GB1585826A

GB1585826A - Circuit breakers

Info

Publication number: GB1585826A
Application number: GB2324777A
Authority: GB
Original assignee: B & R Relays Ltd
Current assignee: B & R Relays Ltd
Priority date: 1978-05-15
Filing date: 1978-05-15
Publication date: 1981-03-11

Description

(54) IMPROVEMENTS IN AND RELATING TO CIRCUIT BREAKERS (71) We, B & R RELAYS LIMITED, a British Company, of Temple Fields, Harlow, Essex, CM20 2BG, 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 improvements in and relating to circuit breakers and particularly concerns current operated earth leakage circuit breakers for domestic or industrial use.

An earth leakage circuit breaker is known from our British Patent Specification No.

1,271,373, which operates by detecting an unbalance between currents supplied to and returned from a load on live and neutral lines of an a.c. supply. A switching means is provided to switch off the current supplied to the load when such an unbalance is detected.

Whilst generally satisfactory, this known circuit breaker suffers from the disadvantage that it needs to be tested periodically to make sure that it will operate correctly in the event of an earth leakage of the supply to the load.

With a view to mitigating this disadvantage, the present invention provides a circuit breaker for connection between an alternating current supply and a load, comprising means for detecting an unbalance in the current flowing in supply lines of an alternating current supply to a load; switching means for disconnecting the supply from the load in response to detection of the said unbalance by the detecting means; and testing means comprising means for introducing an unbalance in the current flowing in said lines, and means adapted to enable current to be supplied through the circuit breaker to the load only after operation of the detecting means and said switching means in response to the said introduced current unbalance.

Preferably, the circuit breaker includes a manually operable means such as a push button for setting the circuit breaker to a condition to conduct current to the load, and the testing means is arranged to operate in response to operation of the manually operable means, in which case operation of the circuit breaker will be tested automatically before it is brought into use.

In one particular form of the invention, the detecting means comprises a differential transformer having primary windings for connection in the live and neutral lines respectively, and a secondary winding arranged to detect any difference between the flux developed by said primary windings.

For use with an alternating current supply having live and neutral supply lines connected to the load, the switching means conveniently comprises a relay having switching contacts for making and breaking an electrical connection in the live line. The testing means in one form comprises a test relay having switching contacts for making and breaking an electrical connection in the live line, means connected to the contacts and arranged to inject a test current through said detecting means upon operation of the test relay to disconnect the live line from the load, the test relay being arranged to connect the live line to the load upon operation of the detecting means and the switching means in response to the test current.

In order that the invention may be more fully understood and readily carried into effect an embodiment thereof will now be described by way of illustrative example with reference to the single Figure of the accompanying drawing.

The circuit breaker is connected to live and neutral lines L and N of an a.c. supply to a load. The circuit breaker comprises a toriodal differential transformer T 1 having opposed primary windings L1, L2; N1, N2 coupled respectively in the live and neutral lines L, N, the transformer also having a secondary winding arranged to detect induc tively any unbalance in the currents flowing in the primary windings.The output of the transformer's secondary winding is connected to operate a switching means in the form of a relay RL1 which has two sets of switching contacts RL1/1; RL1/2. The contacts RL1/2 make or break a connection in the live lead L and the contacts RL1/1 are used in a circuit to hold the relay on after it has been energised, as will be explained later in more detail. The relay RL1 has a centre tapped coil the terminations of which are designated S and F at the start and finish of the coil and C/T at its centre tap: the relay RLI is supplied with an energising current by means of a bridge rectifier BR1 series connected with a capacitor Cl between the live and neutral lines L, N.A capacitor C3 is connected across the output of the rectifier BR1. Passage of the energising current through the coil of the relay RL 1 is controlled by two thyristors SCR1 and SCR2, thyristor SCRI controlling the current flowing in the coil between the centre tap C/T and the start S, the thyristor SCR 2 controlling the current which flows from C/T to F in the coil. The thyristor SCR1 has its gate connected to a potentiometer chain including a potentiometer VR 1 and a resistor R such that the thyristor SCR1 will switch on automatically when the voltage developed across the capacitor C3 rises above a given threshold, for example 200 volts.The purpose of the thyristor SCRI is to make sure that the relay RL 1 is provided with an adequate supply voltage, which will be explained in more detail later.

