920s, and they were fires due to currents flowing to ground. However, itset to operate at 10 to 20 percent of the minimum opera- must be recognized at the outset that the sensitiveting current, or trip value, of the associated overcurrent ground-fault indicating mechanism does not respond todevices. Thus, a power circuit breaker having an overload line-to-line or three-phase faults unless zero phase-trip value of 200 amperes was set to trip on ground faults sequence currents are involved.of only 20 to 40 amperes, which was considered a great To verify that the GFI really will prevent electrocution,achievement of the day. Some ten years later the im- Dr. Archer S. Gordon, of Statham Instruments, Inc.,portance of protecting against low-voltage burndowns in Oxnard, Calif., admninistered 2400 shocks to experi-industrial equipment was recognized in Germany and mental dogs under anesthesia. A commercial 5-mAdevices were developed having a line-to-ground-trip GFI was used, and dogs were connected in series withvalue ofaboutS500mA. the "hot" wire of the 120-volt laboratory circuit andMore recently, about ten years ago, both the French ground. The dogs were given 800 shocks using a current

and the Austrians developed two-wire earth-leakage pathway between right forepaw and left hind paw tocircuit breakers having a trip value of 25 to 30 mA. simulate the frequently experienced arm-to-leg pathway

IEEE spectrum JANUARY 1970

Authorized licensed use limited to: Charles Beck. Downloaded on May 18,2010 at 16:10:37 UTC from IEEE Xplore. Restrictions apply.

Line ° 0 Lirie

N___N L I N L

Tr :.coi' Trip coi. 6l

, S ¢ \i S F~~~~~~~~estTest gi,iIesisto, button

GaPI- mary # ^ -1Magnetic disharcore

core tube"0

4- : v [ ~~~ ~ ~ ~ ~ ~~~ault- FaulN t,n,,-'cdetector detectotz ___ 1, 1 ~~~~~ 1 coil C i

Load Load

FIGURE 1. Schematic diagram, French device. FIGURE 2. Schematic diagram, Austrian device.

in many human electrocutions. No incidence of ventric- It is pertinent that during the past four years severalular fibrillation was observed. Eight hundred shocks technicians and salesmen have received 120-volt shockswere then given to the dogs after electrodes were placed incidental to installing, selling, or demonstrating GFis,on the right forepaw and left forepaw. None of these and no injuries occurred. European experience during the800 shocks produced ventricular fibrillation. However, past nine years has been equally good, and many lives36 fibrillations were produced during the course of 800 have been judged saved by the operation of 30-mA,shocks applied with electrodes placed on opposite sides 220-volt, two-wire devices.of the chest. This result is not important from a safety Types of sensitive ground-fault interruptersviewpoint, since such a pathway is unlikely in humanaccidents. Moreover, since the minimum current for Among the several types of interrupters are the follow-producing ventricular fibrillation in mammals is approxi- ing:mately proportional to body weight, it is evident that the Direct trip. The direct-trip type is used in Europe;GFI will protect human beings, including the very young. it obtains its operating power from the shock or fault

current alone, by means of a differential transformer,which is the leakage-current or ground-fault-currentsensing mechanism. The device has the advantage of

FIGURE 3. First United States GFI. being entirely self-contained, and it can be used wherethe distribution system is grounded at only a few places-such as a system grounded only at the substation. It isalso low cost. However, it has a limited 30-A ampacity.Also the tripping reliability has been questioned becauseof the lack of robust tripping power and the necessarilydelicate mechanisms. This genre comprises:

Tlhe (predwninantly) mechanical type, which consists ofa differential transformer and a polarized relay-tripcoil. Electricite de France has purchased some 30 000units, manufactured by l'lndustrie Electrique de la Seine.for use as entrance switches for residential custorners.The unit is essentially instantaneous in operation, has aminimum trip value of 30 mA, a full-load rating of 30amperes at 50 Hz, 220 volts, and is two-wire, singlephase.

Figure I shows the schematic diagram of this Frenchdevice. A sensitivity of 30 mA is achieved by the use of apolarized, spring-loaded relay trip coil. The deviceoperates on the principle of magnetic balance. As long

56 IEEE spectrulm JANUARY 1970

Authorized licensed use limited to: Charles Beck. Downloaded on May 18,2010 at 16:10:37 UTC from IEEE Xplore. Restrictions apply.

