Improvements relating to overload protection devices

906,594. Electrically-released switches; contact members for switches; protective arrangements. WESTINGHOUSE BRAKE & SIGNAL CO. Ltd. April 4, 1960 [April 6, 1959], No. 11563/59. Class 38 (5). A short-circuiting switch, e.g. for protecting germanium or silicon diode rectifiers, has a moving bridging contact member 1 normally held in the open position against the bias of a spring 3 by the armature 19 of an electromagnet or permanent magnet 20. On the energizing of a trip coil 22, the armature is released and the spring 3 impels the contact member 1 into engagement with resiliently mounted fixed contacts 4, 5 having damping means to prevent rebound. These contacts are pivoted on spurs R, Y extending from bus-bars (not shown), have their movement in one direction limited by adjusting screws 8, 9, and are damped by an assembly comprising a spider 16 slidable on a stud 14 and biased by bow-shaped laminar spring washers 12 with an interposed disc 13. Alternatively, hydraulic damping devices may be used and the spring 3 may comprise a stack of spring washers. The switch is brought into the open position manually or by momentary over-excitation of a holding coil 21. In a three-phase arrangement the contacts may comprise a disc 1 co-operating with three fixed contacts, though a bridging contact member connected to one phase may co-operate with fixed contacts connected to the other two phases. The contacts may be silver faced and gold plated. In the arrangement in which the armature is normally held by a permanent magnet, the armature and magnet may each comprise concentric rings of magnetic and non-magnetic material. If a holding coil 21 is used, it may be energized from a battery through a series resistor having a high positive temperature coefficient. The trip coil 22 may be energized from series-connected single-phase fullwave rectifiers each connected to a current transformer in one of the phases of the protected system, Fig. 2 (not shown).