GB835223A - Improvements in or relating to electrical protective systeí¡í¡ - Google Patents

Abstract

835,223. Protective cut-out arrangements. SIEMENS-SCHUCKERT (GREAT BRITAIN) Ltd., MORTON, J. D., and DIXON, B. J. June 4, 1956 [June 3, 1955], No. 16010/55. Class 38 (5). In an arrangement in which the closing of a contactor 4 under earth fault conditions is prevented by a relay 25 connected in a circuit including a low voltage supply source 65, a star point 89 (or other neutral or " midpoint ") formed on the load side of the contactor, and the fault path, the formation of the star point is prevented when the voltage on the load side of the contactor is above a predetermined value so as to avoid damage should arcing be prolonged at the contactor contacts when opened. In the Figure, the coil 91 of a starting relay 9 is energized on the closing of a starting switch (not shown) associated with a load connected to an outlet socket 1. The resulting closure of contacts 94 of the relay 9 causes the energization of a relay 8 which opens contacts 86 ... 88 to disconnect the star point and closes contacts 84, 85. The contactor coil 41 is now energized over contacts 92 of the relay 9, contacts 84 of the relay 8, contacts 71 of an overload relay 7, a stop button 17 and the contacts 51 of an earth leakage relay 5. In the event of earth leakage, the coil 53 of the relay 5 is energized from the secondary winding 34 of a core-balanced transformer 3 and by opening its contacts 51 causes the opening of the contactor 4 and the disconnection of the auxiliary transformer 6 supplying the relay 9. The relay 9 opens, but as the coil 81 of the relay 8 has been connected to the load side of the contactor through contacts 85 it is not deenergized until arcing ceases at the contactor contacts. If the resistance between earth and any of the phases falls below a predetermined value when the contactor is open and the parts are in the positions shown, then the lock-out relay 25 is energized and by closing contacts 28 connects part of the winding 34 of the corebalance transformer for energization from the transformer 6. As a result relay 5 opens to make it impossible to close the contactor by operating the starting switch. The relay 5 can only be reset by means of a key.