FIGURE 4. GFI protecting author's home since 1962; (A) cover in place, (B) cover removed.

as all of the current from the incoming lines flows in at L 100 mA. Their chief limitation is that they must be usedand exactly the same current flows out at N, there will be only on multigrounded systems (as required in the U.S.A.no flux in the transformer core and no voltage will be by the National Electrical Code) so that a break in theinduced in the fault-detector coil. However, should some neutral wire in the distribution system will not impaircurrent be diverted to ground beyond the load terminals, protection to customers located downstream from thethe magnetic balance will be upset and a flux will be severed neutral.created in the core, which will induce a voltage in the Figure 3 is believed to represent the first successfulsecondary winding. If this induced voltage is suflicient, GFI constructed in the U.S.A. (1961).the device will operate and trip. Figure 4(A) shows the three-wire, 100-ampere, 120/240-

The electrical type, which consists of a differential volt, ground-fault interrupter that has been protectingtransformer, and a single or multistage solid-state recti- this author's home since November 1962; it employs afier-capacitor voltage multiplier that fires a gas tube, single transistor.which energizes the trip coil of the circuit breaker. Al- Figure 4(B) shows the interrupter with the coverthough not quite so delicate as the "mechanical" type, it removed. Note the differential transformer, the epoxy-has considerable inherent time delay. Manufactured bythe Felten and Guilleaume Company of Vienna, it is rated16 amperes, 500 volts, and has a nominal trip value of30 mA.

Figure 2 shows the schematic of the Austrian earth- FIGURE 5. Schematic diagram, three-wire transistorizedleakage circuit breaker. The device functions similarly GFI. Path Q, includes the trip-value control and tempera-to the French apparatus. However, sensitivity is achieved ture-compensation circuits.by the charge stored in the capacitor.

Power-operated. In the power-operated design, the Cdifferential transformer is used only to detect leakage I,.fcurrent or ground-fault current, and tripping force is __ --obtained from the electric power line. The devices are not ci - +- - - .limited in size, and can be readily designed for 15 to 200 LI,amperes, 120 to 480 volts, two- to four-wire, one- or CiCi.1 rre.k -jthree-phase, and with very low trip values. Demonstra- Q.1 RItion models are available having a trip value of only Q31/2 mA. Trip values of commercial units start at S mACiand some units have adjustable trip values from 10 to

Dalziel-Transistorized ground-fault interrupter reduccs shock halzakrdI 57

Authorized licensed use limited to: Charles Beck. Downloaded on May 18,2010 at 16:10:37 UTC from IEEE Xplore. Restrictions apply.

encased sensing circuit, and the three companion beyond the load terminals, the magnetic balance is upsettrip circuit breakers. The series trip in the center or and a voltage appears across the secondary-windingneutral circuit breaker has been replaced by a shunt trip terminals. When this voltage reaches about 1/2 volt, thecoil but the series trips in the line circuit breakers have solid-state switching device Q, becomes conducting andnot been altered. Because of this design, the sensitive the circuit breaker is tripped by energy supplied from thetrip provides a line-to-ground feature in addition to that control transformer. Such a device is capable of highnormally provided for overcurrent or overload protec- sensitivity and large ampacity.tion. If, in a protected circuit, a person were to make contact

There are at least four different types of power- between an energized defective appliance and a groundedoperated GFls: object, causing a ground-current flow in excess of theThe inagnetic-amplifier type, manufactured by F. W. trip value, the circuit breaker will be actuated and

Industries, Ltd., is used in South Africa. The device is the e-lectrocution hazard eliminated. However, theinherently large and heavy because of its magnetic cores. device is actuated only by the current flowing to ground,It is reported to have excellent operating characteristics, and it cannot differentiate between the body-appliancebut has the reputation of being the most expensive of all circuit and any other current pathway to ground havingGFIs. the same resistance. Thus, it is imperative that the

