GB423517A - Improvements in or relating to electric overload protective arrangements - Google Patents

Abstract

423,517. Protective cut-out arrangements. ELECTRICAL IMPROVEMENTS, Ltd., Carliol House, and POYNDER, H. B., 14, Haldane Terrace, both in Newcastle-on- Tyne. Aug. 4, 1933, Nos. 21933 and 28548. [Class 38 (v)] Automatic circuit -breakers.-Relates to overload protective arrangements of the kind in which the trip coil of a breaker is shunted by a resistance, and the shunt circuit is controlled by a thermal or other device so that substantially instantaneous operation on heavy overloads and inverse-time operation for light overloads occurs. To avoid the possibility of the trip coil failing to operate on heavy overloads, due to the increase in its reluctance as its plunger or armature is being raised and to the consequent increase in the amount of current diverted to the shunt circuit, the initial movement of the plunger or armature is arranged to operate an auxiliary switch which controls the energizing circuit of the trip coil to increase its tripping force. As applied to the arrangement disclosed in Specification 389,893, in which the thermal device may be a load-indicating instrument as disclosed in Specification 383,017, the auxiliary contacts are located in the shunt circuit, so as to open @ on heavy overloads and divert all the current through the trip coil, Fig. 1 (not shown). Fig. 6 shows such an arrangement applied to the two-trip-coil protection of three-phase circuits, the current transformers B<1>, B<2>, B<3> being connected in reverse-delta and the trip coils A being connected at one end to the ends of a heating element C and at the other end between two corners of the delta, the third corner being connected to a mid-point tapping of C. The coils A are shunted by the resistances D which are in series with the contacts C<1> operated by C and with contacts E which are opened by A on heavy overload. Fig. 2 shows an auxiliary switch in which the plunger A<2> of the trip coil A normally rests on a contact E<1> and holds it against E<2> which may be adjustable for calibrating A. In another form, Fig. 5 (not shown), the plunger engages one arm of a lever, another arm of which carries the contact, whereas a third arm is acted on by a control spring. In Figs. 7, 8, the coil A operates a change-over switch H so as to increase the tripping current by cutting out some of the turns of the current transformer B and, to avoid open-circuiting of B in doing so, the switch H is constituted by a mercury container pivoted at F<1> and operated by a pin-and-slot connection with A<2>. The latter arrangement is also described and illustrated, Fig. 9 (not shown), in its application to systems as disclosed in Specification 407,454.