Chapter A General rules of electrical installation design

G36
© Schneider Electric - all rights reserved
G - Sizing and protection of conductors
5 Particular cases of short-circuit
current
Example
Is a copper-cored XLPE cable of 4 mm 2 c.s.a. adequately protected by a
C60N circuit-breaker?
Figure G53 shows that the I 2 t value for the cable is 0.3272 x 10 6 , while the maximum
“let-through” value by the circuit-breaker, as given in the manufacturer’s catalogue, is
considerably less (< 0.1.10 6 A 2 s).
The cable is therefore adequately protected by the circuit-breaker up to its full rated
breaking capability.
Electrodynamic constraints
For all type of circuit (conductors or bus-trunking), it is necessary to take
electrodynamic effects into account.
To withstand the electrodynamic constraints, the conductors must be solidly fixed
and the connection must be strongly tightened.
For bus-trunking, rails, etc. it is also necessary to verify that the electrodynamic
withstand performance is satisfactory when carrying short-circuit currents. The peak
value of current, limited by the circuit-breaker or fuse, must be less than the busbar
system rating. Tables of coordination ensuring adequate protection of their products
are generally published by the manufacturers and provide a major advantage of such
systems.
Schneider Electric - Electrical installation guide 2008