GB1020862A - Improvements relating to protective systems - Google Patents

Abstract

1,020,862. Protective arrangements. A. REYROLLE & CO. Ltd. April 26, 1962 [March 27, 1961], No. 11116/61. Heading H2K. A protective arrangement for checking the insulation from earth of a conductor, when it is disconnected by a switch 13 from a supply, comprises two impedances 45, 46 connected in series in a testing circuit between the conductor and earth, and means operative when the switch is closed for passing direct currents through the impedances such as to produce equal and opposite potential differences across them. One direct current is derived from.the supply side and the other from the load side of the switch so that when the switch is open the latter current ceases and the former current tends to pass between the conductor and earth a current representative of the insulation resistance. Thus in the polyphase system shown, an earth fault on one or more of the conductors 14 ... 16, when the switch 13 is open, enables the potential produced across the resistor 45 by transformer 51, 47 and rectifiers 49, to cause current to flow through a resistor 36, a diode 35, a transformer 20, the conductors and the fault path. The voltage produced across the resistor 36 causes a transistor trigger stage 60, 62 to change its state to render an amplifying transistor 69 conductive. A relay 71 is energized and is effective to operate a warning device or to prevent the closing of the switch 13. When the switch 13 is closed, the potential produced across the resistor 46 by a transformer 48, 52 and rectifiers 50 opposes that across the resistor 45. Since there is no resultant D.C. potential at point 43, the transistors 60, 62 and 69 are not energized. Should an earth fault now occur, the A.C. flowing through the testing circuit produces a rectified component across a resistor 38 which biases a transistor 75 to conduction. This causes a transistor 76 to cut-off so deenergizing a relay 77 which can be arranged to actuate a warning 'device and/or open the switch 13. The secondary windings 24 ... 26 of the transformer 20 are connected in delta with a resistor 27 in series in the delta to limit the A.C. earth fault current as claimed in Specification 1,020,863.