Electric Power Generation, Transmission, and Distribution

23.1.6 Calculation of Fault Current

Line Faults

Line-to-neutral fault¼

E

ffiffiffi

3

p

2 Z‘

whereZ‘is the line impedance and 2Z‘is the loop impedance assuming the impedance of the phase

conductor and the neutral conductor are equal (some people use a 1.5 factor).

Line-to-line fault¼E= 2 Z‘

Transformer Faults

Line-to-neutral or three-phase¼

E

ffiffiffi

3

p

ZT

Line-to-line¼

E

2(ZTþZ‘)

whereZ‘¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

RL^2 þXL^2

p

ZT¼

ZT% 10 E^2

kVA

23.1.7 Current Limiting Fuses

Current limiting fuses (CLFs) use a fusible element (usually silver) surrounded by sand (Fig. 23.7).

When the element melts, it causes the sand to turn into fulgerite (glass). Since glass is a good insulator,

600

500

400

300

200

100

0

012

Time (minutes)

345

% of Normal Peak CurrentRelay

Pickup

15-Minute Outage

10-Minute Outage

5-Minute Outage

FIGURE 23.6 Cold load inrush current characteristics for distribution circuits.

FIGURE 23.7 Full range current limiting fuse. (Courtesy T&B. With permission.)