GB1581364A - Circuit for providing immunity from noise and nuisance tripping ground fault detection devices - Google Patents

Description

Application No 38731/79

Divided out of No 1581363 Convention Application No 674714 ( 22) Filed 1 Apr 1977 ( 32) Filed 8 Apr.

( 33) United States of America (US) ( 44) Complete Specification Published 10 Dec 1980 ( 51) INT CL 3 H)02 H 3/05 ( 52) Index at Acceptance H 2 K 208 301 390 463 623 634 771 JK ( 54) CIRCUIT FOR PROVIDING IMMUNITY FROM NOISE AND NUISANCE TRIPPING GROUND FAULT DETECTION DEVICES ( 71) We, SQUARE D COMPANY, of Executive Plaza, Park Ridge, Illinois 60068, United States of America, a corporation organized and existing under the laws of the State of Michigan, 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:

This invention relates to circuit for use in a ground fault detection devices which provides immunity from noise and nuisance tripping This invention has application in the field of ground fault protective devices sensitive enough to protect humans from the hazards of electric shock resulting from ground fault currents In the typical alternating current installation, the neutral conductor is grounded at its source If electrical contact is made with the electrified or hot" conductor by a person standing on the ground, part of the current is diverted from its normal circuit through the electrified and neutral conductor and follows a ground path circuit through the person back to the neutral grounded at the source To protect against such hazard, ground fault interrupting devices have been provided for installation in circuit breakers of branch circuits, and in portable units for temporary field use on constructions sites Attempts have also been made to mount such devices in ordinary household outlet boxes where ground fault protection is most needed, such as bathrooms, kitchens and garages By having the entire ground fault sensing and interrupting mechanism and noise immunity circuit, in the outlet boxes where most needed, the entire branch circuit is not interrupted every time the ground fault mechanism interrupts the circuit.

A disadvantage of the receptacle mounted ground fault interrupting devices which includes noise immunity circuits hitherto known is their size, usually requiring oversize outlet boxes The noise immunity circuit in accordance with this invention is compact enough to be installed in the same outlet boxes as conventional devices without ground fault protection which the detection devices in accordance with this invention replace It is not necessary to remove the original outlet boxes and install new ones of larger volume in bathrooms, kitchens and garages in order to obtain ground fault protection wherever desired by using the new circuits disclosed and described herein.

It is an object of the invention to provide a ground fault detecting devices having miniaturized components and circuitry, in which a rectification bridge is provided having avalanche diodes with a reverse voltage breakdown characteristic selected to prevent the d c voltage appearing across said bridge from exceeding such a preselected value, thus eliminating the need for a separate component to protect from nuisance tripping and damage due to high voltage conditions.

It is an object of the invention to provide a ground fault detecting devices having miniaturized components and circuitry in which a ground fault sensing and interrupting circuit is powered on the line-side of the circuit interrupting contacts, and in which electronic switch means for energizing a trip coil to open said interrupting contacts in connected to commutate off automatically after the ground fault has been cleared, thereby eliminating the need for a separate switch to open the line-side connected coil circuit.

According to the present invention there is provided a circuit for providing immunity from noise and nuisance tripping in a ground fault protection device connected to an electrical distribution system having a plurality of conductors comprising:

PATENT SPECIFICATION " ( 21) Z ( 62) ( 31) ( 11) 1 581 364 ( 19) M 6 in 1 581 364 a) a rectified power supply means connected between two conductors for receiving input power from the distribution system conductors and delivering operating power to the protective device; b) means responsive to a ground fault for producing a fault signal; c) means for tripping; d) switching means respective to the fault signal and inadvertently responsive to a first level of noise voltage for energizing the tripping means; e) means associated with the rectified power supply means and electrically connected with the switching means for clipping the noise voltage when the voltage exceeds a selected second level which is less than the first level so that the switching means does not respond to the noise voltage and inadvertently energize the tripping means; and f) a selected impedance means connected between one of the conductors and the rectified power supply means for limiting current to the rectified power supply means when the noise voltage exceeds the selected second level.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawing which is a circuit diagram of the ground fault detecting device.

Referring now to the Figure the ground fault sensing and grounded neutral detecting components and the tripping mechanism are mounted on the inward facing side a printed circuit board (not shown) These include two differential transformers T-1 and T-2 comprising a ground fault sensing toroid 40 and a coupling toroid 41 respectively, through which a "hot" or electrified conductor L 1 and a neutral conductor NI extend to constitute the primary windings thereof.

The electrified conductor L 1 is connected to line-side terminal 35 a on one side of toroids and 41, and after passing through said toroids, it is connected to load side terminal b The neutral conductor NI is connected to line-side terminal 36 a on one side of toroids 40 and 41, and after passing through said toroids, it is connected to a load side terminal 36 b.