The transistorized type is manufactured by The Rucker normal or standing leakage current of the protectedCompany in the U.S.A. and in Canada, and by the system be as small as possible. Obviously, standingFederal Pacific Electric Company in Canada. This leakage must be less than the trip value or the devicedevice comprises a differential transformer, solid-state will not remain closed. The insulation resistance ofamplifier, and a circuit breaker. modern wiring is very high, and possibly the chief sourceA reed-switch, actuated circuit breaker. manufactured of abnormal leakage is damp, poorly taped joints, and old

by Harvey Hubbell. employs two magnetic cores. The wiring immersed in water-filled conduits. dirty terminalunit is competitively priced. blocks, or defective appliances.A trickle-charged batter) type assures power for Although the National Electrical Code prohibits

tripping a circuit breaker for an extended period even grounding the neutral wire on the load side of the serviceafter the main power supply has failed. A European equipment, such grounds frequently exist, and suchfirm is marketing a transistorized device of this design. grounds will cause the GFI to trip out, much to the con-This is the least expensive of all GFIs, but has the dis- sternation of the electrician making the installation.advantage that when the battery wears out, the device Because of the required grounds on the utility system.becomes inoperative without warning or indication; i.e., such a ground would shunt some of the load currentwhen it fails, it fails stone dead. returning to the differential transformer, upset the mag-

netic balance, and cause variations in the trip value. ToThe transistorized GFI prevent such unwanted tripping, the neutral wire is fed

Figure 5 illustrates a schematic diagram of the three- approximately 100 mV from a third winding on the GFIwire. 120,1240-volt, modern, transistorized, highly sensitive control transformer. If extraneous grounds are presentGFI. As long as all of the current flowing in at LI returns on the load circuits, it will be impossible for the GFIthrough N or L2, the core of the differential transformer to remain closed, and the installer will be immediatelyremains unmagnetized and the voltage of the secondary aware that something is wrong.winding is zero. However. if any current flows to ground Another requirement is that the device withstand

high-current line-to-ground faults (presently prescribedat 5000 amperes rms) for at least the time it takes for thecircuit breaker to clear the fault. The secondary or sensingcircuit is protected from proportionately large currents:

FIGURE 6. The characteristics of a new differential trans- by saturation of the core of the differential transformer.former, illustrating the accuracy with which the device by the secondary winding, which appears as a very high-discriminates between leakage current to ground in impedance source (> 50 000 ohms), and by the sym-milliamperes and the load current in amperes. metrical pair of double diodes, Q2 that clip any transients

to a voltage level within the voltage tolerance of the solid-state device, Qi. Any steep transient-such as that due tolightning or switching surges-is further attenuated by

Lo. c rthe RI C, filter (that does not impair tripping time, butdoes shunt high-frequency, short-duration transient

< ~~~~~~~~~~~~~~~voltagesfrom Q1). These means have been found sufficient3 ~~~~~~~~~~~~~~toeliminate false tripping due to transients coming in the

"front door."Eliminating erroneous tripping due to transients

coming in the "back door"-i.e.. through the controltransformer-is a more difficult problem. Capacitor C2and resistor R2 reduce the susceptibility of the anodegate to respond to unwanted signals. Response to tran-sients is also reduced by inductance Li and resistor R;3and voltage clipper Q;I, which becomes conducting and

0 2040 60 80 IOC) ~~~~prevents the control transformer's secondary voltageLoad cuttent. dmpere, from rising to unacceptable valuies.

58 IEEE spcctrrumi JANUIARY 1970

Authorized licensed use limited to: Charles Beck. Downloaded on May 18,2010 at 16:10:37 UTC from IEEE Xplore. Restrictions apply.

False or nuisance tripping In January 1969, the American National StandardsFalse tripping can be defined as an unwanted power Institute* issued a new Standard for Leakage Current

outage as viewed from either the customer's, the installer's, for Appliances (C101.1), limiting such current (foror the engineer's viewpoint: portable, cord-connected, 120-volt appliances) to 0.5From the customer's standpoint, any power outage mA.2 Although the leakage current of appliances has

will usually be classified as a nuisance. The device may be been gradually reduced over the years, previous regula-performing its intended purpose in every respect, but if tions set the leakage-current limit at 5.0 mA, and it maythe housewife plugs in a defective appliance and the take some time for these usable, old, but leaky ap-circuit goes dead, from her viewpoint the power outage pliances to be discarded. Compounding the problemis a nuisance, notwithstanding that the tripping action is the fact that any mass-produced product must bemay have saved her life. allowed a manufacturing tolerance, and the transistorizedWhen the installer is equipping a home with a ground- GFIs operate between 3.0 and 4.5 mA from zero to full

