GB2366676A

GB2366676A - Residual current device

Info

Publication number: GB2366676A
Application number: GB0107121A
Authority: GB
Inventors: Pat Ward
Original assignee: Shakira Ltd
Current assignee: Shakira Ltd
Priority date: 2000-03-22
Filing date: 2001-03-21
Publication date: 2002-03-13
Also published as: IE20010271A1; IES20010272A2; GB0107121D0

Abstract

A residual current device for an A.C. mains supply includes a circuit 16 for detecting an earth fault current, and contacts 14A, 14B operated by current flowing in a solenoid winding W1 for disconnecting the mains in response to such detection. The circuit 16 is powered from the mains live L and neutral N conductors, and also has a connection to the earth conductor E via a second solenoid winding W2 to maintain power to the circuit 16 if there is a loss of neutral. The second solenoid winding W2 is connected between the mains neutral and earth conductors to disconnect the mains in the event of a reverse live-neutral mains connection.

Description

2366676 RESIDUAL CURRENT DEVICE This invention relates to a residual

current device (RCD) having a loss of neutral circuit. s RCI)s 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 10 tripping functions. The use of VD type RCDs has increased substantially over recent years because of advantages of size, performance and cost.

A drawback of VD, type RCDs is that on a single phase 15 mains installation 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 20 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.

25 The provision of a connection to earth will result in the flow of a current from live to earth within the RCD which could cause problems such as nuisance tripping, etc. Manufacturers generally place some form of impedance within the RCD in the earth connection 30 circuit to limit such current flow to earth. The impedance can be a resistor, capacitor, zener diode, SCR, etc. Whilst all of these circuits and components provide varying degrees of success, they share a common problem in that they ail require additional space on the printed circuit board (PCB) where the RCD circuit elements are assembled.

5 Also, under loss of supply neutral conditions the current limiting effect of the impedance within the RCD earth circuit can substantially reduce the force generated by the solenoid to enable the RCD to trip. The reduced efficiency of the solenoid usually results 10 in the minimum operating voltage of the RCD under a loss of neutral condition being substantially higher than that when the neutral is present. Thus the degree of protection provided by the RCD under a loss of supply neutral condition will be less than that under 15 normal supply conditions.

In addition to loss of neutral protection, a reverse wired live-neutral supply is generally considered to be dangerous and unacceptable. To overcome this problem 20 manufacturers often provide their RCDs with additional circuitry to detect such conditions and bring about automatic tripping of the RCD, thereby alerting the user to the wiring defect. The additional circuitry required for detection of a reverse live-neutral adds 25 further to component cost and space demands which can prove onerous to the RCD manufacturer.

Additional components and circuitry required to perform the various ancillary functions of the RCD such as 30 reverse L-N sensing, etc., add to the cost, complexity and size of the RCD and reduce its overall reliability. In addition, the reduced performance of the RCD under a loss of neutral ccndition may not be acceptable to the user.

The purpose of the present invention is to overcome or 5 mitigate some or all of the above problems.

Accordingly, the present invention provides a residual current device for an A.C. mains having live, neutral and earth conductors, the residual current device 10 including circuit means for detecting an earth fault current and contact means operated by current flowing in a solenoid winding for disconnecting the mains in response to the detection of an earth fault current by the circuit means, wherein the circuit means is powered 15 from the mains live and neutral conductors and further has a connection to the earth conductor via an impedance to maintain power to the circuit means if there is a loss of neutral, and wherein the impedance comprises a further solenoid winding.

Preferably the further solenoid winding is connected between the mains neutral and earth conductors and is operative to disconnect the mains in the event of a reverse live-neutral mains connection.

Most preferably the further solenoid winding is disposed on the same solenoid body as the first winding. In such case the further solenoid winding shares the same. coupling.mechanism to the contact means 30 as the first winding.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which- S Fig. 1 is a circuit diagram of a typical prior art voltage dependent (VD) type RCD with an FE connection;

Figs. 2A and 2B are examples of single and double winding solenoids respectively; Fig. 3 is a circuit diagram of a first embodiment of the invention using the double winding solenoid of Fig. 2B; 15 Fig. 4 is a circuit diagram of a second embodiment of the invention, also using the double winding solenoid of Fig. 2B; and Fig. S is a circuit diagram of a third embodiment of 20 the invention.

Fig. 1 is a circuit diagram of a typical prior art voltage dependent RCD for 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 2S 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 30 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 5 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 10 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 15 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 20 live conductor L via a solenoid S1 and full wave rectifier bridge Dl- 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 to earth E 25 via an impedance Z which can be a resistor, capacitor, zener diode, SCR, etc., as described above. 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 30 presenting ahigher impedance in that path than that of the neutral return path. SCR 18 is connected across the positive and negative sides of the bridge.

