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:-

50 mm^2 cable, maximum current 124A, R= 0. 492 ,X=0.110 ohms/km.

70 mm^2 cable, maximum current 159A, R= 0. 340 ,X=0.106 ohms/km.

95 mm^2 cable, maximum current 193A, R= 0. 247 ,X=0.093 ohms/km.

The 70 mm^2 cable is chosen since the rating of the 50 mm^2 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

Zupis:-

Zup=

440. 0

1. 732 × 100000. 0

= 0. 0 +j 0 .00254 ohms

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 Ipof 25 kApeakfor

a prospective fault current Isof 100 kArms.

f) Find the impedance of the incoming cable.

The impedance Zc 1 of the incoming cable is:-

Zc 1 =

25. 0 ( 0. 340 +j 0. 106 )

1000. 0

= 0. 0085 +j 0 .00265 ohms

g) Find the impedance of the load cable.

The impedance Zc 2 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:-

6mm^2 cable, maximum current 33. 8 A, R= 3. 91 ,X=0.130 ohms/km.

10 mm^2 cable, maximum current 46. 7 A, R= 2. 31 ,X=0.126 ohms/km.

The 6 mm^2 cable is chosen provisionally, since its rating is above the 32 A rating of the MCCB

that feeds it. The impedance Zc 2 of the load cable is:-

Zc 2 =

15. 0 ( 3. 91 +j 0. 13 )

1000. 0

= 0. 0587 +j0.00195 ohms