Ground Fault Circuit Interrupter with Solid State Relay-Controlled Electromagnetic Contactor and Self-Test End of Life (STEOL) Circuits for High-Current Applications
A ground fault circuit interrupter (GFCI) with self-test end-of-life (STEOL) circuitry for 240-volt, 60-ampere applications is provided. An electromagnetic contactor with normally-open contacts on both line conductors is controlled by at least one optically-isolated solid state relay (SSR) connected in series with the contactor coil. A single sense transformer and ground fault detection integrated circuit detect current imbalances and trigger a silicon-controlled rectifier (SCR) to de-energize the SSR and open the contactor. A periodic self-test circuit exercises the fault protection circuit through a coupled winding of the sense transformer without interrupting load power. An auto-monitoring end-of-life circuit permanently denies power upon detecting failure of the fault protection circuit.
1 . A ground fault circuit interrupter (GFCI) for interrupting the flow of current through a pair of lines, wherein one of the pair of lines extends between a first line input and a first line output and the other line extends between a second line input and a second line output, the GFCI comprising:
an electromagnetic contactor rated for at least 60 amperes at 240 volts alternating current and having a coil and a first normally-open switch connected between the first line input and the first line output and a second normally-open switch connected between the second line input and the second line output;
at least one solid state relay (SSR) having an optically-isolated input and a switched output, the switched output connected in series with the electromagnetic contactor coil such that de-energizing the SSR input causes the switched output to open and de-energize the contactor coil, thereby opening the first and second switches; and
a fault detection circuit comprising a single sense transformer configured to detect a ground fault current imbalance between the first and second line conductors and to de-energize the electromagnetic contactor in response thereto.
2 . The GFCI as in claim 1 wherein the at least one SSR comprises a first SSR and a second SSR, and wherein the switched outputs of the first SSR and the second SSR are connected in series with the electromagnetic contactor coil.
3 . The GFCI as in claim 1 wherein the fault detection circuit further comprises a bi-stable electronic latch circuit comprising a silicon-controlled rectifier (SCR) operable in either a conductive or a non-conductive state, the SCR being configured to de-energize the electromagnetic contactor upon detection of a ground fault.
4 . The GFCI as in claim 3 wherein the fault detection circuit further comprises a ground fault detection integrated circuit coupled to the sense transformer and configured to trigger the SCR upon detection of a current imbalance exceeding a predetermined ground fault threshold.
5 . The GFCI as in claim 1 further comprising a simulated ground fault generator, an auto-monitoring logic module, and an end-of-life switch for permanently disabling the electromagnetic contactor if the GFCI fails the self-test.
6 . The GFCI as in claim 5 wherein the simulated ground fault generator comprises a periodic stimulus circuit configured to charge an energy storage element from a supply voltage and to discharge the energy storage element through a self-test winding magnetically coupled to the sense transformer upon the energy storage element reaching a predetermined breakover threshold, thereby periodically inducing a simulated ground fault signal in the sense transformer.
7 . The GFCI as in claim 5 wherein the auto-monitoring logic module comprises:
a monitoring circuit that continuously accumulates an electrical quantity; and
a threshold detector that generates an end-of-life signal when the accumulated quantity exceeds a predetermined threshold,
wherein each successful self-test cycle partially discharges the accumulated quantity maintaining it within a stable bounded range below the predetermined threshold, and wherein failure of the protection chain prevents the discharge causing the accumulated quantity to rise continuously until the predetermined threshold is reached.
8 . The GFCI as in claim 7 wherein the auto-monitoring logic module comprises a discharge circuit connected to the anode of a silicon-controlled rectifier, wherein the duration of each self-test discharge event is determined by the holding current characteristic of the silicon-controlled rectifier, the self-test cycle terminating inherently when the discharge current falls below the holding current threshold.
9 . The GFCI as in claim 8 wherein the silicon-controlled rectifier has a holding current characteristic sufficient to guarantee cessation of conduction prior to the next periodic stimulus event of the simulated ground fault generator at maximum operating temperature, whereby the auto-monitoring logic module maintains stable bounded behavior across the full operating temperature range.