The gate of the thyristor SCR2 is connected to the secondary of the transformer T1 such that the thyristor is fired when an unbalance in the currents flowing in the live and neutral leads occurs.

Two push switches PB1,PB2 are provided, the switch PB1 being for setting the circuit breaker for normal operation, the switch PB2 being for triggering the circuit by simulating a current unbalance across the load through a resistor R5.

Thus, in normal operation of the circuit breaker, the thyristor SCR1 will be fired, causing a current to flow from C/T to S through the coil of the relay RL 1, which actuates the relay so that the contacts RL1/2 are held closed, permitting current to flow to and from the load. However, in the event of a short circuit to earth, an unbalance in the current flowing in the live and neutral lines will occur, which causes the thyristor SCR2 to fire. Then, a current will flow through the relay coil from C/T to F which produces a magnetic field equal and opposite to that produced by the current flowing from CT to S, and thus the relay is released thereby opening the contacts RL 1/2 and preventing the supply of current to the load.

In order to make sure that the circuit breaker is in a condition to operate as described, the circuit breaker includes a testing apparatus. The testing apparatus is arranged to check, upon operation of the push button PB1, that an adequate supply voltage is delivered to the relay RL 1 by the bridge rectifier BR1; this is performed by the thyristor SCR1 which will only fire if the voltage developed across the capacitor C3 exceeds the preset threshold voltage of 200 volts. If this is the case, the testing apparatus is arranged to then introduce a current unbalance into the live and neutral lines, with the load disconnected from the supply, and check whether the relay RL 1 responds to the unbalance of current.If the relay operates in response to the unbalance, the circuit is rendered operative and connects the a.c.

supply to the load.

To this end, the circuit breaker includes a further relay RL2 which operates switching contacts RL2/1; 2/2 and 2/3. The relay RL2 is connected in series with a diode D1 and a resistor R1, between the live line L and one of the contacts of the switching contact RL 1/1. The contact RL2/1 is a self holding contact for the relay RL2, the contact RL2/2 is arranged to render both of the thyristors SCR1, 2 non-conductive temporarily as the contact moves between its two switching positions, and the contact RL2/3 is arranged in the live line L to disconnect the load from the supply and to connect the live line to a path including a diode D3 and a resistor R4.

The resistor R4 is connected to a charging capacitor C2 that is connected in parallel with the relay RL2/3. A neon tube N1 is connected between the live and neutral leads L, N at the output of the circuit breaker to indicate when electrical power is connected to the load.

The push button PB 1 is pressed to initiate operation of the circuit breaker and the testing procedure is performed whilst the push button is depressed. Successful completion of the procedure is indicated by illumination of the neon tube N1 and typically will occur in less than one second from commencement of depression of the push button.

The operation of the circuit is as follows: initially, both the relays RL 1, RL2 are unenergised. Depression of the push button PB1 completes a charging circuit for the capacitor C2 through the diode D1 and the resistor R1. Current also flows from the resistor R1 through the coil of the relay RL2 and the contacts RLl/l but the magnitude of the current is not sufficient to operate the relay RL2.Operation of the push button PB 1 also causes the bridge rectifier BR1 to deliver a charging current to the capacitor C3 and when the voltage across the capacitor exceeds the threshold voltage of 200 volts, the relay RLl will operate and connect the live lead L to the transformer T1 by means of the relay contacts RL1/2. The contacts RLl/l are also operated to disconnect the relay RL2 from the neutral line N.

A current will then flow through the primary coil L1 of the transformer, the contacts RL2/3, diode D3 and resistor R4 to produce an additional charge on the capacitor C2. The flow of this charging current produces an unbalance in the flux produced by the primary coils of the transformer T1, and assuming the relay RLl is working properly, it will respond by becoming deenergised such that the contacts RL 1/2 disconnect the live lead from the transformer, and the contacts RL 1/1 are operated to reconnect the relay RL2 to the neutral line N.

