Medium Voltage Application Guide

13684.A
Maximum cut-off current (kA) peak
13682.A
SWITCHGEAR
Circuit Design (Motor circuit with fuses)
A contactor always needs some form of back-up short circuit protection. Although circuit breakers can be used for
this purpose, it is more common to use MV HRC fuses. Fuses have higher fault breaking capacity, are very fast acting
and are a good current limiting device.
Consider the following direct-on-line motor circuit, which includes a line contactor, short circuit protection fuses and
a motor protection relay, providing overload protection. Assume an operating supply of 6.6 kV/50 Hz and a motor
FLC of 120 A.
Typical DOL circuit with contactor, fuses and relay
6.6 kV /
50 Hz
F1
K1
CT1
PR
3
M1
M
3
(FLC = 120 A)
Step 1: Select the contactor
Rated voltage U r ≥ operating voltage U and AC3 rating I e ≥ motor FLC
A 7.2 kV/50 Hz contactor with an AC3 rating of 200 A will be adequate
Step 2: Select the fuse
The manufacturer will specify the maximum allowable fuse size. As a general rule, the nominal rating of
the fuse should be 1.5 times the motor FLC.
In this case
I (fuse) = 1.5 x 120
= 180 A
Use a 200 A fuse.
On a time-current curve, check that the contactor thermal withstand curve lies outside the total clearing
curve of the fuse.
From the fuse cut-off curve, the "limited" prospective short circuit current of the fuse must be less than the
short-time withstand current rating of the contactor.
I sc' (fuse) ≤ I k (contactor)
Assume the 7.2 kV/50 Hz, 200 A contactor has a short-time withstand current rating I k of 8 kA and the prospective
rms fault current level I sc at the point of installation is 10 kA.
100
50
20
10
315
250
200
160
100
5
63
2
1.0 2 5 10 20 50 100
Prospective current (kA) rms
From the fuse cut-off curve, we can see that a prospective rms fault current of 10 kA (I sc ) will be limited to 4 kA (I sc' )
by the 200 A fuse. Check that I sc' (fuse) ≤ I k (contactor).
4 kA ≤ 8 kA The fuse is suitable for use with the contactor.
Page 98 Medium Voltage Application Guide 710-12280-00A