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 Sl to the full supply voltage.

5 A large current will now flow from live L through the relatively low impedance winding of the solenoid S1, via diodes Dl-D4 and SCR 18, and back to neutral N, activating the solenoid Sl. The solenoid Sl is coupled in known manner to the contacts 14A, 14B in the live 10 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. The coupling mechanism (not shown) typically comprises a plunger slidable in the solenoid body and linked to the 15 contacts.

In the event of a loss of supply neutral and a subsequent residual current fault, the current flowing through the solenoid Sl will now pass via diodes Dl, 20 D2, DS and D6 and SCR 18 through the impedance Z to earth.

DS and D6 are required to provide rectified DC power to the electronic circuit 16 under the loss of neutral 25 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 30 '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 explained previously, regardless of the form that impedance Z may take, it will require space on the PCB, and it may contribute to other problems such as cost, size, solenoid performance and overall reliability.

An embodiment of the present invention, whose circuit diagram is shown in Fig. 3, takes advantage of the fact that in many cases the solenoid Sl which operates the contacts 14A, 14B has room for a second winding to be 10 placed on the same body, and this second winding can be used as the impedance Z. Examples of single and double winding solenoids are shown in Figs. 2A and 2B respectively.

15 Fig. 2A shows a conventional solenoid as used, for example, as the solenoid S1 in the RCD of Fig. 1. In this case the solenoid body 20 has only a single winding W1, connected between the live conductor L and the junction of the diodes D1 and D2. Fig. 2 shows a 20 solenoid which may be used in the embodiment of the invention shown in Fig. 3. In this case, in addition to the original winding Wl there is a second winding W2 overlying the first (alternatively, Wl could overlie W2) . This second winding W2 is connected between the 25 earth conductor E and the junction of the diodes D5 and D6, and the impedance of W2 can be optimised to perform the function of the impedance Z in figure 1.

In figure 3, the second winding W2 replaces Z in the 30 earth path. Under normal supply conditions, the RCD behaves substantially the same as before, but with W2 now acting as the current limiting impedance. Under a loss of neutral condition, when SCR 18 is turned on current will flow from live L via solenoid winding W1, diodes Dl, D2, DS and D6, SCR 18 and solenoid winding W2 to earth, thereby opening the contacts 14A, 14B (the solenoid comprising the body 20 and windings Wl,W2 is 5 coupled to the contacts 14A, 14B as before). Of course, the impedance and number of turns of the winding W2 must be such that the standing current through the winding W2 is insufficient in itself to cause opening of the contacts 14A and 14B under non 10 fault conditions.

The two windings W1 and W2 can be overlaid as shown in Fig. 2 or disposed end to end or in any other convenient arrangement on the body 20. The polarity of 15 the winding W2 is preferably such as to reinforce the magnetic field generated by the winding W1, for reasons to be discussed later, but this is not absolutely necessary provided the net magnetic field generated by the two windings when the SCR 18 is tripped is

20 sufficient to open the contacts.

In the embodiment of Fig. 3 it is necessary to provide means for rectifying the mains supply via live and earth when the neutral is disconnected. This 25 rectification is provided by the diodes DS and D6. Such components add considerably to space, cost and reliability problems for the RCD manufacturer.

These additional components can be avoided by 30 'connecting the windings Wl and W2 as shown in the embodiment of Fig. 4.

In Fig. 4 the winding Wl has been connected between the output of the bridge rectifier circuit (junction of D3/D4) and neutral N and the winding W2 has been connected between neutral N and earth E. In addition, 5 diodes DS and D6 of the previous embodiment have been omitted.

When the neutral N is present and the SCR 18 is turned on, current will preferentially flow from live L via 10 the bridge rectifier circuit, the SCR 18 and winding Wl to neutral N, and the resultant activation of Wl will cause the contacts 14A, 14B to open. The winding W2 provides an impedance in the earth circuit which limits any current flow to earth before or after the SCR 18 is 15 fired. If the SCR 18 is turned on under a loss of neutral condition, current will flow from live L through the bridge rectifier, the SCR 18 and both windings W1 and W2 to earth, and the resultant activation of both solenoid windings will cause the 20 contacts 14A, 14B to open.

With the arrangement of Fig. 4 there is no need for diodes D5 and D6 because either solenoid winding can be activated through the four diode bridge rectifier.

