I recently taught a 2014 National Electrical Code (NEC) class at an industrial facility in Fort Wayne, Ind., where an attendee asked about special-purpose ground fault circuit interrupters (GFCIs) with trip levels above the normal 4–6 milliampere (mA) trip threshold level. I told him that the definition of GFCI in the 2014 NEC only recognizes a Class A device. I also explained that ground-fault protection for personnel were normally 15 and 20A, single-phase, 120-volt (V) receptacles or circuit breakers providing personnel protection. In addition, I told him that I would research the issue further and provide more information at a later date. The following information is the result of this research.
The first mention of a GFCI is in Section 680-4(g) of the 1968 NEC, covering underwater lighting fixtures supplied directly from a branch circuit or by a transformer that would “perform reliably under any likely combination of fault conditions so that there is no shock hazard” to a person.
“Compliance with this requirement shall be assured by one of the following: (1) The design and construction of the fixtures; or (2) The use of a ground-fault circuit interrupter.” The definition of a GFCI was much the same then as it is in the 2014 NEC, except a Class A device was not mentioned in the 1968 NEC. In the 1968 NEC, GFCIs had a trip-threshold level of 20 mA due to the high level of leakage that existed on many swimming-pool circuits and were later referred to as Class B GFCIs. UL 943 is the standard for GFCIs, and any references to Class B GFCIs were removed from the standard during the fourth edition on or about Feb. 1, 2006. The trip threshold of a Class B GFCI device was so much higher at 20 mA than a Class A device at 4–6 mA that additional safety procedures should have been provided involving an assured equipment grounding system for the protected circuit.
The definition in Article 100 of the 2014 NEC for a GFCI is as follows: A device that is intended for the protection of personnel and functions to de-energize a circuit or a portion of the circuit within an established period of time when a current to ground exceeds the values established for a Class A device. An informational note located directly below this definition states that Class A GFCIs trip when the current to ground is 6 mA or higher and do not trip when the current to ground is less than 4 mA. Currently, all GFCI devices covered by the NEC are within the 4–6 mA range. Section 555.3 does require the main overcurrent protective device that supplies a marina to have ground-fault protection that doesn’t exceed 100 mA with that trip threshold considered to be more ground-fault protection of equipment than personnel protection, although some protection could apply to personnel swimming in the water around the marina dock.
On Dec. 18, 2000, Underwriters Laboratories announced the requirements for additional classes of GFCIs, called special-purpose GFCIs. The special-purpose GFCI category is for applications where equipment grounding is provided by the NEC or where the voltage is greater than 150V. Specialty GFCIs trip when the current to ground has a value of 15 to 20 mA. Since the trip threshold can be at or above where a person could normally let go, people touching the protected equipment and ground must have a low-impedance and reliable equipment grounding path that is in parallel with the person’s body and relies on the equipment grounding parallel path for let-go protection. The reliability of the grounding circuit could be demonstrated by a ground monitoring system that monitors the grounding path between the service and the load. If an unacceptable increase in resistance in the grounding path occurs, the circuit will be opened by the ground monitoring system. Double insulation of electrical equipment is another method used for this purpose.
These special-purpose GFCI devices are classified based on voltage and the quality of the equipment grounding path. A Class C is intended for circuits not in excess of 300V AC. A Class D is intended for circuits with one or more conductors over 300V to ground with a reliable ground path to ensure the voltage across the body will not exceed 150V. A Class E is similar to a Class D but has a conventional equipment grounding system or has double insulation, however, the GFCI device opens very quickly before the current has a magnitude and duration of current through the body that exceeds the limits for ventricular fibrillation.