CLASS A GROUND FAULT CURRENT INTERRUPTERS BASICS AND TUTORIALS

Today, hundreds of millions of GFCIs are installed in electrical systems in North America. A harmonized tri-national standard defining Class A GFCIs was jointly issued by ANCE in Mexico as NMX J 520, CSA in Canada as CSA C22.2, and by UL in the United States as UL 943 in 2008.

The standard describes Class A GFCIs as devices designed to protect 95% of normal healthy adults by interrupting a circuit when a ground fault current exceeds 6 milliamps. According to the standard, it must trip at 6 milliamps of leakage current and must not trip below 4 milliamps of leakage current.

Since they are inverse-time devices, they react faster to higher currents. Class B GFCIs were the original GFCIs with a minimum trip current of 20 milliamps that were used in swimming pool lighting

circuits. They have long since been obsolete, but there are still some installed and in use.

Since RCDs and earth leakage circuit breakers (ELCBs) have higher trip currents, they do not meet the standard of a Class A GFCI in North America. They are, however, considered personnel protection devices in some other countries.

Since the development of GFCIs, the live production event industry has been left to its own devices as to whether to use them or not. But now the Entertainment Services and Technology Association (ESTA) is working on BSR E1.19, Recommended Practice for the Use of Class A Ground-Fault Circuit Interrupters (GFCIs) Intended for Personnel Protection in the Entertainment Industry.

This recommended practice spells out when, where, and how to use GFCIs in places of assembly, the production of film, video, and broadcast, theatrical productions, carnivals, fairs, and similar events in North America.

It covers electrical services of 100 amps or less, 120–240 VAC single- and three-phase 60-Hz circuits where the voltage to ground does not exceed 150 VAC. Note that since the voltage to ground in Europe is 230 VAC or 240 VAC, this standard specifically excludes those systems.

In brief, the standard calls for the use of GFCIs in any outdoor, wet, or damp locations unless the circuit is for egress lighting, exit lighting, or emergency lighting systems, or if tripping the GFCI could cause injury.

Since the control circuit in a GFCI requires constant power, the use of standard GFCIs on dimmed circuits is not allowed.  There are, however, special GFCIs with a separate non-dim input designed for use with certain dimmer racks.

There is another potential problem with using GFCIs in a dimming circuit. In a conventional forward phase-control dimmer, the waveform is altered by the switching action of the dimmer.

The resulting waveform has a high third-order harmonic content that the GFCI may interpret as a residual current and then switch off the circuit. In order to avoid this nuisance tripping, dimmer-rated GFCIs must sense peak current instead of RMS current.

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