IE20020242A1 - Residual current device - Google Patents

Abstract

A residual current device for an A.C. mains includes a circuit (16) for detecting an earth fault current and contacts (14A, 14B) operated by a solenoid SI for disconnecting the mains live L and neutral N conductors when an earth fault current is detected. The circuit (16) is powered from the mains live and neutral conductors and also has an FE connection to earth E to maintain power to the circuit if there is a loss of neutral. The FE connection path includes a switch (20) which is opened before the disconnection of mains neutral N when the device is tripped. <Figure 2>

Description

RESIDUAL CURRENT DEVICE This invention relates to a residual current device (RCD) having a loss of neutral circuit.

RCDs can be of. voltage independent (VI) type or voltage dependent (VD) type. VI types depend on the fault current energy to activate a tripping mechanism whereas the VD types use the mains supply for their sensing and tripping functions. The use of VD type RCDs has increased substantially over recent years because of advantages of performance, size and cost.

A possible drawback of VD type RCDs is that they require the presence of both live and neutral supplies for their operation. If a loss of supply neutral occurred it is possible that the RCD may be disabled. Some manufacturers have addressed this problem by providing a separate connection from the RCD to the system earth. This connection, often referred to as a functional earth (FE) connection, can act as an alternative neutral in the event of loss of supply neutral. In effect, the RCD is provided with an FE supply connection in addition to the normal live and neutral supply connections.

As mentioned, VD type RCDs derive their functioning energy from the mains supply. Because of the high impedance of the RCD's electronic circuit the standing current flowing in that part of the circuit will be very low, typically 1-2 mA. However, dependent on its design, the RCD may have a solenoid which requires a substantially larger current flow to enable the RCD OPEN TO PUBLIC INSPECTION | I ί μΜυτλ ύ ι tripping mechanism to operate and trip the RCD. This solenoid current can be of the order of 1 - 2 amperes.

In RCDs fitted with an FE supply connection, when the neutral contact starts to open, the neutral return path effectively starts to increase in impedance, and some of the solenoid current will be diverted to earth through the FE. This relatively large earth current can result in nuisance tripping of upstream RCDs connected on the same installation.

To avoid this problem, manufacturers generally place some form of impedance in the RCD FE connection to limit current flow to earth. Such impedances can be in the form of resistors, capacitors, zener diodes, SCRs, etc. Whilst all of these provide varying degrees of success in their intended use, they share common problems of space, cost, component reliability, and reduced performance of the RCD under loss of neutral conditions .

The purpose of the present invention is to overcome or mitigate some or all of the above problems.

Accordingly, the present invention provides a residual current device for connection to an A.C. mains having live, neutral and earth conductors, the residual current device including circuit means for detecting an earth fault current and solenoid-operated contact means for disconnecting the mains live and neutral conductors in response to the detection of an earth fault current by the circuit means, wherein the circuit means is powered from the mains live and neutral conductors and further has an FE connection to maintain power to the circuit means if there is a loss of neutral, the FE connection path including a switch and the device having means operable when the device is tripped to open such switch before the disconnection of the mains neutral conductor.

Preferably the solenoid has a solenoid body and a plunger which moves relative to the solenoid body when the device is tripped, and wherein the switch is operated by the solenoid plunger.

The switch may be of a type which is biased normally open and is held closed by the plunger until the device is tripped. Alternatively, the switch may be of a type which is biased normally closed and is opened by the plunger when the device is tripped.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of a typical prior art voltage dependent (VD) type RCD with an FE connection; Fig. 2 is a circuit diagram of an embodiment of the invention; Fig. 3 shows a typical solenoid used in the prior art circuit of Fig. 1; Fig. 4 shows an embodiment of a solenoid which may be used in the circuit of Fig. 2; Fig. 5 shows an alternative solenoid used in the prior art circuit of Fig. 1; Fig. 6 shows a further embodiment of a solenoid which may be used in the circuit of Fig. 2; Fig. 7 shows a still further embodiment of a solenoid which may be used in the circuit of Fig. 2, the solenoid being shown in the untripped state of the device; and Fig. 8 shows the solenoid of Fig. 7 after the device has tripped.

Fig. 1 is a circuit'diagram of a typical prior art voltage dependent RCD for connection to an A.C. mains supply having live L, neutral N and earth E conductors. Such devices are very well known, and therefore will only be described briefly for the purposes of the present specification.

On the load 10 side of the RCD the live and neutral conductors L and N respectively are passed through a current transformer CT having a secondary winding 12. Under normal conditions, when relay contacts 14A, 14B in the live and neutral conductors are closed, the current flowing in the live conductor L from the mains supply to the load 10 will equal the current returning in the neutral conductor N from the load to the supply. There are, therefore, equal and opposite currents flowing through the transformer CT so that the current induced into the secondary winding 12 is zero.

