264 Electrical Power Systems Technology
ning or line-to-ground shorts.
Single-phase 120/240-volt or 240/480-volt systems are grounded in
a manner similar to a three-phase ground. The neutral of the single-phase
three-wire system is grounded by a metal rod (grounding electrode) driv-
en into the earth at the transformer location. System grounding conduc-
tors are insulated with white or gray material for easy identification.
Equipment Grounding
The second type of ground is the equipment ground, which, as the
term implies, places operating equipment at ground potential. The con-
ductor that is used for this purpose is either bare wire or a green insulated
wire. The NEC describes conditions that require fixed electrical equipment
to be grounded. Usually, all fixed electrical equipment located in industri-
al plants or commercial buildings should be grounded. Types of equip-
ment that should be grounded include enclosures for switching and pro-
tective equipment for load control, transformer enclosures, electric motor
frames, and fixed electronic test equipment. Industrial plants should use
120-volt, single-phase, duplex receptacles of the grounded type for all por-
table tools. The grounding of these receptacles may be checked by using a
plug-in tester.
GROUND-FAULT PROTECTION
Gr ound-fault interrupters (GFIs) are used extensively in industrial,
commercial, and residential power distribution systems. It is required by
the NEC that all 120-volt, single-phase, 15- or 20-ampere receptacle outlets
that are installed outdoors or in bathrooms have ground-fault interrupters
connected to them. These devices are also called ground-fault circuit inter-
rupters (GFCIs).
GFI Operation
These devices are designed to eliminate electrical shock hazard re-
sulting from individuals coming in contact with a hot AC line (line-to-
ground short). The circuit interrupter is designed to sense any change in
circuit conditions, such as would occur when a line-to-ground short exists.
One type of GFI has control wires that extend through a magnetic toroi-
dal loop (see Figure 10-5). Ordinarily, the AC current flowing through the
two conductors inside the loop is equal in magnitude and opposite in di-