fault interrupter, the GFI will not stay in the "on" load, over a temperature range of -35 ° to 660C, and withposition unless the system is free of inadvertent ground- an applied 102-132 volts. Thus, there may not be muching of the neutral conductor on the load side of the device. margin between a 3.0-mA-trip-value device, and theLikewise, the GFI will not stay in the closed position if sum of normal wiring leakage and likely leakage currentthe system has an excessive amount of standing leakage. of appliances-especially some of the older appliances.If the device does not stay "on," and trips repeatedly Fortunately, as far as is known, this effect has not been aeither because of grounding of the neutral conductor or source of complaint, and field experience indicates thatexcessive leakage, it is only natural if the installer con- the 5-mA trip value is satisfactory for protecting indi-siders this tripping a nuisance. vidual appliances and 15- or 20-ampere branch circuits.From the engineer's standpoint, if the device has been It is time to dispel two rather widespread misconcep-

installed and responds to a transient-due to switching, tions concerning GFIs. There is an erroneous impressionlightning, or whatever-and the device interprets these that the GFI reduces the magnitude of the ground-faultsignals incorrectly and responds in the same manner as it current to a safe value. Actually the GFI does not limitresponds to a ground fault constituting a shock hazard, current at all; if it did it would seriously limit the load-then truly the device has falsely tripped. carrying capability of the protected circuits. The GFITo summarize, tripping may be classified into at least achieves safety by limiting the exposure time and hence

three categories: the energy in the shock. Danger of electrocution de-1. Intended operation. Tripping in the intended manner pends upon energy, not current. The shorter the dura-

when a shock or fire hazard exists is the normal operating tion of the current flow, the less energy that has coursedmode for the device. through the victim and the greater the current that can

2. Warranted nuisance tripping. In this category, be tolerated by the human body.consider tripping of the device in its intended manner, It is necessary to stress that the trip value of the GFIbut where the very act of tripping constitutes a nuisance has no relationship whatever to let-go currents. Al-to the customer. Included in this category are operation though the GFI has to be set to operate at a prescribedof the device under grounded neutral conditions and current, the device actually monitors the impedance toexcessive standing leakage in the system. Here the unit is ground of the protected circuit. Thus a GFI set to tripfunctioning in its intended manner, however, when inter- at 5 mA will cause circuit interruption when the circuitpreted from the standpoint of the customer or the in- impedance to ground drops to Z = E/I = 24 000 ohms;staller, the operation of the device does in fact constitute for a trip value of 10 mA, the critical impedance is 12 000a nuisance. ohms; and for a trip value of 15 mA, impedance = 8000

3. Unwarranted or false tripping. This category com- ohms.prises tripping due to a response of the device to signalsthat are not line-to-ground faults constituting a shock Why GFI?hazard. In 1930, a series of tests was conducted at Under-

It is readily apparent that the first two cases cannot be writer's Laboratories, Inc. 3, in an effort to determine thecircumvented by any design refinement. In fact, the power maximum current that an individual could withstandoutages resulting from either are usually consistent with for a short time and still have voluntary control of histhe design parameters required to produce a device that muscles. In these tests, members of the laboratories'provides shock- and fire-hazard protection to the user. staff were used as subjects. The electrodes consisted ofAny compromise to reduce the so-called nuisance trip- pliers held in each hand. (Although the tests were re-ping in these cases would compromise the ability of the corded for alternating current, tests with direct currentdevice to respond in a shock situation. indicated slightly higher values could be withstood for a

Truly false tripping (3) is a result of inadequate engi- short time until a hot spot occurred at the point of con-neering. Earlier in the development of these devices, this tact.) The results obtained for 13 subjects were as follows:was a considerable problem. However, continuingeffort has produced a very reliable and accurate device; Current, Resistance,see Fig. 6. Subject V, ac mAQ

Trip value Maximum 40.0 10.0 6670It is evident that the trip value of the GFI is a com- Average 27.8 7. 8 3560

promise between providing maximum protection and Minimum 20.0 6.0 2330avoiding unwanted tripping due to abnormal leakagecurrents-the sum of wiring and appliance leakage. $ Formerly USA Standards Institute.