The capacitor C2 then discharges through the coil of the relay RL2, causing its energization. The relay RL2 is then held on by its contacts RL2/1, and its contacts RL2/3 are operated to a position to connect the live lead L to the load. Thus, the relay RL2 will only operate in response to a detection of the current unbalance by the relay RL 1.

Operation of the relay RL2 also causes the contacts RL2/2 to render both of the thyristors SCRI and SCR2 instantaneously nonconductive, thereby disconnecting the relay RL 1. The capacitor C3 then becomes recharged and the thyristor SCR1 is again rendered conductive to cause re-energisation of the relay RL 1 such that power from the live lead is then connected to the load through the contacts RL1/2, and the relay RL 1 is then held on by the self holding contact RLl/l.

A successful completion of the test procedure is indicated by illumination of the neon tube N 1 by the power connected to the load, and the push button PB 1 can then be released.

Thus, the circuit breaker is adapted to rest the operation of the relay RL 1 in response to a current unbalance in the transformer before power is connected through the circuit breaker to the load, the testing being performed automatically each time the power is connected to the load by depression of the set push button PB 1.

Conveniently, the circuit breaker of the invention described herein can be mounted in a domestic socket outlet as is described in our application no. 43371/76, (Serial no.

1545600) but of course can be utilised in other domestic and industrial applications.

WHAT WE CLAIM IS: 1. A circuit breaker for connection between an alternating current supply and a load, comprising means for detecting an unbalance in current flowing in supply lines of an alternating current supply to a load; switching means for disconnecting the supply from the load in response to detection of the said unbalance by the detecting means; and testing means comprising means for introducing an unbalance in the current flowing in said lines, and means adapted to enable current to be supplied through the circuit breaker to the load only after operation of said detecting means and said switch- ing means in response to the said introduced current unbalance.

2. A circuit breaker in accordance with claim 1 including manually operable means for setting the circuit breaker to a condition to conduct current to the load and wherein said testing means is arranged to operate in response to operation of said manually operable means.

3. A circuit breaker in accordance with claim 1 or 2 wherein the detecting means comprises a differential transformer having primary windings for connection in said lines respectively, and a secondary winding arranged to detect any difference between the flux developed by said primary windings.

4. A circuit breaker in accordance with any preceding claim and adapted to be connected in live and neutral lines of an alternating current supply.

5. A circuit breaker in accordance with claim 4 wherein said switching means comprises a switching relay having switching contacts for making and breaking electrical connection in the live line.

6. A circuit breaker in accordance with claim 4 or 5 wherein the testing means comprises a test relay having switching contacts for making and breaking an electrical connection in the live line, means connected to the contacts of the test relay and arranged to inject a test current through the detecting means upon operation of the test relay to disconnect the live line from the load, the test relay being arranged to connect the live line to the load upon operation of the detecting means and the switching means in response to the test current.

7. A circuit breaker in accordance with claim 6 wherein said test current injecting means includes a capacitor connected to the contacts of the test relay and for being charged from said live line.

8. A circuit breaker in accordance with claim 5, 6 or 7 including a bridge rectifier for deriving a rectified direct current from said supply lines, said switching relay having an energising coil provided with a centre tapping connected to receive said direct current from the bridge rectifier, the ends of said coils having respective thyristors connected thereto, the gate of one of the thyristors being connected to said detecting means and the gate of the other thyristor being connected to a potentiometer chain connected to receive said direct current.

9. A circuit breaker in accordance with

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Claims

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lead L to the transformer T1 by means of the relay contacts RL1/2. The contacts RLl/l are also operated to disconnect the relay RL2 from the neutral line N.

