Page 185 - Handbook of Electrical Engineering

168 HANDBOOK OF ELECTRICAL ENGINEERING

Solution:
a) Choose the upstream MCCB at the switchboard and its settings.
From a manufacturer’s data sheet a 125 A MCCB with an adjustable 100 A thermal release is
chosen. The thermal release is set to 90 A to match the total load.
b) Choose the incoming feeder cable.
From a manufacturer’s data sheet several cables can be compared for the same ambient conditions
and laying arrangements. Their details are:-

2
50 mm cable, maximum current 124 A, R = 0.492,X = 0.110 ohms/km.
2
70 mm cable, maximum current 159 A, R = 0.340,X = 0.106 ohms/km.
2
95 mm cable, maximum current 193 A, R = 0.247,X = 0.093 ohms/km.
2
2
The 70 mm cable is chosen since the rating of the 50 mm cable is just too low.
c) Choose the downstream load MCCB and its settings.
From a manufacturer’s data sheet a 32 A MCCB with an adjustable 32 A thermal release is
chosen. The thermal release is set to 29 A to match its load.
d) Find the upstream fault source impedance.
For a prospective symmetrical fault current of 100 kA rms the upstream fault source impedance
Z up is:-
440.0
Z up = = 0.0 + j 0.00254 ohms
1.732 × 100000.0
e) Find the cut-off, or let-through, current from the switchboard.
From a manufacturer’s data sheet a 125 A MCCB has a let-through current I p of 25 kA peak for
a prospective fault current I s of 100 kA rms .
f) Find the impedance of the incoming cable.
The impedance Z c1 of the incoming cable is:-

25.0(0.340 + j 0.106)
Z c1 = = 0.0085 + j 0.00265 ohms
1000.0
g) Find the impedance of the load cable.
The impedance Z c2 of the incoming cable is:-
From a manufacturer’s data sheet several cables can be compared for the same ambient conditions
and laying arrangements. Their details are:-

2
6mm cable, maximum current 33.8 A, R = 3.91,X = 0.130 ohms/km.
2
10 mm cable, maximum current 46.7 A, R = 2.31,X = 0.126 ohms/km.
2
The 6 mm cable is chosen provisionally, since its rating is above the 32 A rating of the MCCB
that feeds it. The impedance Z c2 of the load cable is:-
15.0(3.91 + j 0.13)
Z c2 = = 0.0587 + j 0.00195 ohms
1000.0