GB2231215A - Residual current circuit breaker - Google Patents

Abstract

A plug-in electrical circuit breaker has a socket 3 into which is inserted the plug of an electrical appliance. A relay 30 is opened in response to a detected fault in the appliance thereby disconnecting it from power supply. A reset button and a test button associated with contacts 99, 82 are both located in the region of the socket 3 and are concealed by the plug when inserted. A transformer 33 is coupled with live conductor 21 and neutral conductor 22 and has a detector winding 34 responsive to imbalance between current in the conductors. A pair of transistors 51, 52 are connected with their bases across the detector winding 34. A detected fault causes a change in voltage across a resistor 54 between the collectors of the transistors 51, 52 which causes the coil 40 of the relay 30 to be de-energised and the relay to open. <IMAGE>

Description

ELECTRICAL CIRCUIT BREAKERS This invention relates to electrical circuit breakers.

Electrical circuit breakers are increasingly being used to protect the users of electrical equipment from faults in the equipment or associated power line that could otherwise cause electric shocks. The circuit breakers operate by isolating the equipment from the power supply in response to imbalance in the live and neutral conductors, such as caused by leakage of current to earth.

Imbalance is usually detected by means of a toroidal transformer core around which are wound both the live and neutral conductors. During correct operation, the vector sum of the current in the two conductors is zero so that no emf is induced in the transformer. If earth leakage occurs, however, there will be imbalance in the currents leading to a non-zero vector sum which causes an emf to be induced in the transformer. This emf is detected by a third winding and used to open a relay which isolates the appliance from the power supply. Where these circuit breakers are intended to plug into existing mains sockets they are generally referred to as Portable Residual Current Devices or PRCD's; breakers of this kind incorporated into a socket are referred to as Residual Current Protected Socket Outlets.

Normally, disconnection of the circuit breaker will lead to opening of the relay contacts which must be manually closed when the breaker is subsequently used.

It is important, however, to ensure that power cannot be supplied to a faulty appliance by manually holding the relay contacts closed whilst resetting the breaker.

Various arrangements have been-proposed in the past, but these generally have the disadvantage of being relatively complex or of requiring the breaker to be disconnected from the power supply, thereby preventing it from being tested at the same time. Also, the circuits previously used to detect the induced emf have tended to be expensive, in order to produce the required sensitivity and reliability.

It is an object of the present invention, to provide a circuit breaker that can be used to avoid some at least of the above mentioned disadvantages.

According to one aspect of the present invention there is provided an electrical circuit breaker for connection between electrical equipment and a power supply including a transformer that is coupled with at least a live and neutral current conductor, a detector winding responsive to imbalance between current in said conductors, a pair of transistor devices having their emitters and collectors connected together and their bases connected across said winding, resistance means connected in the collector circuit between the two transistor devices, a relay and means responsive to change in voltage across the resistance means to open the relay and thereby interrupt flow of current through the breaker.

The breaker preferably includes a capacitor connected between the detector winding and the transistor devices to ac couple the transistor devices to the winding. The relay may have a coil arranged to hold the relay closed, a parallel arrangement of a capacitor and switching device being controlled by the pair of transistor devices such that the capacitor is discharged and the winding is de-energised when the switching device is closed. The switching device may include a thyristor.

The breaker may include an electrical socket adapted to receive therein a plug connected with the electrical equipment. The breaker may include an electrical plug coupled with the relay, the plug being adapted for insertion into an electrical supply socket such that power is supplied from the electrical supply socket to the electrical equipment via the relay.

A circuit breaker in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of the circuit breaker; Figure 2 is a circuit diagram of the breaker; and Figure 3 is a perspective view of a part of the circuit breaker.

With reference first to Figure 1, the circuit breaker is in the form of a plug-in unit or PRCD contained within a moulded plastics casing 1. A three-pin plug 2 projects from the lower face of the casing 1 which is, in use, pushed into a mains power supply outlet socket 7. A socket 3 is provided in the upper face of the circuit breaker for receiving a standard three-pin plug 4 connected with electrical equipment 5 by a cable 6.

The circuit breaker isolates the socket 3 from the power supply in response to an electrical fault in the cable 6 or equipment 5 which causes imbalance in the live and neutral currents.

The construction of the circuit breaker will now be described in more detail with reference to Figure 2.

The live, neutral and earth pins, L N and E respectively of the plug 2 are connected via conductors 21, 22 and 23 to respective openings in the socket 3. The earth conductor 23 extends uninterrupted between the plug 2 and socket 3. The live and neutral conductors 21 and 22 extend through respective contacts 31 and 32 of a two-pole relay 30. Between the relay 30 and the socket 3, each conductor 21 and 22 has a winding 24 and 25 respectively which makes one turn around the core of a toroidal transformer 33. The transformer 33 also has a third winding 34, of a thousand turns, which is connected to an imbalance detection circuit, indicated by the numeral 50.

