Single-phase and Three-phase Distribution Systems 267
vices are constructed according to standards developed by the Underwrit-
ers’ Laboratories. The GFI circuit breakers combine ground-fault protec-
tion and circuit interruption in the same over-current and short-circuit pro-
tective equipment as does a standard circuit breaker. A GFI circuit break-
er fits the same space required by a standard circuit breaker. It provides
the same branch-circuit wiring protection as the standard circuit breaker,
as well as ground-fault protection. The GFI sensing system continuously
monitors the current balance in the ungrounded (hot) conductor and the
grounded (neutral) conductor. The current in the neutral wire becomes
less than the current in the hot wire when a ground fault develops. This
means that a portion of the circuit current is returning to ground by some
means other than the neutral wire. When an imbalance in current occurs,
the sensor (a differential current transformer) sends a signal to the solid
state circuitry, which activates a trip mechanism. This action opens the hot
line. A differential current as low as 5 mA will cause the sensor to send a
fault signal and cause the circuit breaker to interrupt the circuit.
Ordinarily, GFI receptacles provide ground-fault protection on 120-,
208-, or 240-volt AC systems. The GFI receptacles come in 15- and 20-am-
pere designs. The 15-ampere unit has a receptacle configuration for use
with 15-ampere plugs only. The 20-ampere device has a receptacle config-
uration for use with either 15- or 20-ampere plugs. These GFI receptacles
have connections for hot, neutral, and ground wires. All GFI receptaclesTable 10-1. Body Reaction to Alternating Current
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Amount of Current Effect on Body
1 mA or less No sensation (not felt).
More than 5 mA Painful shock.
More than 10 mA Muscle contractions; could cause “freezing” to the electrical circuit
for some people.
More than 15 mA Muscle contractions; could cause “freezing” to the electrical circuit
for most people.
More than 30 mA Breathing difficult; could cause unconsciousness.
50 to 100 mA Ventricular fibrillation of the heart is possible.
100 to 200 mA Ventricular fibrillation of the heart is certain.
Over 200 mA Severe burns and muscular contractions; the heart is more apt to
stop beating than to fibrillate.
1 ampere and above Permanent damage to body tissues.
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