Page 187 - Handbook of Electrical Engineering

170 HANDBOOK OF ELECTRICAL ENGINEERING

Figure 7.9 Coordination of MCCBs at a distribution board.

Add Z c2 to Z fc to give the fault impedance Z fd as:-

Z fd = Z fc + Z c2 = 0.0143 + j 0.002759 + 0.0587 + j 0.00195
= 0.073 + j 0.00471 ohms

The fault making current I fd is:-

V p 440.0
I fd = = = 3473A rms
Z fd 1.732(0.073 + j 0.00471)
Locate the point U for 3473 A on the prospective curve in Figure 7.9.
k) Check the peak making capacity and peak let-through capacity of the MCCBs chosen above.
The following manufacturer’s data are typical for 125 A and 32 A MCCBs:-

MCCB Making capacity Let-through capacity
rating kA peak (cut-off)
kA rms kA peak
Note 1
32 A 95 209 6.0
125 A 132 290 25.0
Note 1: Approximate values of the doubling factor taken to be
2.2.