The differential transformer T-1 which includes toroid 40 functions as a so-called zero sequence transformer to sense the occurrence of a ground fault on the load side of the electrified conductor Ll When no ground fault is present the magnetic fields resulting from current flow in electrified conductor Ll in one direction and in neutral conductor NI in the opposite direction are of opposite polarity and equal The magnetic fields thus cancel out However, when a ground fault occurs in the electrified conductor Ll on the load side of toroid 40, a portion of the current returns to source through a ground path rather than through neutral conductor N 1 Thus, the respective magnetic fields of electrified conductor L 1 and neutral conductor Ni are unbalanced as they pass through toroid 40 where they constitute the primary winding of this differential transformer Accordingly the magnetic fields do not cancel out, and a net amount of magnetic flux is available to be picked up in secondary winding 44 on toroid and thus induce a voltage signal therein.

The detection and interruption circuit is powered as follows A full wave bridge 45 having avalanche characteristics is connected across line conductor L 1 and neutral conductor N 1 on the line or source side of interrupting contacts 103 and 104, by means of conductor 48 connected to one side of the bridge and conductor 49 connected to the other side and extending to terminal 50 of trip coil 51 Conductor 52 extends from trip coil terminal 53 to neutral conductor Ni.

Bridge 45 provides a rectified power supply to an integrated circuit components 54 through conductors 55 and 56 connected to pins 57 and 58 respectively of I C components 54.

The I C component 54 includes therein an operational amplifier, voltage and regulator and level detector The pins designated 57 and 58 represent the voltage regulator portion, pins designated 59, 60, 61 62 and 63 represent the operational amplifier portion, and pin 64 represents the level detector portion.

As stated above, a rectified power supply is fed from bridge 45 to I C 54 and connected to pins 57 and 58 which represent the voltage regulator portion of the I C.

chip 54 and which sets the appropriate voltage level for the operational amplifier portion of the chip.

When a ground fault occurs in line conductor LI, on the load side of the toroid 40, part of the current returns to source through a ground path rather than through the neutral conductor NI, creating an imbalance in the respective magnetic fields of L 1 and NI where they pass through toroid 40.

As described above, a net amount of magnetic flux is thus available to induce a voltage signal in secondary winding 44 This voltage signal is transmitted to the operational amplifier stage of I C 54 by way of an input circuit comprising conductor 65 leading to pin 59 (an inverting input terminal of the operational amplifier stage), and conductor 66 leading to pin 60 (a non-inverting input terminal of the operational amplifier stage) Conductor 67 leads from terminal 68 of secondary winding 44 to the junction 69 with conductor 65 Conductor 70 leads from the other terminal 71 of secondary 44 to the junction 72 with conductor 66.

1 581 364 Diodes 73 and 74 are connected in parallel with secondary winding 44 to prevent saturation of the transformer toroid core 40 during very high values of ground fault current.

When the induced voltage signal transmitted from secondary winding 44 is received on pins 59 and 60 of the op amp stage of I C.

54, it is transmitted to the output pin 61, from which it flows through a negative feed back path, comprising conductor 75 to junction 76 and then to an inverting input 62 through conductor 77 ahnd a one megohm potantiometer 78 This negative feedback path controls the gain of the amplifier stage, and the potentiometer may be adjusted to control the ground fault trip current response For example, it may be adjusted so when there is 5 milliamps different between the currents in line conductor L 1 and neutral conductor N 1, the amplifier output peak voltage exceeds the reference voltage of the level detector stage, supplied by the voltage regulator stage which in turn receives a d c voltage supply on pins 57 and 58 from bridge 45 through a voltage dropping resistor 79 When the output peak voltage of the amplifier stage exceeds the reference voltage, a d c voltage is produced at pin 64 of the level detector stage which triggers silicon controlled rectifier (SCR) 80 into conduction.

Capacitor 81 is connected across input terminals 62 and 63 of the op amp stage of I C 54 to provide a low pass filter for additional immunity to spurious noise signals.

The d c voltage from pin 64 of I C 54 is fed to the gate of SCR 80 through conductor 82 Capacitor 83 is connected across the cathode-gate circuit of SCR 8 O to prevent it from triggering and tripping the circuit due to noise on the circuit which could be amplified by I C 54.

When SCR 80 is triggered into conduction line voltage is applied to trip coil 51 causing it to trip thus opening contacts 103 and 104 to interrupt the power line circuit When SCR 80 conducts, a circuit is completed from Ni through conductor 52, terminals 50 and 53 of trip coil 51, conductors 49 and 84 to SCR 80, conductors 85 and 56, sener diode 86 and conductor 48 to Li.

The full wave rectifier bridge 45 includes zener diodes 86, 87, 88 and 89 The diodes are selected to avalanche with a reverse voltage between 200 and 300 volts peak If a voltage transient in excess of 300 volts peak occurs between conductors L 1 and NI of the power circuit, the avalanche or zener diodes 86-89 will avalanche and clip the voltage at a safe amplitude to protect SCR 80 nd I C 54 from damage The impedance of trip coil 51 acts as a choke to limit current sufficiently on occurrence of such high transients to protect the diodes 86-89 from being damaged By using a rectification bridge of this type with avalanche or zener diodes, an additional component such as a metal oxide varister (MOV) used in other ground fault protector circuits is not needed for impulse protection The avalanche rectification bridge performs the dual functions of recification and protection from high voltage transients.

