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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


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