Figure 2-1. GFCI-protected circuits are one way of providing protection of personnel

DOE-HDBK-1092-2004

Figure 2-1. GFCI-protected circuits are one way of providing protection of personnel

using electric hand tools on construction sites or other locations.

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.

Differential transformers continuously monitor circuits to ensure that all current that flows out to

motor or appliances returns to the source via the circuit conductors. If any current leaks to a

fault, the sensing circuit opens the circuit breaker and stops all current flow.

A GFCI will not protect the user from line-to-line or line-to-neutral contact hazards. For example, if

an employee using a double-insulated drill with a metal chuck and drill bit protected by a GFCI

device 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 GFCIs in branch circuits for other than dwelling units is defined in the NEC 410.4.

2-6