GB1311719A - Fault-detection circuit for use in the protection of an electrical system - Google Patents

Abstract

1311719 Protective arrangements SIEMENS AG 11 Nov 1970 [11 Nov 1969] 53733/70 Heading H2K A fault-detection circuit for use in the protection of an electrical system comprises a threshold device UG1 responsive to undervoltage, a threshold device IG1 responsive to a lower overcurrent, a threshold device IG2 responsive to a higher overcurrent and a logic circuit arranged to provide an output signal when the devices UG1, IG1 respond to a lower overcurrent accompanied by undervoltage and when the means IG2 responds to a higher overcurrent irrespective of the voltage. The devices UG1, IG1, IG2 provide outputs which cease when the specified conditions arise so that a NOR gate N1 or a signal inverter N2 then provides a signal to an "order transmitter" B. The resulting output signal may control either directly or through one or more protective devices, signalling devices and/or circuitbreakers. A relay U with changeover contacts increases the voltage value at which the means UG1 ceases to provide an output when zerosequence currents arise and an overcurrent relay UA disconnects the NOR gate N1 should a short-circuit fault occur in the circuit supplying the undervoltage means UG1. A polyphase system would be protected by a plurality of such fault-detection circuits with a single selecting circuit to select for operation the relay U associated with the phase having the smallest phase voltage and/or the largest phase current. This selecting circuit may operate in response to a quotient or difference measurement. It may comprise a plurality of polarized relays each having a winding carrying a D.C. current proportional to a phase current and a winding carrying a D.C. current proportional to the sum of the phase currents.