The relay 30 has a coil 40 which is connected across a capacitor 41. The capacitor 41 is connected in series with a diode bridge 42, a parallel arrangement of a capacitor 43A and resistor 43B, and a further resistor 43C between the neutral line 22 and the live line 21, at points between the relay 30 and the socket 3. The coil 40 is energised, via the diode bridge 42 when the relay 30 is closed, sufficiently to hold both contacts 31 and 32 closed. The capacitor 41 serves as a reservoir to smooth current to the coil 40 from the diode brige 42.

The detection circuit 50 includes a pair of transistors 51 and 52 connected together, back-to-back.

The two emitters of the transistors are connected together via a resistor 53, the two collectors being connected via a resistor 54. The base of transistor 51 is connected directly to one terminal of the winding 34, the base of the other transistor 52 being connected to the opposite terminal of the winding 34 via a capacitor 55. A pair of diodes 56 and 57 are connected in parallel, in opposite senses, across the winding 34 intermediate the capacitor 55 and the winding to limit the signal voltage applied across the transistors 51 and 52.

A resistor 58 is connected between the neutral line 22 and the emitter of transistor 51. A resistor 59 is connected between the neutral line 22 and the base of transistor 51. Another resistor 60 is connected between the bases of the two transistors 51 and 52. A further resistor 61 is connected between the base and collector of the transistor 52. The resistors 59, 60 and 61 provide quiescent bias for the transistor pair 51 and 52, whilst the resistor 58 sets the total current drawn by both the transistors 51 and 52. The value of the resistor 53 is selected to reduce imbalances between the transistors and enables the quiescent voltage across the resistor 54 to be set during manufacture. A capacitor 62 is connected between the base of transistor 51 and neutral line 22.

A series combination of a transistor 70 and zener diode 71 is connected across the resistor 54. The collector of the transistor 70 is connected to a first electrode of a capacitor 72 via a resistor 73, the second electrode of the capacitor being connected to neutral line 22. A resistor 74 is connected in parallel with the capacitor 72, the first electrode of which is connected to the gate of a thyristor 75 or similar switching device.

The thyristor 75 is connected in series with a resistor 76 in parallel across the capacitor 41. A resistor 77 is connected in series between the resistor 76 and the zener diode 71, the resistor 77 and zener diode together providing a stable power supply for the detection circuit 50 which enables it to function correctly down to 70% of the nominal supply voltage.

A test circuit 80 comprising a series connection of a resistor 81 and a contact 82 is connected between the live line 21, at a point between the transformer 33 and the socket 3, and the neutral line 22, at a point between the relay 30 the transformer 33.

With reference now also to Figure 3, the circuit breaker has two buttons 91 and 92 marked ON and 'TEST' respectively which are both located in recesses in the region of the socket 3 so that they will be concealed by the plug 4 when inserted. The two buttons 91 and 92 are formed on a common plastics frame 93 which has a horizontal central shaft 94 midway between the buttons about which the frame can pivot. The ON button 91 projects upwardly from a side arm 95 that extends outwardly to the left of the shaft 94 at one end, the 'TEST' button 92 projects upwardly from a similar side arm 96 that extends outwardly to the right of the shaft.This arrangement, it can be seen, prevents the TEST and ON buttons been depressed simultaneously and thereby enables the resistor 81 in the test circuit 80 to be a relatively cheap, transiently rated resistor instead of the more expensive resistor that would be needed if it was possible to close both the ON and TEST buttons together.

At the opposite end of the shaft 94, a reset finger 97 projects outwardly to the left to the relay 30. A test finger 98 projects outwardly of the shaft 94 on the right hand side to engage a contact spring 99. The spring 99 forms one of the test contacts and is located above a fixed test contact 82, so that when the 'TEST' button 92 is pushed down, the finger 98 swings down to push the spring down to touch the fixed contact 82. When the ON' button 91 is pushed down, the frame 93 swings in the opposite sense, causing the finger 97 to swing down and close the contacts 31 and 32 in the relay 30. The spring 99 normally urges the frame 93 to a position in which the two buttons 91 and 92 are flush with the surface of the housing 1 and in which neither finger 97 or 98 exerts any downward pressure.

The armature of the relay 30 is coupled directly by a mechanical linkage to a visual indicator 39 which is visible through a window in the housing 1. The indicator 39 is marked ON' and OFF', the indicator being positioned so that the ON' is visible when the contacts 31 and 32 are closed. The OFF' is only visible once both contacts 31 and 32 are in an open condition. Instead of the printed legends ON' and OFF', the indicator could have different portions of different colour, such as red and green.