Dalziel-Transistorized ground-fault interrupter reduces shock hazard 59

Authorized licensed use limited to: Charles Beck. Downloaded on May 18,2010 at 16:10:37 UTC from IEEE Xplore. Restrictions apply.

1000 in 1968. the UL research group presented data to theResearch Sub-Conmmittee of the American NationalStandards Committee CIOI, on reaction and perception

Eetztf @ fW 40ff< currents involving casual contacts with various metallicobjlects energized at 120 volts but with the current strictly

f~r~4ults limiited to preset values. The contacts consisted of placingthe left hand in a vessel of saline solution, and the righthand. wrist. underarm, or lingers were SUbj1ected to

100 u~~~~~~~~~~~~~~nexpected, programmed, chanCe-tou1Ch contatcts with100 0 b1 _ energized electrodes. Rather extensive tests using cur-

rents of 4 to 4.5 mA were observedi on three subl1jectS,tand the corresponditng body-circuit resistances for theCaSUal contacts ranged frohmoa minimum of 89(0 tO amiaximuim of' 17 800 ohmis.

Table I of a 19Y41 AIEE Tranisactionv paper-I givesresults for 60-Hz let-go current tests made onl 114 Imlen.

* 12 ooo ~~~~ Let-g ofrengIn these tests, the SUhIjeCtS were repeatedly given shocks to10 f _ deterimiine the largest Current that they' COLld tolerate

,Ind still let go of' the condUctor b) Lusing the imuLsclesstimUlated by that Current. The electrodes consisted of anio. 7 or 8 AWG copper wire heldi in the right hand, and a

Trip value hat brass plate Was used as the return electrode. Thez~~~~ OOO~~~~~~I ~~mean valueW for the group WaIS 15.5 m1A. The body cirCu)it

resistatnces wNith moist contacts were Measured on 54 ofthe grOuIp; the miinimumti valuewaWIS 1570) ohmis with 19.1nmA and the matxinlLtll was 4431 ohms with 14.2 mA.

0.01 0.1 1.0 An extension of this work' included Ict-go current testsmade on 134 men and 28 women. Although resistancesSere not determined, the range of currents voluntarily

FIGURE 7. Relationship between trip current and shockduration for a typical Rucker GFI. The electrocutionthreshold and the let-go (freezing) threshold for adu:tsis included to give proper perspective. The horizontaldashed lines indicate body current for variously assumedbody-circuit resistances. It is generally accepted thatthe minimum likely body resistance in low-voltageaccidents for a current pathway between major extremi- FIGURE 9. A two-wire, 30-ampere, 120-volt, 5-mA ground-ties with liquid contacts is 500, and for the perspiring trip GFI for protecting permanent wiring.hands of a technician, 1500 ohms. Corresponding resis- -.tances for dry hands or casual contacts are too variableto mention precise figures. Note that the current-timecurves rise almost vertically for currents in excess oftwice the trip value.

FIGURE 8. Portable GFI two-wire, 15-ampere, 120-volt,5-mA ground-trip value. Principal application is protect-ing 120-volt hand-held tools and portable appliances usedin damp or wet locations.

Authorized licensed use limited to: Charles Beck. Downloaded on May 18,2010 at 16:10:37 UTC from IEEE Xplore. Restrictions apply.

tolerated by the men was 9.7 to 26 mA; for women it was ground fault on L,. Fortunately, this extreme (or worst)7.5 to 13.5 mA. condition should be of infrequent occurrence.

Meaningful tests on children are difficult to perform In recent years, the trip value of European devicesand only a few tests have been reported. The following has been reduced from 30 to 25 mA. However, even thistests may be of interest. A boy nearly I1 years old had a value has been of serious concern to safety-mindedlet-go current of 9 mA, whereas a well-developed nine- Europeans for some time.year-old boy weighing 28 kg had a let-go current of 7.6 There is simply no question that the sensitive GFImA. Slightly different tests were made at the University provides a tremendous increase in protection over presentof Wisconsin in which a current of 7 mA paralyzed thegrip of a five-year-old boy. No experimental data for let-go or tolerance currents for any other children areavailable.

