Describes supplementary requirements for enclosed distribution boards (DBU), which are stationary, type tested assemblies (TTA) for indoor use, containing protective devices and designed for either domestic purposes or for places where unskilled persons have access for their use. Control and/or signalling devices may also be included. They are for use on a.c., with a voltage to earth not exceeding 300V. The outgoing circuits contain short-circuit protective devices, each rated current not exceeding 125 A with a total incoming load current not exceeding 250 A.