3.7.4 - Conductors of reduced current-carrying capacity

Short circuit protection must be positioned at every point where a reduction in cable-current carrying capacity occurs, as for overload protection {Fig 3.20}. However, if short circuit protection on the supply side of the point of reduction (for example, at the incoming mains position) has a characteristic that protects the reduced conductors, no further protection is necessary {Fig 3.22 }.

Even if not protected by a suitable device on the supply side, short circuit protection may be positioned on the load side of the reduction in rating if the conductors do not exceed 3 m in length, and are protected by trunking and conduit, and are not close to flammable materials, This reduction is particularly useful

Fig 3.22 Short circuit protective device protecting a circuit of reduced cross-sectional area

when connecting switchgear {Fig 3.23}. It should he noted that the 'tails' provided to connect to the supply system should always be of sufficient cross-sectional area to carry the expected maximum demand, and should never be smaller than 25 mm² for live conductors and 16 mm2 for the main earthing conductor.

Fig 3.23 Short-circuit protection not required for short switch gear connections