Except for its nuisance value, or for its "startle" effectin causing an involuntary movement triggering an acci-dent, the smallest electric shock of importaince is thatcurrent which causes loss of voluntary control of thehand when an electrified object is grasped. As the magni-tude of the alternating current is increased, the first sensa-tions of tingling give way to contraction of the muscles.The muscular contractions and accompanying sensationsof heat increase as the current is increased, and finally avalue of current is reached at which the subject cannot re-lease his grasp of the conductor.

It is important, if a GFI is used, that person-to-con-ductor contact resistances should be especially low,thereby initiating a fast-trip action. When a person cannotlet go and "freezes" to the conductor the resistance shouldbe much lower than the values previously given becausethe tighter the grip, the lower the contact resistanceand the greater the current. Moreover, during contact,the current stimulates sweating, which again reducesthe resistances. Soon blisters form, again the resistancelowers, and the current increases. Therefore, upon con-tact with a faulty appliance protected by a GFI, aperson either lets go immediately, or if he freezes to thecircuit, the current should increase rapidly and assureoperation of the GFl, as shown in Fig. 7.

FIGURE 10. A commercial installation of four, three-wire

Realistic limits 70-ampere, 120/240-volt, 5-mA ground GFI's protectingRealistic limits ~~~~~~~panel, boards.Field experience indicates that the 5-mA trip value is

satisfactory for protecting individual branch circuitssupplying appliances. Although a trip value of 5 mA is FIGURE 11. Protected power outlet, three-wire, 120/240highly desirable, currents to the level of 12.5 mA are toler- volt, 70-ampere 5-mA ground-trip value for temporaryable and may have to be considered for large units, such power for construction jobs.as main-circuit breakers-especially three-wire devices forprotecting panel boards. This fact is so because, withsuch setuips, greater lengths of conduictors are needed,resulting in higher standing leakage, and because of thelikelihood of a greater number of appliances. (The nuM-ber of these larger GFIs that has been installed in thefield is quite limited and the trip value for these devicesshould not be frozen until more experience is obtained.)In this author's opinion, the upper limit of trip valuie,however, should not exceed 12.5 mA.A three-wire GFI set to trip at 12.5 mA under cer-

tain conditions may require a current of 25 mA to trip.For example, a 12.5-mA device protecting a three-wirecircuit having either zero leakage, or exactly equal leak-ages on wires Li and L2 (Fig. 5), will require 12.5 mA totrip regardless of whether a ground fault occurs on L,or L2. However, if the leakages of LI and L2 are unequal,the GFI may require a greater fault current to trip. Forexample, if the leakage on LI were zero, and the leakageon L2 were just less than 12.5 mA, it would take slightlymore than 25 mA to cause the GFI to trip for a line-to-

Daizici-Tranisistorized grounid-fatult interrupter reduces shock hlzazird 61

Authorized licensed use limited to: Charles Beck. Downloaded on May 18,2010 at 16:10:37 UTC from IEEE Xplore. Restrictions apply.

6 ~~~~~~~~~~~~~manypotentially dangerotus situations that occur in andaround the all-electric homec. Some typical installationand performance characteristics of available commercial

5 ~~~~~~~~~~~~~~GFIsare shown in Figs. 8 -12.Operation on reduced voltage is also of considerable

importance. Tests on commercial units indicate that4 ~~~~~~~~~~~~~~reliabletripping is obtained from rated voltage down to

E ~~~~~~~~~~~~~~75volts. The present emphasis is to study and developimpulse-testing techniques suitable to application of these

3 ~~~~~~~~~~~~~~~devices.Caution2The ground-fault interrupter is a safety device, and not

a toy. The public, especially children, must be patientlyeducated in the proper and safe use of electric appliances

1 and tools. The installation of a GFI is no license forimproper, careless, or unsafe practices either in the homeor in industry.

