Handbook of Electrical Engineering

SWITCHGEAR AND MOTOR CONTROL CENTRES 169

h) Find the fault current at the distribution board, point B.

From a manufacturer’s data sheet the contact impedance data for low voltage MCCBs are:-

MCCB Resistance Reactance

Rating in amps in ohms in ohms at 60 Hz

16 0.01 neglect

20 0.008 neglect

25 0.0065 neglect

32 0.005 0.000009

50 0.0027 0.000016

63 0.002 0.000025

80 0.0014 0.000042

100 0.0011 0.00007

125 0.0008 0.0001

160 0.00055 0.00015

200 0.0004 0.0002

250 0.00029 0.00027

320 0.0002 0.0004

Hence the upstream MCCB impedance Zm 1 is 0.0008+j 0.0001 ohms.

Therefore the fault impedance Zfbis:-

Zfb=Zc 1 +Zm 1 = 0. 00093 +j0.00275 ohms

The fault making current Ifbis:-

Ifb=

Vp

Zfb

=

440. 0

1. 732 ( 0. 0093 +j 0. 00275 )

= 26 , 195 Arms

Where Vpis the line-to-neutral voltage. Locate the point R for 26,195 A on the prospective curve

in Figure 7.9.

i) Find the fault current at the beginning of the load cable, point C.

Hence the downstream MCCB impedance Zm 2 is 0. 005 +j 0 .000009 ohms. Add this to Zfbto

give the fault impedance Zfcas:-

Zfc=Zfb+Zm 2 = 0. 00093 +j 0. 00275 + 0. 005 +j 0. 000009

= 0. 0143 +j0.002759 ohms

The fault making current Ifcis:-

Ifc=

Vp

Zfc

=

440. 0

1. 732 ( 0. 0143 +j 0. 02759 )

= 17 , 443 Arms

Locate the point S for 17,443 A on the prospective curve in Figure 7.9.

j) Find the fault current at the end of the load cable, point D.