Monitoring three=phase power cable connected to appliance e.g. motor - detecting interruption caused by at least one cable conductor or short circuit between at least two conductors using current source connected to motor supply mains

摘要

An overall feed current (IR) is led from the current source (7) of the monitor circuit via a series circuit of a first electronic steady current relay (9), a first overcurrent switch (15.1) and a first cable conductor (22), to the motor (1), distributed in the resistance network, to give two partial currents (IR1,IR2). The first part current is led back across a series circuit of the second cable conductor (23) and a second steady current relay (10) to the current source (7). The second part current is led back over a series circuit of a third conductor (24), a second overcurrent switch (15.2), a third steady current relay (11) and an adjustable resistance (13) to the current source (7). The impedances of the current paths are adjusted by the resistance (13) so that different size current flows are produced. The trip currents of the overcurrent switches and the response currents of the relays (9,10,11) are so tuned to each other, that relatively small current value ranges for the steady permitted current flows are produced. The response due to undercurrent activates at least one relay to operate a fault alarm. ADVANTAGE - Practically monitors all alterations in condition of cable. Current in monitor circuit kept low.