Page 186 - 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 Z m1 is 0.0008 + j 0.0001 ohms.
Therefore the fault impedance Z fb is:-

Z fb = Z c1 + Z m1 = 0.00093 + j 0.00275 ohms

The fault making current I fb is:-
440.0
V p
I fb = = = 26,195A rms
Z fb 1.732(0.0093 + j 0.00275)
Where V p is 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 Z m2 is 0.005 + j0.000009 ohms. Add this to Z fb to
give the fault impedance Z fc as:-

Z fc = Z fb + Z m2 = 0.00093 + j 0.00275 + 0.005 + j 0.000009
= 0.0143 + j 0.002759 ohms

The fault making current I fc is:-
440.0
V p
I fc = = = 17,443A rms
Z fc 1.732(0.0143 + j 0.02759)
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.