An electrical by-pass unit, for use with residual current devices (RCD) to permit testing of equipment receiving its electrical supply from a circuit controlled by an RCD, comprising a rigid, electrically-insulating body (19, 20), adapted for releasably secure attachment to an RCD and incorporating within the body one or more electrical conductors (1, 2), which are inaccessible to touch when the by-pass unit is attached to the RCD, and are adapted to safely electrically connect respective input and output terminals of the RCD.