Electrical Power Systems Technology

Single-phase and Three-phase Distribution Systems 265

rection. Any change in this equal and opposite condition is sensed by the

magnetic toroidal loop. When a line-to-ground short occurs, an instanta-

neous change in circuit conditions occurs. The change causes a magnetic

field to be induced into the toroidal loop. The induced current is amplified

to a level sufficient to cause the circuit breaker mechanism to open. Thus,

any line-to-ground short will cause the ground-fault interrupter to open.

The operating speed of the GFI is so fast that the shock hazard to individu-

als is greatly reduced, since only a minute current opens the circuit.

Figure 10-5. Simplified schematic of a ground-fault interrupter

GFI Applications

Construction sites, where temporary wiring is set up, are required to

use GFIs for the protection of workers using electrical equipment. Ground-

fault protection of individuals and commercial equipment must be pro-

vided for wye-connected systems of 150 to 600 volts for each distribution

panelboard rated at over 1000 amperes. In this situation, the GFI will open

all ungrounded conductors at the panelboard when a line-to-ground short

occurs. Now, GFIs are used for all types of residential, commercial, and in-

dustrial applications.

Types of Ground-fault Protection Systems

There are four basic types of ground-fault protection systems in use

today. They are: hospital applications, residential applications, motor pro-

tection applications, and specific electrical power distribution system ap-

plications. These ground-fault systems can be classified either by what

they are to protect, or by the type of protection they are to provide. Hos-