The most important thing to understand about ground fault protection devices is that one type is to protect personnel and the other is to protect electrical equipment, the latter having different trip levels for different types of protection.
Three basic ground fault systems or leakage current protection devices are used in the electrical system: ground fault protection for personnel that are UL Listed in accordance with UL 943, immersion detection protection for appliances that are UL Listed in accordance with UL 1664, and ground fault protection for equipment that is UL Listed in accordance with UL 1053.
The basic definition of a ground-fault circuit-interrupter (GFCI) is a general-use device whose function is to interrupt the electric circuit to a load within an established period of time. There is a Class A GFCI that trips when a ground fault current exceeds 5 milliamps and there is a Class B GFCI that trips when a ground fault current exceeds 20 milliamps.
A Class B GFCI with a 20 milliamp trip level is to be used only for protection of underwater swimming pool lighting fixtures installed before adoption of the 1965 National Electrical Code (NEC). When using a Class B GFCI, the swimming pool lighting circuit must be disconnected before servicing or relamping the lighting fixture. Often the electrical equipment in older pools had a leakage current that was greater than the 5 milliamps required for a Class A GFCI and this leakage current would cause nuisance tripping of the circuit. Therefore, a Class B device with a higher trip current was permitted.
Another GFCI-Type device is an appliance leakage current interrupter (ALCI). It is a device intended to be used in conjunction with an electrical appliance and is designed to interrupt the circuit when a ground fault current exceeds 6 milliamps. It is not the intent to replace a GFCI with an ALCI where a GFCI is required by the NEC and it does not take the place of the branch circuit overcurrent protection device in the circuit.
An immersion-detection circuit-interrupter (IDCI) is another device intended to be used with electric appliances and designed to interrupt circuit to the load when an appliance is unintentionally immersed in water. Again, it is not the intent to replace a GFCI with an IDCI where a GFCI is required by the NEC and it does not take the place of the branch circuit overcurrent protection device in the circuit. Section 422-41 in the NEC requires cord- and plug-connected freestanding hydromassage units and hand-held hair dryers subject to immersion in water to be protected by an IDCI.
Ground fault protection equipment (GFPE) can either consist of an equipment leakage current interrupter (ELCI) or ground fault sensing and relaying equipment.
An ELCI is a device intended to provide leakage current protection in electrical appliances and electrical utilization equipment. This device will open all ungrounded conductors of the supply circuit to electrical equipment if the current in excess of the trip current occurs between live parts and the grounded enclosure or other grounded parts of the system.
Again, it does not take the place of the branch circuit overcurrent protection device in the circuit. Section 426-28 requires ground fault protection of equipment for fixed outdoor electric deicing and snow-melting equipment and Section 427-22 requires the same for electric heat tracing and heating panels. The trip ratings for these devices are usually in the 30 milliamp and higher range (anything higher than the 6 milliamps for a Class A GFCI device).
Ground-fault sensing and relaying equipment is intended for use in power distribution systems rated at 600 volts maximum. These devices are considered to be equipment protection devices and not personnel protection devices. They operate to cause a disconnecting means to function at a predetermined minimum value of ground fault in accordance with the NEC.
Ground-fault protection of equipment is further defined in the NEC in Article 100 as “a system intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent protection device.”
An example of this type of protection requirement is found in Section 230-95 of the NEC. Section 230-95 requires solidly grounded wye electrical services of more than 150 volts to ground but not exceeding 600 volts phase-to-phase with main disconnecting means rated at 1000 amperes or more, to be provided with GFPE. The setting of the GFPE must not be greater than 1200 amperes and the maximum time delay before operation shall not exceed one second for ground-fault currents of 3000 amperes or greater. It is plain to see by these values that this system is not designed for personnel protection.
In conclusion, make sure to use the proper type of system when designing and installing a ground fault sensing system, since there are many different purposes and functions for these devices.
ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or via e-mail at firstname.lastname@example.org.