The ground fault protection circuit is powered from the line side of the interrupting contacts 103 and 104 In this way, the ground fault protection circuit remains powered even after the power circuit has been interrupted by opening of contacts 103 and 104 In other devices of this type which power the ground fault protective circuit from the line side, a separate switch is used to de-energize the trip coil after tripping for a ground fault In the present invention, a separate switch is not needed for this purpose The problem of de-energizing the trip coil after tripping for a ground fault is solved by connecting the anode end 90 of SCR 80 to the a c side of bridge 45 rather than to the d.c side as is customary in prior art devices.

The anode end 90 is connected to the a c.

power line circuit through conductors 84, 49, coil 51 and conductor 52 to the neutral conductor N 1 This means that the SCR 80 operates in the half wave mode and commutates off once every cycle Therefore, when the fault is removed by opening of the contacts at the next line voltage excursion through zero, the SCR 80 turns off.

A test circuit is provided which includes a kilohm resistor 91 in series with test switch 92 in conductor 93 extending from junction 94 with line conductor L 1 on the load side of the differential transformer toroid 40 to junction 95 with conductor 49 leading to neutral conductor Ni on the line side of toroid 40 Thus, when test switch 92 is closed, current imbalance occurs at toroid as a result of current bypassing the neutral conductor at that point returning to neutral from junction 94 through the test circuit If the ground fault protection circuit is working properly, such current imbalance will induce a pick-up signal which will be amplified and trip the circuit in the manner described.

The ground fault protection circuit also includes protection against a ground on the neutral conductor which if not detected and cleared would adversely affect the sensitivity of the circuit Protection against a grounded neutral is provided as follows.

A coupling transformer T-2 including toroid 41 having a winding 96 is connected to the output 61 of the oprational amplifier stage of I C 54, by means of a feedback circuit Conductor 97 extends from terminal 98 of the winding 96 to junction 76 to 1 581 364 receive an output from terminal 61 of the op amp A capacitor 99 and resistor 100 are connected in series in conductor 97 to complete a regenerative feedback path from the output of the operational amplifier stage of I C component 54 to the transformer winding 96 on toroid 41 of transformer T-2.

The other terminal 101 of winding 96 is connected through conductor 101 a to terminal 71 of the secondary winding 44 on toroid 40 of the differential transformer T-1.

Transformer T-2 and the circuit in which it is connected are quiescent when conditions in the power line circuit are normal and no ground is present on conductor N 1 on the load side of transformer T-2 However, if the neutral wire N-i is grounded on the load side of toroids 40 and 41 through an impedance of 4 ohms or less, a feedback circuit then exists through the one turn loop created by neutral wire N-1 passing through both toroids 40 and 41, which thereby magnetically couples the transformers T-1, and T-2 This feedback loop causes the operational amplifier stage of the I C to oscillate Such oscillation is detected by the internal level detector stage of the I C in the same manner as a signal voltage resulting from occurrence of a ground fault An output voltage thereupon appears on pin 5 of the level detector stage of the I C, which gates the SCR 80 into conduction thereby causing trip coil 51 to open contacts 46 and 47, thus interrupting the circuit.

The circuit and components described above, will therefore interrupt the power line circuit both on occurrence of a ground fault on the load side of the toroids 40 and 41 and on occurrence of a grounded neutral on the load side of toroids 40 and 41 Reference is made to our co-pending Patent Application No 13768/77 (Serial No.

1581363) in which claims are directed to a ground fault disconnect assembly.

Claims (4)

WHAT WE CLAIM IS:

1 A circuit for providing immunity from noise and nuisance tripping in a ground fault protective device connected in an electrical distribution system having a plurality of conductors comprising:

a) a rectified power supply means connected between two conductors for receiving input power from the distribution system conductors and delivering operating power to the protective device; b) means responsive to a ground fault for producing a fault signal; c) means for tripping; d) switching means responsive to the fault signal and inadvertently responsive to a first level of noise voltage for energizing the tripping means; e) means associated with the rectified power supply means and electrically connected with the switching means for clipping the noise voltage when the voltage exceeds a selected second level which is less than the first level so that the switching means does not respond to the noise voltage and inadvertently energise the tripping means; and f) a selected impedance means connected between one of the conductors and the rectified power supply means for limiting current to the rectified power supply means when the noise voltage exceeds the selected second level.

2 A circuit as claimed in claim 1 wherein the means for tripping includes the selected impedance means.

3 A circuit as claimed in Claim 1 wherein the switching means comprises SCR electrically connected to the fault means at the gate for receiving, the fault signal and an anode and cathode, said switching means being inadvertently responsive to the first level of noise voltage applied across the anode and cathode.

4 A circuit, as claimed in Claim 1 wherein the means for clipping the noise comprises a full wave rectifier bridge including avalanche diodes selected to reverse when the noise voltage exceeds the second level which is between 200 and 300 volts.

A circuit substantially as hereinbefore described with reference to the accompanying drawing.

FITZPATRICKS, (Chartered Patent Agents), 14/18 Cadogan Street, Glasgow G 2 6 QW.

and Warwick House, Warwick Court, London WC 1 R 5 DJ.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited Croydon, Surrey, 1980.

Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A i AY, from which copies may be obtained.