Before the circuit breaker is plugged into the mains supply socket 7, the resilience of the contacts 31 and 32 in the relay 30 urge them to an open state, as shown in Figure 2, because there is no power supply to the relay coil 40. When the breaker is initially plugged into the mains supply socket, the open contacts 31 and 32 in the relay 30 will still prevent power reaching the coil 40. To close the relay 30, the user has to depress the ON button 91; this can only be done before the plug 4 is connected to the breaker. Pressing the ON' button 91 closes the two contacts 31 and 32 and causes energisation of the coil 40 via the diode bridge 42 which is sufficient to hold the contacts 31 and 32 in the closed state after the ON' button 91 is released.

As long as the contacts 31 and 32 are closed, power is supplied to the circuit breaker's socket 3 on lines 21 and 22 via the windings 24 and 25 of the transformer 33. The plug 4 can now be pushed into the socket 3 so that power is supplied to the electrical equipment 5. When the plug 4 is inserted, it covers both the TEST button 92 and the ON button 91. This ensures in a very simple way that the relay cannot be held manually closed when any load is connected to the breaker. It also ensures that the TEST button 92 cannot be used to disconnect loads, thereby reducing wear on the relay contacts. Concealing the ON button 91 and TEST button 92 by the appliance plug 4 reduces the risk of tampering, such as by children.

During correct operation of the equipment, the current flowing through the neutral winding 25 will be equal to, and in the opposite sense from, the current flowing through the live winding 24. The vector sum of the currents will therefore be zero and there will be no current induced in the detector winding 34. In this state, the transistor pair 51 and 52 is biased by the resistors 59, 60 and 61 to a quiescent state in which a voltage of approximately 2.2 volts is dropped across resistor 54.

If, however, a fault occurs in the equipment 5 or the associated cable 6 which results in an earth leakage current, the current flowing in the neutral line 22 will drop below that in the live line 21, thereby resulting in a non-zero vector sum of current. This will cause a voltage to be produced across the winding 34. This voltage is a.c. coupled via capacitor 55 to the transistor pair 51 and 52 forcing it from its quiescent state sufficiently to turn on the series combination of the transistor 70 and zener diode 71. After a delay resulting from the charging of capacitor 72 via resistor 73, the thyristor 75 is turned on, thereby discharging capacitor 41. The peak discharge current is limited by resistor 76.

When the voltage across the capacitor 41 falls below the holding voltage of the relay 30, the relay contacts 31 and 32 spring open, disconnecting the appliance 5 from the mains supply and also disconnecting the circuit breaker from the mains supply.

The capacitor 62 biases transistor 51 off during the initial energisation on setting the circuit breaker, preventing the relay 30 being de-energised during this process.

The use of a pair of transistors 51 and 52 in this way enables very low induced emf to be detected reliably.

The detection circuit is relatively simple and can be made at low cost.

Before the plug 4 is pushed into the socket 3 on the breaker, it is possible to test for correct operation of the breaker. This is done by first switching the breaker on using the ON' button 91 and then pressing down the TEST' button 92 which causes the spring contact 99 to close onto the fixed contact 82 and the test circuit 80 to conduct. Current is then drawn from the live line 21, from a point on that side of its transformer winding 24 closer the socket 3, and supplied to the neutral line 22 to a point on that side of its transformer winding 25 remote from the socket 3.

This causes a current to flow in the live winding 24 but no current in the neutral winding 25, thereby producing imbalance in a similar way that would be produced with a fault. If the circuit breaker is functioning correctly, the relay 30 will open and the indicator 39 will change from ON' to 'OFF', or change colour, signalling correct operation to the user. The circuit breaker can then be reset using the 'ON' button 91.

It will be appreciated that the circuit breaker need not have a plug 2, but could be incorporated in a mains socket unit.

Claims (6)

1. An electrical circuit breaker for connection between electrical equipment and a power supply including a transformer that is coupled with at least a live and neutral current conductor, a detector winding responsive to imbalance between current in said conductors, a pair of transistor devices having their emitters and collectors connected together and their bases connected across said winding, resistance means connected in the collector circuit between the two transistor devices, a relay and means responsive to change in voltage across the resistance means to open the relay and thereby interrupt flow of current through the breaker.

2. An electrical circuit breaker according to Claim 1, wherein the breaker includes a capacitor connected between the detector winding and the transistor devices to ac couple the transistor devices to the winding.

3. An electrical circuit breaker according to Claim 1 or 2, wherein the relay has a coil arranged to hold the relay closed, wherein a parallel arrangement of a capacitor and switching device is connected across the coil, and wherein the switching device is controlled by the pair of transistor devices such that the capacitor is discharged and the winding is de-energised when the switching device is closed.

4. An electrical circuit breaker according to Claim 3, wherein the switching device includes a thyristor.

5. An electrical circuit breaker according to any one of the preceding claims, wherein the breaker includes an electrical socket adapted to receive therein a plug connected with the electrical equipment.

6. An electrical circuit breaker according to any one of the preceding claims, wherein the breaker includes an electrical plug coupled with said relay, and wherein the plug is adapted for insertion into an electrical supply socket such that power is supplied from the electrical supply socket to the electrical equipment via the relay.