Means for dual level ground fault protection of a.c. circuits

Claim 1

A multi-stage relay circuitry for dual-level detection of ground fault currents in alternating current circuits of one or more phases and for protectively interruptiong the circuits, comprising in combination: first unitary-circuit means for deriving and for responding to a monitoring phase signal of which the deriving circuitry is common to any or all phases monitored, the monitoring phase signal being representative of fault current and the magnitude thereof in at least one of the phases of the monitored alternating current circuits, said first unitarycircuit means being comprised of a signal minimum and maximumthreshold means including connected in parallel (a) a minimumthreshold circuit including in series a minimum-resistance resistor and a solid-state light-emitting diode LED, in parallel with (b) a maximum-threshold resistor means, and in parallel with (c) a maximum-threshold short-circuiting means for shortcircuiting the signal of a magnitude greater than the signal of the maximum-threshold of the maximum-threshold resistor means, said solid-state light-emitting diode LED including a power input lead and a transistor output lead, there being also included a power-reducing resistor in series with the input lead, and connected to said input lead between said input lead and said power-reducing resistor there being a time-delay resistorcapacitor combination means, and including a gate transistor having the time-delay resistor-capacitor combination means connected operatively to gate terminal of the gate transistor, with the input lead of the gate transistor connected to a power source, and there further being included each of an alarm relay means and a circuit-breaking relay means each connected in series with the output lead of the light-emitting diode LED and each also connected to the output lead of the gate transistor, and additionally there being in series between the circuit-breaking relay means and the light-emitting diode LED a second resistor means limiting the maximum current therethrough prior to current exceeding the resistance of the second resistor means being activatable of said circuit-breaking relay means; and including a current-switch means and circuitry thereof operatively positioned relative to and as a part of the first relay means as a seal-in circuit for maintaining the first relay in a closed stateposition following activation of the first relay means, and the switch means including a manually operable circuit-breaking switch for breaking the circuit of the switch means manually.