A current will then flow through the primary coil L1 of the transformer, the contacts RL2/3, diode D3 and resistor R4 to produce an additional charge on the capacitor C2. The flow of this charging current produces an unbalance in the flux produced by the primary coils of the transformer T1, and assuming the relay RLl is working properly, it will respond by becoming deenergised such that the contacts RL 1/2 disconnect the live lead from the transformer, and the contacts RL 1/1 are operated to reconnect the relay RL2 to the neutral line N.

The capacitor C2 then discharges through the coil of the relay RL2, causing its energization. The relay RL2 is then held on by its contacts RL2/1, and its contacts RL2/3 are operated to a position to connect the live lead L to the load. Thus, the relay RL2 will only operate in response to a detection of the current unbalance by the relay RL 1.

Operation of the relay RL2 also causes the contacts RL2/2 to render both of the thyristors SCRI and SCR2 instantaneously nonconductive, thereby disconnecting the relay RL 1. The capacitor C3 then becomes recharged and the thyristor SCR1 is again rendered conductive to cause re-energisation of the relay RL 1 such that power from the live lead is then connected to the load through the contacts RL1/2, and the relay RL 1 is then held on by the self holding contact RLl/l.

A successful completion of the test procedure is indicated by illumination of the neon tube N 1 by the power connected to the load, and the push button PB 1 can then be released.

Thus, the circuit breaker is adapted to rest the operation of the relay RL 1 in response to a current unbalance in the transformer before power is connected through the circuit breaker to the load, the testing being performed automatically each time the power is connected to the load by depression of the set push button PB 1.

Conveniently, the circuit breaker of the invention described herein can be mounted in a domestic socket outlet as is described in our application no. 43371/76, (Serial no.

1545600) but of course can be utilised in other domestic and industrial applications.

WHAT WE CLAIM IS: 1. A circuit breaker for connection between an alternating current supply and a load, comprising means for detecting an unbalance in current flowing in supply lines of an alternating current supply to a load; switching means for disconnecting the supply from the load in response to detection of the said unbalance by the detecting means; and testing means comprising means for introducing an unbalance in the current flowing in said lines, and means adapted to enable current to be supplied through the circuit breaker to the load only after operation of said detecting means and said switch- ing means in response to the said introduced current unbalance.
2. A circuit breaker in accordance with claim 1 including manually operable means for setting the circuit breaker to a condition to conduct current to the load and wherein said testing means is arranged to operate in response to operation of said manually operable means.
3. A circuit breaker in accordance with claim 1 or 2 wherein the detecting means comprises a differential transformer having primary windings for connection in said lines respectively, and a secondary winding arranged to detect any difference between the flux developed by said primary windings.
4. A circuit breaker in accordance with any preceding claim and adapted to be connected in live and neutral lines of an alternating current supply.
5. A circuit breaker in accordance with claim 4 wherein said switching means comprises a switching relay having switching contacts for making and breaking electrical connection in the live line.
6. A circuit breaker in accordance with claim 4 or 5 wherein the testing means comprises a test relay having switching contacts for making and breaking an electrical connection in the live line, means connected to the contacts of the test relay and arranged to inject a test current through the detecting means upon operation of the test relay to disconnect the live line from the load, the test relay being arranged to connect the live line to the load upon operation of the detecting means and the switching means in response to the test current.
7. A circuit breaker in accordance with claim 6 wherein said test current injecting means includes a capacitor connected to the contacts of the test relay and for being charged from said live line.
8. A circuit breaker in accordance with claim 5, 6 or 7 including a bridge rectifier for deriving a rectified direct current from said supply lines, said switching relay having an energising coil provided with a centre tapping connected to receive said direct current from the bridge rectifier, the ends of said coils having respective thyristors connected thereto, the gate of one of the thyristors being connected to said detecting means and the gate of the other thyristor being connected to a potentiometer chain connected to receive said direct current.
9. A circuit breaker in accordance with

claim 8 and including a test capacitor connected between the outputs of said bridge rectifier.
10. A circuit breaker for connection between an alternating current supply and a load, substantially as hereinbefore described with reference to the accompanying drawing.