A 3-phase 4-wire (230/400 V) installation is TN-C earthed. A circuit is protected by a
type B circuit-breaker rated at 63 A, and consists of an aluminium cored cable with
50 mm2 phase conductors and a neutral conductor (PEN) of 25 mm2.
What is the maximum length of circuit, below which protection of persons against
indirect-contact hazards is assured by the instantaneous magnetic tripping relay of
the circuit-breaker?
Figure F46 gives, for 50 mm2 and a 63 A type B circuit-breaker, 603 metres, to which
must be applied a factor of 0.42 (Figure F44 for m Sph
SPE = = 2)..
The maximum length of circuit is therefore:
603 x 0.42 = 253 metres.