10 . The GFCI as in claim 1 wherein the at least one SSR is a MOSFET-output solid state relay.
11 . The GFCI as in claim 1 further comprising a transient voltage suppressor network connected across the at least one SSR.
12 . A ground fault circuit interrupter system for protecting a 240-volt, 60-ampere AC circuit, the system comprising:
a toroidal sense transformer through which a first line conductor and a second line conductor pass, the sense transformer further comprising a sense winding and a self-test winding wound on the toroidal core, all magnetically coupled, and wherein the system does not comprise a second sense transformer;
a ground fault detection integrated circuit connected to the sense winding, configured to detect current imbalances indicative of a ground fault;
a silicon-controlled rectifier (SCR) having its gate connected to an output of the ground fault detection integrated circuit;
a pair of optically-isolated solid state relays, each having an LED input and a MOSFET output, the LED inputs connected in series in a circuit controlled by the SCR;
a 240-volt, 60-ampere rated electromagnetic contactor having a coil connected in series with the MOSFET outputs of the pair of solid state relays, and having a first normally-open contact on the first line conductor and a second normally-open contact on the second line conductor;
a relay synchronized switch circuit configured to maintain energization of the electromagnetic contactor coil and the pair of solid state relays during each self-test event;
a self-test circuit comprising a periodic stimulus generator and the self-test winding, configured to periodically inject a simulated ground fault signal into the sense transformer at intervals determined by circuit parameters; and
an end-of-life circuit configured to accumulate a monitored quantity responsive to self-test cycle activity, to compare the monitored quantity against a predetermined end-of-life threshold, and to permanently deny power to the load when the monitored quantity exceeds the end-of-life threshold.
13 . The system of claim 12 , wherein the self-test circuit and the end-of-life circuit comprise discrete analog and passive components.
14 . A ground fault circuit interrupter (GFCI) for interrupting the flow of current through a pair of lines, wherein one of the pair of lines extends between a first line input and a first line output and the other line extends between a second line input and a second line output, the GFCI comprising:
A 240-volt, 60-ampere rated electromagnetic contactor having a coil and a first normally-open switch connected between the first line input and the first line output and a second normally-open switch connected between the second line input and the second line output;
at least one solid state relay (SSR) having an optically-isolated input and a switched output, the switched output connected in series with the electromagnetic contactor coil such that de-energizing the SSR input causes the switched output to open and de-energize the contactor coil;
a capacitor bus configured to supply operating current to the SSR input, the capacitor bus being charged from a power supply connected across the first and second line inputs;
a power-on delay circuit configured to inhibit energization of the SSR input until the capacitor bus voltage has risen to a predetermined operating threshold, thereby ensuring controlled turn-on of the electromagnetic contactor upon application of line voltage; and
a fault detection circuit configured to de-energize the SSR input upon detection of a ground fault current imbalance between the lines.
15 . The GFCI as in claim 14 wherein the power-on delay circuit comprises a comparator having a first input connected to a voltage divider across the capacitor bus, a second input connected to an internal reference voltage, and an output configured to enable the SSR input when the voltage divider output exceeds the internal reference voltage.
16 . The GFCI as in claim 15 wherein the comparator is a precision comparator and the internal reference voltage is a bandgap reference voltage.
17 . The GFCI as in claim 14 further comprising a relay synchronized switch circuit activated simultaneously with a simulated ground fault stimulus event and configured to maintain energization of the electromagnetic contactor coil and the at least one SSR for the duration of the self-test window, such that the electromagnetic contactor remains closed and load power is uninterrupted during each self-test cycle.
18 . The GFCI as in claim 17 wherein the relay synchronized switch circuit is activated by the same stimulus event that generates the simulated ground fault signal, such that activation of the relay synchronized switch circuit and injection of the simulated ground fault signal are inherently synchronous.