Page 259 - Handbook of Electrical Engineering

CABLES, WIRES AND CABLE INSTALLATION PRACTICES 243

Figure 9.10 Overcurrent protection of a cable and motor.

For the circuit shown the following design conditions should apply,
• Cable ‘derated’ current I c should be greater than the rated motor current I m .
• In practice the motor running current I run would normally be slightly less than I m , since the motor
should have a rating greater than that of its driven machine. The margin in current would depend
upon the rating of the motor, see sub-section 1.6.

The following worked example illustrates how (9.12) and (9.13) are applied.

9.4.4.1 Worked example

2
Consider the worked example in sub-section 9.4.3.5.2 for a 160 kW motor, a 3-core 300 mm XLPE
◦
cable, and a 400 amp fuse. Assume the cable is routed in air at an ambient temperature of 35 C.
The motor has a full-load current I m of 245.4 amps.
The fuse rating is 400 amps and a near-asymptotic current I 2 of between 600 and 800 amps.
The ‘derated’ current of the cable is 0.92 × 666 = 613 amps (see Tables 9.15, 9.16 and 9.17).
This is the current I c .
Now I B should be the least of I m and I c ,i.e. I B = 245.4amps.
I n should be the rated current of the fuse, i.e. I n = 400 amps
I z should be the cable ‘derated’ current I c i.e., I z = 613 amps.
From (9.12),
I B [245.4] ≤ I n [400] ≤ I z [613]

which is satisfactory.
From (9.13):
I 2 [600 to 800] ≤ 1.45I z [889]