Earth isolation monitoring circuit for AC supply network - uses diodes for coupling measuring bridge to AC power lines

摘要

The insulation of a non earthed mono or polyphase AC network and the DC circuits powered from it is monitored by a circuit which utilises diodes in place of an inductance for coupling to the network. This allows accidental groundings of the DC circuit to be detected with safety. One arm (6) of a bridge (5) contains a resistor (7) in series with a DC supply (8). The other arms contain resistors (10,14,16) while a measuring resistor (19) is connected across one diagonal. The other diagonal (11,12) is connected to the AC network (UV) via diodes (24,25). The signal from the measuring resistor is coupled to an impedance matching amplifier (27) followed by an average value circuit (28). A threshold circuit (31) is adjusted so that a fall in voltage across the measuring resistor is detected when accidental grounding of the AC network occurs.