Graund Fault Circuit Interprises

DOE-HDBK-1092-98

consideration. Therefore, where there are conflicts between the mandatory requirements of the above

codes, standards, and regulations, the requirements that address the particular hazard and provide the

greater safety shall govern.

2.7 GROUND FAULT CITCUIT INTERRUPTERS

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.

2-8