Patent GB-718973-A (GB)
Title: Improvements in or relating to protective circuit arrangements for operating low pressure electric discharge lamps
IPC: H05B41/46
Filed: 1951-07-31
Abstract: 718,973. Protective cut-out systems. GENERAL ELECTRIC CO., Ltd., and WILLIAMS, C. E. July 31, 1952 [July 31, 1951], No. 18096/51. Class 38(5) [Also in Group XXXV] In a circuit arrangement for operating a number of low pressure discharge lamps wherein two or more transformers have their primary windings serially connected with each other across the supply terminals and each secondary winding is adapted to feed one or more lamps, means are provided to bring about a protective action should the effective impedance of one or more of the primary windings change due to the occurrence of a fault. For example, a short circuit in one of the transformer secondary circuits would give a low transferred impedance in the corresponding primary winding, whilst an open circuit would give a high transferred impedance. The protective means includes at least one bridge circuit formed by two or more impedances serially connected with each other in parallel with the transformer primary windings, and by a connec tion from the junction between two adjacent primary windings and a point on the series combination of the impedances normally at the same potential as the junction, the operating coil or coils of an electromagnetic relay being arranged so that the relay is operated, to bring about a protective action, by the out of balance current flowing in the bridge circuit due to a fault. Normally the supply to the primary windings would include a variable dimming resistance, and the invention permits the resistance value to be varied without bringing about the protective action. According to one embodiment Fig. 1, the serially-connected primary windings 1, 2 of two similar transformers 3, 4 are connected to the supply 6, 7 through a dimming resistance 5 and contactor 17, each secondary winding 8, 9 feeding four low pressure discharge lamps 18, 19. The protective arrangement comprises a relay 10 having two high impedance operating coils 11, 12 arranged to act in opposition, the coils being serially connected across the windings 1, 2 and the junctions between the coils and the windings being interconnected to form a bridge circuit. A hold-on coil 13 of the relay 10 and operating coil 16 of the contactor 17 are connected in series with the relay contacts 14 across the supply 6, 7 on the live side of the contactor. In normal operation the relay contacts 14 remain open since the same current flows through each coil 11, 12 and these coils act in opposition. Should, fault occur in one of the lamp circuits the same voltage will not be dropped across each of the primary windings 1, 2 and the currents in the coils 11, 12 will differ. The relay contacts 14 will thus be closed and coil 15 energized to open the contactor 17, coil 13 maintaining the circuit in this condition. According to Fig. 2 (not shown) the high impedance coils 11, 12 are each replaced by a low impedance coil in series with a negative glow tube. Normally the voltage across each of the glow tubes is insufficient to ionize it, but on fault in one of the lamp circuits an increased voltage across one of the primary windings causes the corresponding tube to conduct and a relay coil is energized to close the contacts 14. According to Fig. 3 (not shown) the relay 10 is operated by a single coil inserted in the inter-connection between the junctions of the impedances 11, 12 and windings 1, 2 there being no current flowing through the interconnection in normal operation. In the modification of Fig. 4 (not shown) the impedances of Fig. 3 are constituted by two negative glow tubes. In the arrangement of Fig. 5 two additional transformers 30, 31 are provided to permit operation of sixteen lamps from two pairs of series connected secondary windings 8, 33, and 9, 36, the protective arrangement being the same as that shown in Fig. 1, but, alternatively, the arrangements of Figs. 2- 4 may be employed. In another arrangement for operating sixteen lamps, Fig. 7, the primary windings 38, 39 of two additional transformers 40, 41 constitute the impedances shunting the windings 1, 2, the secondary windings 8, 9, 42, 43 being .so connected in pairs (called " inverse " connection) that a fault in either secondary circuit similarly affects the primary windings in the opposite arms of the bridge and a current flows through the relay operating coil 46. In the arrangement according to Fig. 8 two groups of four transformers 48-51 and 56-59 are connected in parallel with each other to feed thirty-two lamps from four pairs of '' inversely " connected secondary windings 64-71. the junctions between adjacent primary windings of each group being interconnected through relay operating coils 73-75 which close the contacts 14 under fault conditions in any of the secondary circuits. The transformers may be of the leaky inductance type designed to withstand additional heating under fault, in which case the relay 10 may, alternatively, protect the dimming resistance 5 by short-circuiting it. Further, the contactor 17 may be omitted and the relay provide the protective action directly. Specification 715,693, [Group XXXV], is referred to.