Page 196 - Handbook of Electrical Engineering

FUSES 179

8.4.1 Worked Example

An example of fuse selection:-
A 6600 volt induction motor is fed from a fuse-contactor starter. Find the most appropriate fuse
rating and the appropriate size of a PVC cable for the motor. The following data are known:-
Motors:
• Rated kW = 760 kW.
• Rated cos Ø = 0.9.
• Rated efficiency = 0.96.
• Starting current = 4 times rated current.
• Starting cos Ø = 0.3.
• Starting time = 5 seconds.
Cable:
• Route length is short and volt-drops are negligible for starting and running.
• Derating factor to account for grouping, burying, racking, ambient
temperature is 0.65.
• 3-core cable sizes available are 25, 35, 50, 70, 95 mm sq, their nominal
current ratings are, 100, 125, 155, 190, 235 amps respectively.
2
• I t characteristics can be found by using a ‘k’ value of 110 for PVC cables
with copper conductors.
Power system:
• Fault level 150 MVA.
• Assume a three-phase fault at the motor.
• Assume an X-to-R ratio of 25.
• Fuse characteristics as shown in Figure 8.4.
The calculations can be carried out in various sequences; the following is just one sequence.
Step 1. Calculate the motor running and starting current.
P
Running current = √
3V cos Ø
7,60,000
= √ = 76.95 amps
3 × 6600 × 0.96 × 0.9
Starting current = 4 × 76.95 = 307.8amps

Step 2. Scale-down the cable ratings to suit the derating factors, prepare a revised table:-
Core size mm sq 25 35 50 70 95
Scaled-down ratings A 65 81 101 124 153

Hence the ‘minimum’ cable core size to suit the motor running current is 35 mm sq.
Step 3. Calculate the prospective symmetrical and asymmetrical fault currents.
Fault MVA of system (S f ) S f
I sym = √ = √
3 rated voltage (V ) 3V
1,50,000,000
= √ = 13,122 amps rms
3 × 6600