However, if an earth fault occurs on the load side of the RCD there will be some current flow to ground, leading to an imbalance in the currents flowing in the live and neutral conductors. This induces a non-zero current in the secondary winding 12 which is measured by electronic circuit 16. · If the current induced in the secondary winding 12 exceeds a pre-determined threshold, indicative of an unacceptable level of earth fault current, the circuit 16 will cause a silicon controlled rectifier SCR 18 to be triggered (turned on) .

Power is supplied to the electronic circuit 16 from the live conductor L via a solenoid SI and full wave rectifier bridge D1-D4 with a return to the neutral conductor N. An FE connection is provided by two additional diodes D5 and D6 which form a third arm of the bridge and whose junction is connected by way of an FE connection to earth E via an impedance Z which can be a resistor, capacitor, zener diode, SCR, etc. SCR 18 is connected across the positive and negative sides of the bridge. The electronic circuit 16 is powered via a dropper resistor R from the positive side of the bridge. The impedance Z limits the standing current flowing to earth by presenting a higher impedance in that path than that of the neutral return path.

However, impedance Z may not be necessary for that function for RCDs having a low standby current threshold, e.g.

D5 and D6 are required to provide rectified DC power to the electronic circuit 16 under the loss of neutral condition. Alternatively, a single diode could be used to provide power to the electronic circuit 16 via live L and neutral N, and a second diode could be used to provide power to the electronic circuit via live L and earth E under a loss of neutral condition. The bridge arrangement has the advantage of using both half cycles of the mains supply to provide power to the electronic circuitry 16 and the SCR 18.

As mentioned, if there is a residual current (earth fault current) exceeding a predetermined threshold the electronic circuit 16 will turn on SCR 18, effectively subjecting the solenoid SI to the full supply voltage.

A large current will now flow from live L through the relatively low impedance winding of the solenoid SI, via diodes Dl—D4 and SCR 18, and back to neutral N, activating the solenoid SI. The solenoid SI is coupled in known manner to the contacts 14A, 14B in the live and neutral conductors L, N such that activation of the solenoid causes the contacts 14A, 14B to open and thereby disconnect the mains from the load 10.

A potential problem may arise when the neutral contact starts to open. At this point the impedance of the neutral path will increase and solenoid current will be diverted to earth via the FE. If Z is of negligible impedance, the FE current can attain a large value and cause an upstream RCD to trip. The user will see this as nuisance tripping of the upstream RCD because the downstream RCD should have cleared the fault without causing the upstream RCD to trip. To overcome this problem, Z will need to have a relatively high impedance value.

In the event of a loss of supply neutral and a subsequent residual current fault, the current flowing through the solenoid SI will now pass via diodes DI, D2, D5 and D6 and SCR 18 through the impedance Z to earth .

In this case, the impedance of Z will need to be sufficiently low to ensure that the solenoid can generate sufficient energy at the specified minimum operating voltage. Furthermore, any value of Z above zero ohms will result in some degradation in the performance of the RCD compared to its performance when the neutral is connected. For example, the minimum operating voltage with neutral present could be lower than when the neutral is absent, etc.

Therefore, the impedance value of Z has to be a tradeoff between the requirement to have a high value Z to minimise earth current flow when the neutral is present, and a low value Z to provide for an assured level of performance of the RCD under loss of neutral conditions .

In addition to the above conflict in requirements in terms of the impedance value of Z, additional problems arise in terms of accommodating Z in the RCD. Impedance Z will impact on factors such as space, cost and product reliability.

The embodiment shown in Fig. 2 overcomes these problems whilst providing the RCD with an efficient and effective loss of neutral operating capability.

The embodiment uses a switch 20 in the FE circuit which remains closed until activation of the solenoid, at which point it opens just before the neutral contact 14B opens. Advantageously, in this embodiment the switch 20 is combined with and is operated by the solenoid SI. This is now explained with reference to Fig. 3 which shows a conventional solenoid and Fig. 4 which shows a solenoid fitted with the switch 20= The conventional solenoid comprises a solenoid body or bobbin 22 on which is wound an electrical winding 24 terminated on two pins 26A, 26B. Two pins 28A, 28B at the other end of the solenoid can be used to secure the solenoid in position. A plunger 30 is positioned in an axial·through-bore of the bobbin. When the solenoid is energised, the plunger 30 is drawn into the bobbin (in the direction right to left as seen in Fig. 3) . The energy imparted to the plunger by the solenoid is such that the plunger continues its movement in the bobbin to strike a trip lever 32 and open the RCD live and neutral contacts 14A, 14B.

