Handbook of Electrical Engineering

170 HANDBOOK OF ELECTRICAL ENGINEERING

Figure 7.9 Coordination of MCCBs at a distribution board.

Add Zc 2 to Zfcto give the fault impedance Zfdas:-

Zfd=Zfc+Zc 2 = 0. 0143 +j 0. 002759 + 0. 0587 +j 0. 00195

= 0. 073 +j0.00471 ohms

The fault making current Ifdis:-

Ifd=

Vp

Zfd

=

440. 0

1. 732 ( 0. 073 +j 0. 00471 )

= 3473 Arms

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

kArms kApeak

kApeak(cut-off)

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.