GB1604711A

GB1604711A - System for protecting power distribution circuit against ground faults

Info

Publication number: GB1604711A
Application number: GB19698/78A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1977-09-09
Filing date: 1978-05-16
Publication date: 1981-12-16
Also published as: IT7827131A0; FR2402959A1; DE2838804A1; JPS5453239A; CA1121497A; IT1098755B

Description

(54) SYSTEM FOR PROTECTING POWER DISTRIBUTION CIRCUIT AGAINST GROUND FAULTS (71) We, GENERAL ELECTRIC CoM- PANT, a corporation organised and existing under the laws of the State of New York, United States of America, of 1 River Road, Schenectady 12305, State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: Ground fault protection for personnel and electrical equipment is an ever increasing concern in both home and industry.

For residential circuit applications, ground fault circuit interrupting (GFCI) devices are now widely avallable in compact configurations for convenient installation in existing service entry equipment. These GFCI devices are primarily intended to protect people from the hazards of electrical shock caused by leakage current emanating from ground faults, however they do afford a measure of equipment protection in terms of acting to halt ground fault current which can be damaging to insulation.

For high current applications found in industry, ground fault protection is available in basically two configurations. With the event of so-called " static trip" circuit breakers, it has become economically possible to combine overcurrent and ground fault signal processing circuitry in a compact electronic trip unit package which can be integrated with the circuit breaker to achieve comprehensive circuit protection.

Alternatively, traditional circuit breakers having thermal-magnetic and dualmagnetic trip units for overcurrent protection can be utilized with so-called ground fault relays " in providing ground fault protection as well. These relays respond to a ground fault signal developed by a suitable sensor, such as a zero sequence transformer coupled with the load carrying conductors, by energizing (or de-energizing) a solenoid which, in turn serves to trip the breaker to initiate circuit interruption.

Our U.S. Patent No. 4,044,395, issued in U.S.A. on August 23, 1977, discloses a system for protecting relatively large power distribution circuits against ground faults by utilizing as its principle operating component a conventional GFCI device normally used in low power, residential circuit applications. As disclosed and claimed therein, the GFCI device is installed in a control circuit for an undervoltage release solenoid adapted to a conventional circuit breaker protecting the distribution circuit or in the control circuit for the holding coils of a contactor operating in the distribution circuit. A ground fault sensor in the form of a zero sequence transformer coupled with the load current carrying conductors of the distribution circuit develops a current signal in its secondary winding in response to a ground fault on the distribution circuit.This current signal is injected into one side of the control circuit to create a current imbalance of the nature to which the GFCI device is responsive. The GFCI device trips to interrupt the control circuit, and the undervoltage release solenoid drops out to trip the circuit breaker or the holding coil becomes de-energized to open the contactor. In either case, the distribution circuit is interrupted.

The system configuration of that patent, while quite satisfactory for most applications, has one drawback in that it is sensitive to control circuit voltage fluctuations.

That is, should the control circuit voltage drop into an abnormally low level, the undervoltage release solenoid could drop out, causing the circuit breaker to trip despite the fact there is no ground fault on the distribution circuit. The same nuisance interruption of the distribution circuit could likewise result from control circuit voltage dips if a contactor is substituted for the circuit breaker.

According to the present invention there is provided a system for protecting an electrical power distribution circuit against ground faults by energizing a solenoid to trip a circuit breaker connected in said circuit, said system comprising: an energization circuit for a said solenoid; a normally open switch connected in said energization circuit; a power supply circuit; a differential current transformer comprising first and second primary windings connected one each in series with each side of said power supply circuit; and a second ary winding for development of a signal indicative of an imbalance in currents flowing in said primary windings; means responsive to said signal being of a predetermined magnitude for a predeter mined duration to effect closure of said normally open switch; and a ground fault sensor for coupling to a said distribution circuit and having output terminals connected across one of said primary windings; whereby in use a ground fault on a said distribution circuit causes a current signal to be developed by said sensor, said current signal flowing in said one primary winding to create a said imbalance in said transformer primary windings so that said responsive means effects closure of said switch, and said switch closure causes energization of said solenoid by said energization circuit to trip said circuit breaker.

Turning to the drawing, a ground fault protection system according to the present invention is depicted in its application to a high voltage, idustrial-type electrical distribution circuit including a grounded neutral source 10 supplying three-phase power over phase or line conductors 12 to a load 14. Included in this distribution circuit is a conventional three-pole circuit breaker, generally indicated at 16, having separable contacts 18 connected in series with each line conductor 12. The circuit breaker also includes, is diagrammatically illustrated in FIGURE 1, a trip unit 20 of known construction responsive to the levels of current flowing in the three line conductors for effecting automatic opening of the breaker contacts 18 under overload and short circuit conditions.

Operatively associated with circuit breaker 16 in a well known manner is a solenoid 22, commonly referred to as a "shunt trip" solenoid. As is well understood in the art, a shunt trip solenoid in its adaptation to a circuit breaker is normally de-energized, but when it is desired to trip the circuit breaker, its coil is energized.