25 This obviates the need for rectification circuitry to be provided specifically for loss of neutral operation.

Under a reverse live-neutral condition, the winding W2 will be connected across the mains supply and will 30 automatically be activated causing the contacts to open provided the winding W2 has the correct polarity on the body 20. The winding W2 effectively provides the reverse live-neutral sensing and activation functions for this circuit.

The arrangement of Fig. 4 suffers from a possible 5 disadvantage in that when contact 14B starts to open the impedance in the neutral line starts to increase rapidly due to arcing and the widening gap in contact 14B. Dependent on the speed of opening, the point on the A.C. wave of opening, etc., and the impedance of 10 W2, some of the current flowing through W1 may flow through W2, with the resultant risk of tripping upstream RCDs. This problem can be overcome by the arrangement of Fig. 5 which shows an alternative means for connection of the two solenoid windings on the body 15 20. In this arrangement the electronic circuit 16 and winding W1 are connected on the supply side of contacts 14A and 14B, while winding W2 remains connected on the load side as for Fig 4. The current through winding Wl will no longer see an increase in the impedance in the 20 neutral path and current will be less likely to be diverted through winding W2.

In conventional loss of neutral circuits typified by 25 Fig. 1 the provision of an impedance in the earth path not only limits the current flow in that path but also reduces the efficiency of the solenoid under a loss of neutral condition. The energy produced in a solenoid is dependent on the ampere turns generated in its 30 windings. When the solenoid is energised with the live and neutral present, the ampere turns produced by the solenoid will be a maximum. Under a loss of supply neutral condition the solenoid will be activated via the earth circuit, but the current limiting impedance in the earth circuit will reduce the current flow through the solenoid. This in turn reduces the ampere turns and reduces the efficiency of the solenoid.

In the circuits of Figs. 3 to 5 the impedance in the earth circuit comprises the second winding W2 on the solenoid and when the neutral is present this impedance limits the current in the earth circuit, as normal.

10 However, under a loss of neutral condition current will flow through both windings W1 and W2 when the SCR 18 is turned on. Now, instead of behaving as a power dropping impedance which reduces the efficiency of the solenoid, the additional winding W2 supplements the 15 winding W1 so as to increase the overall number of turns such that the ampere turns of the solenoid remains much the same as that produced under normal supply conditions. Thus the efficiency of the solenoid is substantially the same whether it is activated via 20 neutral or earth.

Furthermore, in respect of the Figs. 4 and 5 embodiments, the dual winding solenoid obviates the need for rectification circuitry to be provided 25 specifically for loss of neutral operation, and it provides automatic tripping under a reverse liveneutral condition. It is highly cost effective, space efficient, and improves the overall reliability of the RCD.

In the foregoing embodiments the additional winding W2 is disposed on the same solenoid body as the winding W1. As discussed, this has advantages of saving space as well as allowing the winding W2 to share the same coupling mechanism to the contacts 14A, 14B as the winding W1. However, the winding W2 could be disposed on a separate solenoid body and independently coupled 5 to the contacts 14A, 14B for disconnecting the mains in the event of a reverse live-neutrai mains connection.

The invention is not limited to the embodiments described herein which may be modified or varied 10 without departing from the scope of the invention.

Claims

1. A residual current device for an A.C. mains having live, neutral and earth conductors, the residual 5 current device including circuit means for detecting an earth fault current and contact means operated by current flowing in a solenoid winding for disconnecting the mains in response to the detection of an earth fault current by the circuit means, wherein the circuit 10 means is powered from the mains live and neutral conductors and further has a connection to the earth conductor via an impedance to maintain power to the circuit means if there is a loss of neutral, and wherein the impedance comprises a further solenoid 15 winding.

2. A residual current device as claimed in claim 1, wherein the further solenoid winding is connected between the mains neutral and earth conductors and is 20 operative to disconnect the mains in the event of a reverse live- neutral mains connection.

3. A residual current device as claimed in claim 2, wherein the first solenoid winding is in the path 25 between the circuit means and the neutral conductor.

4. A residual current device as claimed in claim 1, 2 or 3, wherein the circuit means is powered from the mains live andneutral conductors on the supply side of 30 the contact means.

5. A residual current device as claimed in any preceding claim, wherein the further solenoid winding is disposed on the same solenoid body as the first winding.

6. A residual current device as claimed in claim 5, 5 wherein the first and further windings are overlaid on the body or disposed end to end on the body.

7. A residual current device as claimed in claim 6, wherein the polarity of the second winding is such as 10 to reinforce the magnetic field generated by the first winding.

8. A residual current device for an A.C. mains having live, neutral and earth conductors, substantially as 15 hereinbefore described and/or as shown in any one of: Figure 2B and Figure 3; Figure 2B and Figure 4; and Figure 2B and Figure 5.