GB2181599A

GB2181599A - Residual current circuit breaker

Info

Publication number: GB2181599A
Application number: GB08624002A
Authority: GB
Inventors: Douglas John Akehurst
Original assignee: Duraplug Electricals Ltd
Current assignee: Duraplug Electricals Ltd
Priority date: 1985-10-07
Filing date: 1986-10-07
Publication date: 1987-04-23
Also published as: DE3633787C2; GB2181599B; IT8621927A0; GB8624002D0; ES2002795A6; IT8621927A1; IT1197365B; NL8602442A; DE3633787A1; FR2588415B1; FR2588415A1; GB8524705D0

Abstract

In a residual current circuit breaker having a tripping solenoid (11) for opening switch contacts (RC1, RC2) to interrupt current flow to a load (10) in response to a load current imbalance being detected (4,3) a Triac (54) is included to interrupt the current flow more rapidly than the time taken to open the switch contacts, whereby enhanced safety is achieved. <IMAGE>

Description

SPECIFICATION Residual current circuit breaker This invention relates to residual current circuit breaker, also known as earth leakage circuit breakers.

Residual current circuit breakers generally include a solenoid switch for disconnecting a load from an electrical supplywhen a fault, i.e. an earth leakage condition, is detected. The advantage of a solenoid switch is that it ensures complete electrical isolation ofthe load from the supply by virtue ofthe gap opened up between the switch contacts, which means greater safety. Typically, the minimum time taken for the mechanically actuated switch contacts to open is around 0.04 seconds. Although the operation time is short, a significant amount of electrical energy can flow in this period.Depending on the current flowing the dangerto person offibrillation is determined by the time ittakesto isolate the power, and if itshould happen that the line conductor is shorted directly to earth sufficient current can pass for a 13 amp fuse to blow before the switch contacts are opened.

The present invention aims at a solution to this problem and in accordance with the invention there is provided a residual current circuit breakercom- prising a pairofconductorsforconnecting a load to an electric power supply, meansforsensingwhen thecurrentsflowing in the conductors differ by a predetermined amount, a mechanical switch device having contacts capable of being opened apartto interruptthe currentflowto the load, means coupled to the sensing means for energising a solenoid to open said switch contacts when said predetermined difference in currents is sensed, and a solid state switching device having a control input coupled to the sensing means for said device to interruptthe flow of electric current to the load more rapidly than the timetaken forthe solenoid to open the switch contact when said predetermined difference in currents is sensed.

The solid state switching device may comprise a thyristor and most conveniently and preferably is a Triac. If the A.C. power supply has a mains frequency of 50 c.p.s., the Triac will operate to turn the current off within 0.01 seconds, i.e. not more than one half cycle of the supply. This time is substantially less than the minimum of 0.04 seconds taken forthe switch contacts to open.

Afull understanding ofthe invention will be had from the following detailed description in which reference is made to the accompanying drawing which is a schematic representation of a residual current circuit breaker embodying the invention.

The residual current circuit breaker includes a sensor device comprising a magneticcore having the form of a toroid except for a single narrowair gap. Located in the air gap, preferably with a sliding fit orwith a small clearance between the opposed pole pieces ofthe core is a Linear Hall Effect integrated circuit element 2. The element 2 has such a size thatsubstantially all the magnetic flux passing between the pole pieces is transmitted through the element.Two conductor coils 3, 4arewound with an equal number of turns on opposite sides of the core land in use the conductors are connected so that the currents they carry induce oppositely directed magnetic fluxes through the closed magnetic circuit defined by the core 1 and intersected by the Hall element 2.Thus, when the currents flowing in the respective conductors are equal there is no resultant flux in the magnetic circuit and no output signal is produced bytheHallelement2. lfthe currents car- ried bytheconductorcoils differ, however, a mag- neticflux is induced in the core 1 and will have an intensity dependent upon the difference between the electric currents, and the Hall element 2 will emit a signal which is proportional to the resultant mag netic flux. To retain the components of the sensor device in current position it may be embedded in a suitable encapsulation material, and for magnetic screening purposes it may be encased within a layer of ferrous material.