Its plunger is magnetically attracted from an inactive to an actuated position, in the process striking a latch associated with the trip unit. The latch releases the breaker mechanism which operates under the power ofa mechanism spring to abruptly open the breaker contacts.

The illustrated system further includes a ground fault circuit interrupting (GFCI) device, generaly indicated at 26, of the type widely available for use in low voltage residential-type circuits for protecting humans from the hazards of electrical shock due to ground faults. As diagrammaticaly illustrated in FIGURE 1, the GFCI device 26, energized from a conventional 120 volt AC source 24, includes a differential current transformer consisting of a toroidal core 28, a first single turn primary winding 30a connected in series with the line side of a control power circuit fed from source 24, a second single turn primary winding 30b connected in series with the neutral side of the control power circuit, and a multi-turn secondary winding 32 connected to the input of an electronic module 34.As is well understood in the art, the differential transformer develops a signal in its secondary winding 32 in response to a differential in the currents flowing in the primary windings 30a and 30b, as would be occasioned by an imbalance in the currents flowing in the two sides of the control power circuit. This signal is processed by electronic module 34, and, if found to exceed a predetermined magnitude and duration, an electronic switch is triggered to complete an energization circuit for a solenoid (not shown). Energization of this solenoid initiates the opening of contacts 36 to interrupt the control power circuit.

It will be understood that the GFCI device may be constituted in a single pole circuit breaker configuration. The GFCI device is illustrated as being equipped to break both sides of the control power circuit. To break both sides of the control power circuit using a circuit breaker configuration, a conventional single pole GFCI circuit breaker is equipped with one or more additional breaker poles which may or may not have overcurrent tripping capability. So-called "switching neutral " GFCI circuit breakers now being offered by our Circuit Protective Devices Department, Plainville, Connecticut, provide the capability of breaking both sides of the control power circuit.

To sense a ground fault on the distribution circuit downstream from circuit breaker 16, a ground fault sensor, generally indicated at 38 and typically a zero sequence transformer, is utilized. Thus, as illustrated in FIGURE 1, sensor 38 comprises a toroidal core 40 which embraces the three line conductors 12 of the distribution circuit. As long as the vectorial sum of the currents flowing in the three line conductors 12 equals zero, the net flux induced in core 40 is also zero and no voltage is induced on its multi-turn secondary winding 42, which is connected across either primary winding (winding 30b in the illustrated embodiment) of GFCI device 26. It is seen that in the absence of an induced voltage in sensor secondary winding 42, the GFCI differential current transformer remains balanced.On the other hand, if a ground fault should exist on the distribution circuit, such as illustrated at 44, the vectorial sum of the currents in line conductors 12 no longer equals zero, and a voltage is induced in secondary winding 42. This induced voltage, as impressed across primary winding 30b, causes additional current to flow therethrough, and the GFCI different transformer becomes unbalanced, resulting in the opening of the GFCI device contacts 36. While the distribution circuit is illustrated as simply a three phase, three wire circuit, it will be appreciated that it may also in clude a fourth, neutral wire, in which case all four wires would be embraced by core 40.Moreover, the sensor 38 may be constituted by separate current transformers inductively coupled with each distribution circuit wire and with their secondary windings connected in an appropriate parallel fashion to sense ground fault currents.

To translate the tripping of GFCI device 26 in response to a ground fault on the distribution circuit into tripping of circuit breaker 16 to clear the ground fault, the GFCI device is provided with a switch 50. This switch may be in the form of an auxiliary switch mechanically coupled with the GFCI circuit breaker mechanism and operated thereby to assume a normally open condition while the GFCI breaker contacts are closed and to assume a closed condition whenever the GFCI breaker contacts are opened. Preferably however, switch 50 is of the so-called "trip or bell alarm " type physically adapted to a GFCI circuit breaker in the same manner as currently being adapted to conventional residential-type circuit breakers.A bell alarm switch is mechanically adapted to a circuit breaker so as to be insensitive to manual opening of the circuit breaker, but is actuated to its closed condition in response to tripping of the circuit breaker.

In this connection, reference is made to the disclosure in U.S. Patent No. 3,256,407.

Switch 50 may have its contacts positioned for closure in response to engagement by a movable contact carrier as the carrier springs to its open circuit position incident to a ground fault trip function. In either case, switch 50 is mechanically coupled to the GFCI device contacts 36 such that the switch is left open as long as the device contacts are closed. When the device contacts are tripped open, the switch is actuated to its closed condition. Switch 50 is wired into the control power circuit at the line side of the GFCI device, but could be fed from a source separate from source 24. It is thus seen that until the GFCI device is tripped, the switch is open and thus energization of the shunt trip solenoid from source 26 is inhibited.However, when the GFCI device trips in response to a ground fault on the distribution circuit, switch 50 closes to effect energization of the shunt trip solenoid and tripping of circuit breaker 16.

From the foregoing description of the disclosed embodiment of the invention, it is seen that by using the combination of three low-cost components, namely, a residential-type GFCI device equipped with a trip alarm or auxiliary switch, a shunt trip solenoid and a ground fault sensor, to control a conventional industrial circuit breaker, economical ground fault protection is afforded to a high current power distribution circuit.