In the embodiment of Fig. 4, the switch 20 is fitted to the two rear pins 28A, 28B on the bobbin 22. A conductive spring 34 is fitted to the pin 28A such that its free end is normally biased against the pin 28B so as to short the pins 28A, 28B together (the latter are, of course, connected into the FE circuit as shown in Fig. 2). Thus the pins 28A, 28B and the spring 34 constitute the switch 20. When the solenoid is energised, the plunger 30 is drawn into the bobbin as before. As the plunger 30 moves inwards, an arm 36 carried by the plunger initially pushes against the extended free end of the spring 34 pushing it away from the pin 28B and thereby opening the electrical connection between the two pins 28A, 28B. The plunger will continue its forward travel and activate the trip lever 32 as in the conventional solenoid.

Thus, when the solenoid is activated, the FE circuit will be opened fractionally before the trip lever 32 is activated with the result that the FE circuit will open just before the neutral contact opens. Once the FE circuit is open, none of the solenoid current can be diverted to earth.

In the event of a loss of neutral condition, the RCD will be powered between live and earth. When the solenoid is energised, all of the solenoid current will flow through the FE circuit until the RCD is tripped. The effect of the switch 20 is to maximise the impedance in the FE circuit when solenoid current can flow through the neutral, and to minimise the impedance in the earth path when the neutral is broken.

In some RCDs, the plunger 30 needs to be biased to maintain part of the plunger body outside the bobbin 22. This is done, for example to ensure that the plunger is drawn away from the trip lever to facilitate resetting of the RCD, or to ensure that when the solenoid is energised the solenoid will exert a pulling force on the plunger, etc. Biasing the plunger in. such cases usually requires a separate spring 40 to be fitted to the plunger, as shown in Fig. 5. The plunger spring 40 ensures that the plunger is partially withdrawn from the solenoid body as required. 8C The spring switch of Fig. 4 can be designed in such as way as to achieve the dual functions of providing a normally closed switch 20 in the FE circuit and also biasing the plunger. This arrangement obviates the need for a separate plunger spring 40. It is a relatively simple procedure to mechanically couple the spring 34 to the plunger to achieve both functions .

One example is shown in Fig. 6. Here the free end of the spring 34 has an extension 42 which is brought around to engage behind the enlarged head 44 of the plunger 30 and bias it partially out of the bobbin.

In each of the preceding embodiments the switch 20 is biased normally closed by the inherent resilience in the spring 34, and is forced open by the plunger 30 when the device is tripped.

Equally, however, the switch 20 can be one that is biased normally open but which is held closed by the plunger until the device is tripped. An example of such an arrangement is shown in Fig. 7.

In Fig. 7, the FE switch 20 comprises contacts 20A, 20B which are biased normally open by inherent bias in the contact 20A. The switch 20 is positioned behind the bobbin 22 such that the plunger 30, biased by the spring 40, acts on the switch contact 20A to close the two normally open contacts. The contacts 20A, 20B can be fitted on a printed circuit board or any convenient location to achieve the desired action.

When the solenoid is activated, the plunger 30 is drawn into the bobbin 22 and away from the FE switch contact 20A, allowing the switch 20 to open. This condition is shown in Fig. 8. Through the energy provided by the solenoid, the plunger will continue its forward momentum and activate the trip lever 32 to open the mains contacts 14A, 14B as before.

It will be understood that, depending on the 10 circumstances, the FE circuit may include only the switch 20, i.e. the FE circuit has no impedance other than the inherent impedance of the conductors forming the connection to earth, or it may include an impedance Z in series with the switch 20.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.

Claims (6)

1. A residual current device for connection to an A.C. mains having live, neutral and earth conductors, the residual current device including circuit means for detecting an earth fault current and solenoid-operated contact means for disconnecting the mains live and neutral conductors in response to the detection of an earth fault current by the circuit means, wherein the circuit means is powered from the mains live and neutral conductors and further has an FE connection to maintain power to the circuit means if there is a loss of neutral, the FE connection path including a switch and the device having means operable when the device is tripped to open such switch before the disconnection of the mains neutral conductor.

2. A residual current device as claimed in claim 1, wherein the switch is operated by the solenoid.

3. A residual current device as claimed in claim 2, wherein the solenoid has a solenoid body and a plunger which moves relative to the solenoid body when the device is tripped, and wherein the switch is operated by the solenoid plunger.

4. A residual current device as claimed in claim 3, wherein the switch is biased normally open and is held closed by the plunger until the device is tripped.

5. A residual current device as claimed in claim 3, wherein the switch is biased normally closed and is opened by the plunger when the device is tripped.

6. A residual current device as claimed in claim 5, wherein the switch also biases the plunger partially out of the solenoid body in the untripped state of the 5 device.