The sensor device is connected in the electric circuit of a residual current circuit breaker which controlsthe supply of electric power to a load orappliance 10 (shown in dotted line) from an electric power source, such as the mains AC supply. The circuit breaker includes a switching device constituted bya tripping solenoid 11 and a double pole switch having two sets of contacts RC1 and RC2 through which the conductors 3,4 are respectively connected to there minal connections 12, for connection to the Line and Neutral sides of the power supply.The switch is operable manuallyto open and closethe contacts RC1, and RC2, and the solenoid is arranged totripthe switch contacts from the closed condition to the open condition when the solenoid is energised. The load 10 is connected between output terminal connections 14,15 at the other ends of the conductors 3, 4. The outputterminal of the Hall Effectelement2is connected to the inputofa signal processor l6which is described in detail below.

To provide a DC power supply forthe Hall Effect elementand the signal processor 16, a resistor 17 and a rectifier bridge 18 are connected in seriesac- ross the power input terminals 12, through conductor lines 19,20. The DC output voltage from the bridge 18 is supplied to a pair of DC lines 21,22 and is stabilised e.g. at 5 volts, by a Zener diode 23 and capacitor 24 connected in parallel between the lines 21, 22. The stabilised DC voltage isfed to the Hall Effect element2, and to the signal processor(bycon- nections notshown)through the lines 21,22.

The signal processor includes an AC coupling 25, AC amplifier 26, precision rectifier 28, a comparator 29 and an optical isolator 30 arranged in sequence.

The AC coupling 25, conveniently a simple capacitor, filters any DC component from the signal emitted by the Hall Effect element 2 before transmitting the signal to the input side of the AC amplifier 26. The amplified signal from the output amplifier 26 is supplied to the input of the precision rectifier 28 which converts it into a DC voltage signal. Thus, the signal emitted from the output of the rectifier is proportional totheAC signal from the Hall element 2, and hence to the flux in the magnetic core 1 and thereby to the difference in the electric currents flowing in the supply and return conductors 3,4. The comparator 29 compares the output signal from the rectifier with a reference voltage conveniently obtained by means of a voltage divider connected across the DC lines 21, 22.As shown the voltage divider takes the form of a potentiometer 31 having its wiper contact connected to the comparator, but it could instead be provided by a pair of resistors. The reference voltage is set at such a ievel, e.g. about3volts,that it is normally greaterthan the signal from the rectifier 28, but the latter signal exceeds the reference voltage when a predetermined imbalance occurs between the currents in the conductors 3,4. The comparator com paresthetwo inputsignalsfedto it and emits an output only when the signal from rectifier 28 is greaterthan the reference voltage. The output signal from the comparator 29 is supplied to the input of a discriminator device 33.

The discriminator 33 has two outputs 34,35 the first one of which is connected through a resistor 50 to the control in put of a Triac 51 connected in series with the energising coil ofthetripping solenoid 11 across the main current conductors 3,4. The second output of the discriminator 33 is fed through a resistor 53 to the control input of a Triac 54 connected directly in the line conductor 3.

The signals from the two outputs of the discrimina tor 33 are 1800 out of phase. In other words, there is no signal atthefirstoutput 34 when a signal is emitted from the second output 35, and vice versa. Under normal conditions, i,e, when the same currentflows in conductors 3 and 4, there is no signal from comparator29 andthe discriminator33 produces a signal at the second output 35 that the Triac 54 remains in a conducting state for mains current to be supplied to the load 10. There is no signal atthe first output 34 so Triac 51 is non-conducting and the solenoid 11 is not energised. In this normal condition of the solenoid 1 1,the contact sets RC1, RC2 can be closedforthe load 10 to be connected to the powersupply.The Triac 51 is operable to energise the solenoid to open the contact sets RC1, RC2 and thereby disconnectthe load l0fromthepowersupply.

In response to a fault condition, e.g. a difference in current levels of 1 Oma in the conductors 3,4, the signal from the Hall element 2 increases to such a level that the voltage signal from the rectifier 28 exceeds the reference voltage and an output is emitted by the comparator 29. Consequently, the second ouput350fthe discriminator33 isterminated and simultaneously a signal is emitted atthe first output 34.TheTriac54 responds by becoming nonconductive to interrupttheflow of mains current to the load 10,this interruption taking place rapidly within one half cycle of the A.C. supply, i.e. 0.01 seconds for a 50 c.p.s. mains supply.Thus, the current passing to the load 10 10 is cut off before the contact sets RC1, RC2, actuated bythe solenoid 11 upon the triggering ofthe Triac 51, have been opened to break the connection between the load 10 and the mains supply, it tasking a minimum 0.04 seconds for switch contacts to be opened.

