Medium Voltage Application Guide

13745.A
Fuse rating (A)
13697.A
Fuse rating (A)
SWITCHGEAR
Motor circuits
Special motor rated fuses are used for motor starting. These fuses can sustain repeated motor start overload
currents without degradation. Fuses are installed to provide short circuit protection only and the motor circuit
must have separate overload protection.
Fuse selection for a motor application is typically carried out using graphs provided by the fuse manufacturer. These
graphs consider motor starting current (A), motor run-up time (s) and starts per hour.
Typical fuse ratings for 2, 4 or 8 starts per hour, starting time 60 seconds
2 x 315
2 x 250
2 x 200
315
250
200
160
100
1
8
4
2
Starts per hour
63
200 1000 10000
Motor starting current (A)
Typical fuse ratings for 2, 4, 8, 16 or 32 starts per hour, starting time 15 seconds
2 x 315
2 x 250
2 x 200
315
250
1
32
16
8
4
2
Starts per hour
200
160
100
63
200 1000 10000
Motor starting current (A)
Source: example curves based on ABB CMF fuse links
Exercise
A 3.3 kV motor has a full load current of 150 A. Its expected start current is 5.5 times full load current for 10 seconds
and it operates at 2 starts per hour. Select the required protection fuse.
Use the graph for Starting time 15 seconds. The start current will be 5.5 x 150 A = 825 A.
For motor starting current of 825 A, at 2 starts per hour, the required motor rated fuse is 250 A/3.6 kV.
Page 108 Medium Voltage Application Guide 710-12280-00A