There are 2 classes of ground-fault circuit interrupters and each class has a distinct function. A Class A ground-fault circuit interrupter trips when the current to ground has a value in the range of 4 through 6 milliamperes and is used for personnel protection. A Class A ground-fault circuit interrupter is suitable for use in branch circuits. A Class B ground-fault circuit interrupter (commonly used as ground fault protection for equipment) trips when the current to ground exceeds 20 milliamperes. A Class B GFCI is not suitable for employee protection.

Ground-fault circuit protection can be used in any location, circuit, or occupancy to provide additional protection from line-to-ground shock hazards because of the use of electric hand tools. There are four types of GFCIs used in the industry:

1. Circuit breaker type 2. Receptacle type

3. Portable type

4. Permanently mounted type.

The condition of use determines the type of GFCI selected. For example, if an electrician or maintenance person plugs an extension cord into a nonprotected GFCI receptacle, the easiest way to provide GFCI protection is to utilize a portable-type GFCI.

See NEMA 280-1990, "Application Guide for Ground Fault Circuit Interrupters."

2.7.1 HOW A GFCI WORKS

See Section 4.14 for ground-fault protection of equipment. GFCIs are devices that sense when current-even a small amount-passes to ground through any path other than the proper conductor. When this condition exists, the GFCI quickly opens the circuit, stopping all current flow to the circuit and to a person receiving the ground-fault shock.

Figure 2-1 shows a typical circuit arrangement of a GFCI designed to protect personnel. The incoming two-wire circuit is connected to a two-pole, shunt-trip overload circuit breaker. The loadside conductors pass through a differential coil onto the outgoing circuit. As long as the current in both load wires is within specified tolerances, the circuit functions normally. If one of the conductors comes in contact with a grounded condition or passes through a person's body to ground, an unbalanced current is established. This unbalanced current is picked up by the differential transformer, and a current is established through the sensing circuit to energize the shunt trip of the overload circuit breaker and quickly open the main circuit. A fuse or circuit breaker cannot provide this kind of protection. The fuse or circuit breaker will trip or open the circuit only if a line-to-line or line-to-ground fault occurs that is greater than the circuit protection device rating.

Figure 2-1. GFCI-protected circuits is one way of providing protection of personnel using electric hand tools on construction sites or other locations.

A GFCI will not protect the user from line-to-line or line-to-neutral contact hazards. For example, an employee is using a double insulated drill with a metal chuck and drill bit protected by a GFCI device. If the employee drills into an energized conductor and contacts the metal chuck or drill bit, the GFCI device will not trip (unless it is the circuit the GFCI device is connected to) as it will not detect a current imbalance.

2.7.2 USES

The use of GFCI's in branch circuits for other than dwelling units is defined in NEC Section 210-8(b), for feeders in NEC Section 215-9, and for temporary wiring in Section 305-6.

Ground-fault protection for personnel shall be provided for temporary wiring installations utilized to supply temporary power to equipment used by personnel during construction, remodeling, maintenance, repair or demolition activities.

For temporary wiring installations;

a) All 120-V, single-phase, 15- and 20-A receptacle outlets that are or are not a part of the permanent wiring of the building or structure and that are in use by employees shall have GFCI protection for personnel [See 29 CFR 1926.404(b) and NEC Section 305-6(a) and (b)] or an assured equipment grounding program (See Section 8.2).

b) GFCI protection or an assured equipment grounding program (See Section 8.2) for all other receptacles to protect against electrical shocks and hazards. [See NEC 305-6(a) and (b)].

c) Receptacles on a two-wire, single-phase portable or vehicle-mounted generator rated not more than 5 kW, where the circuit conductors of the generator are insulated from the generator frame and all other grounded surfaces, need not be protected with GFCIs. (See Figure 2-2 and Section 6.4).

Portable GFCIs shall be trip tested according to the manufacturers instructions.