When the contacts RC1, RC2 have been opened the outputs from the discriminator 33 revert to the normal condition and, assuming thatthe fault re sponsibleforthe earth leakage current has been re ctified,the double pole switch can be reset manually to close the contacts and re-establish the electric cur- rent paths between the load and the power source.

The more rapid interruption ofthecurrent achieved through the inclusion of the Triac 54 has the following advantages: a)therewill be much less shock sensationtothe nervous system if a person is subjected to the earth fault leakage current because of the shorter time to effect isolation; b) much less electric power will pass and, in particular, even if the line conductor is shorted directly to earth the powerwill not be enough two blow a 13 amp fuse; and c) a residual current circuit breaker adapted to re spondto an earth leakage currentofsay lOmawill operate to cut off the power supply before afurther circuit breaker located upstream and adapted to respond to a higher leakage current, e.g. 30ma, will be operated.

While the invention has been described with a Triacto ensure rapid current interruption twill be understood that alternative thyristor devices could be used, such as two SCRs connected back-to-back.

TheACcoupling 25 included atthe input ofthe signal processor acts as a safety device by making the circuit breaker impossible to over-ride by positioning a permanent magnet in the vicinity ofthe sensor.

In order to enable the circuit breaker to be tested an auxiliary load 36 may be connected in series with a push button switch 38 between the conductors 3, 4to by-pass the coiled portion of one conductor 3. Upon pressing the button to close the switch 38 different current levels will be established in the coiled portions of conductors 3 and 4 and, provided the circuit breaker is functioning correctly, the solenoid 11 will be energised to open the contacts RC1, RC2. After releasing the button to reclose the switch 38 the double pole switch can be operated manually to reclose the contacts RC1, RC2 and thus complete the test procedure.

Claims

1. A residual current circuit breaker comprising a pair of conductors for connecting a load to an electric power supply, means for sensing when the currents flowing in the conductors differ by a predetermined amount, a mechanical switch device having contacts capable of being opened apartto interruptthe current flow to the load, means coupled to the sensing means for energising a solenoid to open said switch contacts when said predetermined difference in cuit rents is sensed, and a solid state switching device having a control input coupled to the sensing means for said device to interruptthe flow of electric current to the load more rapidlythanthetimetakenforthe solenoid to open the switch contacts when said predetermined difference in currents is sensed.

2. A residual current circuit breaker according to claim 1 ,wherein the solid state switching device comprises a Triac.

3. A residual current circuit breaker according to claim 1 or 2, wherein said means for energising the solenoid comprises a solid state device having a con trol input coupled to the sensing means.

4. Aresidual currentcircuit breakeraccordingto claim 1 or 2, wherein the means for energising the solenoid comprises a Triac connected in series with the energising coil of the solenoid between the two conductors.

5. A residual current circuit breaker according to claim 4 and claim 2, wherein the sensing means includes a signal emitting means for producing an output signal when said predetermined difference in currents is sensed, and a discriminator device having an input connected to said signal emitting means and having two outputs respectively connected to the control inputs of the Triacs, said signals produced at the two outputs being 180 out of phase and said discriminator reversing said signals in response to an output signal from the signal emitting means.

6. A residual current circuit breaker according to any one ofthe preceding claims, wherein the sensing meanscomprises a sensordeviceforproducing a signal dependent upon the difference in currents flowing in the conductors, and signal processing means including means for providing a voltage signal proportional to the signal from the sensor de vice, and meansforcomparing saidvoltagesignal with a constant reference signal and for emitting an output signal when said voltage signal exceeds said constant signal.

7. A residual current circuit breaker according to claim 6, wherein means are connected across said conductors for producing a stabilised DC power supplyforsaid sensor device and signal processing means, and said reference voltage is derived from a voltage divider connected across the DC supply.

8. A residual current circuit breaker according to any one of the preceding claims, wherein the sensing means comprises a sensor device having a core member defining a substantially closed magnetic circuit having an air gap, and a Hall Effect element located in the air gap,thetwo conductors having coiled portions wound upon the core member.

9. A residual current circuit breaker according to claim 8, wherein an AC coupling is provided to remove any DC component in the output signal from the Hall Effect element.

10. A residual current circuit breaker su bsta nti- ally as herein described with reference to the accompanying drawings.