Electrical Power Systems Technology

Single-phase and Three-phase Distribution Systems 279

connects directly to the branch circuits. Neutrals should never be opened

(fused). This is a safety precaution in electrical wiring design.

Three-phase Branch Circuits

A diagram for a three-phase, four-wire (120/208-volt) power distribu-

tion panel is shown in Figure 10-9. There are three single-phase 120-volt

branch circuits, and two three-phase 208-volt branch circuits shown. The

single-phase branches are balanced (one hot line from each branch). Each

hot line has an individual circuit breaker. Three-phase lines should be con-

nected so that an overload in the branch circuit will cause all three lines to

open. This is accomplished by using a three-phase circuit breaker, which

is arranged internally as shown in Figure 10-9.

Figure 10-9. Diagram of a power distribution panel for a three-phase, four-wire

branch circuit