Electric Power Generation, Transmission, and Distribution

however, lines are generally 10 miles or less. Short-circuit levels at the substation are dependent on
voltage level and substation size. The average short-circuit level at a distribution substation has been
shown, by survey, to be about 10,000 A. Feeder load current levels can be as high as 600 A but rarely
exceed about 400 A with many never exceeding a couple of hundred amperes.

23.1.2 Fault Levels

There are two types of faults, low impedance and high impedance. A high impedance fault is considered
to be a fault that has a highZdue to the contact of the conductor to the earth, i.e.,Zfis high. By this
definition, a bolted fault at the end of a feeder is still classified as a low impedance fault. A summary of
findings on faults and their effects is as follows.

S

R

138 kV Distribution

Substation Transformer

ISC = 10,000 A 13.8 kV

Feeder Breaker

Peak Load = 600 A Three-Phase, 4-Wire,

Multi-grounded Fuse Cutout

Normally Open Tie Switch

Single-Phase Sectionalizer

Distribution

Transformers

4–15 Holmes/ Transformer

Fixed Capacitor Bank

Three-Phase Recloser

Switched Capacitor

Bank (= 600 kVAR)

Pothead

Faulted Circuit Indicator

Elbow Disconnect

Normally Open Tie

Normally Open Tie

Underground Lateral

FCI FCI

FIGURE 23.1 Typical distribution system.