0

1.25l557l,Theauthor wishLes to acknowledge and thank the severalTemperatture. C enginceers of The Rucker Company, Oakland, Calif., for their

splcfn(lidi cooperaitioni in furniishintg techniical data and the phioto-FIGURE 12. Characteristics of a temperature-compensa- gratphs.ting circuit. Note the satisfactory response over the con-siderable range in temperature. REFERENCES

1. Daltziel, C. F., anid Lee, W. R., "Recvaluation of lethail electriccurrenits,'' IEEE Tra,iis. ladulistri enaid Genieratl Applications, vol.IGA-4, pp. 4e7-476, Sept,Oct. 1968, and vol. IGA-4, pp. 676-677,Nov./Dec. 1968.2. "Proposed USA standard for leakage current for appliainces,"USA Standard C101.1, Jan. 1969.

conventional overcurrent protective devices, which will 3. Whitaker, It. B., "Electric shock as it pertains to the electricnot operate at all on the currents that flow through the fence," Bulletini of Research No. 14, Underwriters' Laboratories,human body during accidental contact with the 15- and Inic., Dcc. 1939.20-ampere circuits used in the U.S. home or on the farm. 4. Dalziel, C. F., Lagen, J. B., and Thurstoni, J. L., "Electricslhock.'" Elee. Ens,g., vol. 60, pp. 1073-1079, Dec. 1941. (Discussioni,Application of these sensitive GFls should result in same vol., pp. 1295 -1297.)little inconvenience if properly wired, except for possible 5. Dalziel, C. F., Ogden, E., and Abbott, C. E., "Ettect of fre-interruption of illumination. Certainly it is undesirable queilcy on let-go curreints," Elec. EDig., vol. 62, pp. 745-750, Dec.

to bewithutlghts n th dea of ight hen ttemting 1943. (Discuissioni, satme vol., pp. 998-1000.)to be without lights in the dead of night when attempting 6. Dalziel, C. F., "Tranisistorized, residual-current trip device forto locate trouble. Here, the answer is to provide protec- low-voltage circuit breakers," AIEE Tranis. Pouer Apparatas anidtion only where protection is a must-that is. for the SJ stems, vol. 81, pp. 978-983 (Feb. 1963 sectioin).circuits supplying wall and floor receptacles. (It is widelyrecognized that the electric shock hazard is largelyassociated with portable appliances in contrast to ceiling Charles Francis Dalziel (F) showed an early flair for elec-or wall-bracket illuminating fixtures.) tricity. As a teenager, he became the 381st licensed ama-A requirement that receptacle outlets be suppliedl from teur radio operator after World War I. During high

school and college, he served as a substation operatora subpanel protected by two- or three-wire GFIs should with a utility company. After receiving the B.S. degreeresult in a great improvement in the electrical safety ot1 from the College of Mechanics, University of Californiathe U.S. home. This should protect the barefooted hedge (there was no Electrical Engineering Department) in 1927,clipper, the college student studying near a metal desk he took a brief excursion through Europe and then joinedlamp with his bare feet on the hot-air register, the family the General Electric Company as a testman. In 1929, he leftormpwithplayingb leethontneliving r begistweerlnelG.E. for the San Diego Gas and Electric Company where hesitting or playing in the lving room between the metal was soon given charge of system protection. In 1932, hefloor lamp and steam radiator, the daughter acciden- joined the then new Department of Electrical Engineering,tally dropping a radio in the bathtub, the father working University of California, Berkeley, subsequently receivedon the toaster on the stainless-steel sink work area. the the M.S. degree (1934) and the E.E. degree (1935), andmother vacuum-cleaning the house with a 25-year-old ecame a professor emeuritus iny has centered about elec-machine, the son cutting the lawn with an electric lawn trical safety. Professor Dalziel has volunteered his servicesmower, the sister playing the record player at the to many professional, advisory, and public service groups,edge of the swimming pool, the baby chewing the frayed including duty as chairman ofinsulation of the sewing-machine cord with wet diapers the San Francisco Section of theor bare knees making contact with a grounded metallic San Francisco Section of the

object. and the professor sitting on an aluminum chair IEEE Power Group. He hasin the back yard dictating his next article on electric lectured throughout the worldshock into the tape recorder. and has authored 120 pa-Such protective means would also protect the me- pers-some having earned

commendations (recently, firstchanic using the electric hand tool in the wet garage or prize, IGA Group Transactions,basement and the fix-it man crawling under the floor with 1969). Professor Dalziel is a reg-a brass-shell socket extension lamp. It would also mitigate istered professional engineer.

62 Dalzihl-Transistorized ground-fault iiterrupter reduces shock hazard

Authorized licensed use limited to: Charles Beck. Downloaded on May 18,2010 at 16:10:37 UTC from IEEE Xplore. Restrictions apply.