The control power circuit may be utilized to power an electrical load 52 connected downstream from the GFCI device. This load may include indicator light, alarms, and other devices as may be incorporated in typical industrial electrical power delivery installations. Since load 52 is fed via the GFCI device, it, as well as the control power circuit downstream from the GFCI device, is afforded ground fault protection. This is a significant feature as well from a personnel safety standpoint. If the GFCI device is in a circuit breaker configuration, then load 52 is also afforded overcurrent protection.

It will be appreciated that control power source 24 may be in the form of a 120/240 or 120/208 AC volts supply feeding two line conductors and a grounded neutral conductor. In this case, the GFCI device 26 would be provided by a two pole GFCI circuit breaker, such as is offered by our Circuit Protective Devices Department, Plainville, Connecticut. This can be desirable from the standpoint that load 52 may require a 240 volt supply or be constituted by a variety of 120 and 240 volt load. In practice, it would be preferable to connect secondary winding 42 of sensor 38 directly across one of the primary windings 30a, 30b, rather than across the one primary winding and one set of GFCI contacts 36 as shown, so as to avoid opencircuiting the sensor secondary winding.

Also to be noted is the fact that the GFCI device contacts 36 are not electrically involved in the shunt trip solenoid energization circuit and need not be directly mechanically involved with switch 50, as in the case of a bell alarm switch. Consequently, if a particular installation does not call for a load 52 or that such load and its associated circuit does not require ground fault protection, then the GFCI device may be devoid of contacts 36. In this case, the shunt trip solendid energization circuit including switch 50 could be connected to the load side of the GFCI device.

WHAT WE CLAIM IS: - 1. A system for protecting an electrical power distribution circuit against ground faults by energizing a solenoid to trip a circuit breaker connected in said circuit, said system comprising: an energization circuit for a said solenoid; a normally open switch connected in said energization circuit; a power supply circuit; a different current transformer comprising first and second primary windings connected to one each in series with each side of said power supply circuit, and a secondary winding for development of a signal indicative of an imbalance in currents flowing in said primary windings; means responsive to said signal being of a predetermined magnitude for a predetermined duration to effect closure of of said normally open switch; and a ground fault sensor for coupling to a said distribution circuit and having output terminals connected across one of said primary windings; whereby in use a ground fault on a said distribution circuit causes a-current signal to be developed by said sensor, said current signal flowing in said one primary winding to create a said imbalance in said transformer primary windings so that said responsive means effects closure of said switch, and said switch closure causes energization of said solenoid by said energization circuit to trip said circuit breaker.

2. A system according to claim 1 wherein said solenoid energization circuits connected to said power supply circuit in parallel with said primary windings.

3. A system according to claim 1 or claim 2 including circuit interrupting means having contacts connected in at least one side of said power supply circuit, said responsive means responding to said imbalance signal being of said predetermined magnitude for said predetermined duration also to effect operaion of said circuit interrupting means to open said contacts.

4. A system according to claim 3 wherein said power supply circuit includes a line side and a grounded neutral side, said contact being connected at least in said line side.

5. A system according to claim 4 wherein a pair of said contacts are connected in each of said line and grounded neutral sides of said power supply circuit, said contact pairs being mechanically coupled together to open and close in concert.

6. A system according to any one of claims 3 to 5 including means for connecting an electrical load across said power supply circuit downstream from said primary windings and from said contacts, whereby said responsive means serves also to respond to a said imbalance signal developed as a result of a ground fault on said power supply circuit downstream from said primary windings to effect operation of said circuit interrupting means to open said contacts.

7. A system according to any one of claims 1 to 6 wherein said normally open switch is a mechanical switch mechanically coupled to said responsive means.

8. A system for protecting an electrical power distribution circuit against ground faults by energizing a solenoid to trip a circuit breaker connected in said circuit, said system being substantially as described herein with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. ground fault protection, then the GFCI device may be devoid of contacts 36. In this case, the shunt trip solendid energization circuit including switch 50 could be connected to the load side of the GFCI device. WHAT WE CLAIM IS: -

1. A system for protecting an electrical power distribution circuit against ground faults by energizing a solenoid to trip a circuit breaker connected in said circuit, said system comprising: an energization circuit for a said solenoid; a normally open switch connected in said energization circuit; a power supply circuit; a different current transformer comprising first and second primary windings connected to one each in series with each side of said power supply circuit, and a secondary winding for development of a signal indicative of an imbalance in currents flowing in said primary windings; means responsive to said signal being of a predetermined magnitude for a predetermined duration to effect closure of of said normally open switch; and a ground fault sensor for coupling to a said distribution circuit and having output terminals connected across one of said primary windings; whereby in use a ground fault on a said distribution circuit causes a-current signal to be developed by said sensor, said current signal flowing in said one primary winding to create a said imbalance in said transformer primary windings so that said responsive means effects closure of said switch, and said switch closure causes energization of said solenoid by said energization circuit to trip said circuit breaker.