GB800938A - Improvements in and relating to the control of electrical heating apparatus - Google Patents

Abstract

800,938. Switching systems for heating resistances. METROPOLITAN - VICKERS ELECTRICAL CO., Ltd. April 18, 1956 [April 18, 1955], No. 11139/55. Class 38 (4). In a circuit of the type whereby two or more heating elements are normally arranged in series, but for rapid initial heating, one or more of the elements are connected directly across the supply under the control of a thernal relay, devices are included to ensure that in the event of failure of the thermal relay, the circuit will revert to normal series working. In the system shown in Fig. 1, HI, H2 are heating elements connected together and to contact b of changeover relay R1. On-off switch CS energizes three parallel paths; (a) a circuit comprising coil RW4 of magnetic relay R4, heater 4 of thermal relay R3 and contact 5 of thermal cutout R2, (b) heater of energy regulator S over contacts 1, and (c) H1, H2 in series also over contacts 1 and normally closed contacts CC of R1. Energization of RW4 closes contacts 6, completing circuit of RW1 over contact 7 of R3, thus connecting H1, H2 in parallel, with the common connection b fed from the mains through thermal cut-out R2 and heavy current fuse F2. After a predetermined interval, bimetallic contacts 7 open under the action of heater 4, releasing RW1 and reverting H1, H2 to series connection under the control of energy regulator S and fed through light current fuse F1 In the event of heater 4 becoming opencircuited, coil RW4 will lose its supply, releasing RW1. Should contacts 7 of R3 fail to open, thermal cut-out R2, which has a longer delay period than R3, will operate, thus again releasing RW4, and hence RW1, and, as an additional precaution, breaks the main supply to common connection b. In a simplified circuit, Fig. 2 (not shown), heater H1 is short-circuited by operation of RW1, thus connecting H2 directly across the supply, and in another modification, a switch CSA may be inserted in the neutral, disconnecting H1 and limiting the rapid heating to that given by H2 only. The heaters may or may not be of equal rating. Specification 517,538, [Group XXXVII], is referred to.