WO2012110279A1

WO2012110279A1 - Electromechanical relay circuit

Info

Publication number: WO2012110279A1
Application number: PCT/EP2012/050911
Authority: WO
Inventors: Patrick Ward
Original assignee: Shakira Limited
Priority date: 2011-02-15
Filing date: 2012-01-20
Publication date: 2012-08-23

Abstract

A relay circuit comprises an electromechanical relay (RLA) having contacts (SW1) in an AC supply to a load (LD). The relay contacts (SW1) automatically close when a current greater than a predetermined closing current is passed through the relay (RLA) and are maintained closed by passing a holding current, less than the closing current, through the relay. The contacts (SW1) automatically open if the current through the relay falls below the holding current. First and second charge storage devices (C1, C2) are connected via rectifiers (X1 or X1, D3) to the AC supply in parallel with the relay (RLA) such that, upon application of power from the AC supply, current flows to the charge storage devices to charge them up. The first charge storage device (C1) is charged to a voltage (Vh) sufficient to provide a holding current for the relay at least when the AC supply is at or above a minimum operating voltage but insufficient to provide a closing current. The second charge storage device (C2) is charged to a voltage (Vc) sufficient to provide a closing current for the relay at least when the supply is at or above the minimum operating voltage. A manually operable reset switch (MR) is connected between the second charge storage device (C2) and the relay (RLA) to discharge the second charge storage device through the relay. A fault detector (CT, 100) detects a fault in the AC supply to the load (LD) and provides a corresponding output (40), the output causing an interruption in the holding current flow through the relay (RLA).

Description

Electromechanical Relay Circuit

This invention relates to an electromechanical relay circuit for disconnection of an AC supply in the event of a fault being detected in the supply.

For the purposes of the present specification an

electromechanical relay is an electrical switch with mechanical contacts which are operated (i.e. opened and/or closed) by a magnetic field produced by current flowing in a coil, usually a solenoid. A basic electromechanical relay is shown in Figure 1.

The basic relay comprises a bobbin 10 which is fitted to a ferromagnetic pole piece 12 and frame 14. A solenoid coil 16 is wound on the bobbin. A pivoting ferromagnetic armature 18 is fitted to the top of the frame and is biased into a first, open position (as shown in Figure 1) by a spring 20. An electrical contact 22 is fitted to the armature 18 via a flexible contact 24, the contact 22 being movable towards and away from a fixed contact 26 by

pivoting of the armature 18.

When a current of sufficient magnitude (known as the closing current) is passed through the coil 16 the

resultant magnetic flux induced into the ferromagnetic parts 12/14/18 of the relay will cause the armature 18 and moving contact arm 24 to be drawn towards the top of the pole piece 12, with the result that the moving contact 22 engages the fixed contact 26 to complete a current path from an input conductor 28 to an output conductor 30.

Thereafter, the contacts 22, 26 will remain closed provided a minimum holding current, less than the closing current, continues to flow through the coil 16. However, should the current flowing in the relay fall below the holding current Electromechnical Relay Circuit

For the purposes of the present specification an

pivoting of the armature 18.

Thereafter, the contacts 22, 26 will remain closed provided a minimum holding current, less than the closing current, continues to flow through the coil 16. However, should the current flowing in the relay fall below the holding current the spring 20 will open the contacts 22, 26 which can then only be re-closed manually (if a manual reset, not shown, is provided) or by increasing the magnitude of the current through the relay at least to the closing current.

This relay design is simple and well proven. The relay can operate with a DC or an AC supply to the coil, and can operate several contacts which can be normally open, normally closed or used as changeover contacts between two circuits.

The relay of Figure 1 is known as an electrically latching relay because it needs a constant current flow through the coil to maintain the contacts in the closed position.

Although it offers many benefits, such relays are rarely used in residual current devices (RCDs) or arc fault detectors (AFDs) or similar applications because they consume a relatively large amount of power and because of the difficulty of meeting the requirements of the relevant product standards in relation to closing, opening and manual reset operations.

Figure 2 shows a basic relay in an RCD application. In Figure 2, an AC mains supply comprising live and neutral conductors L, N is connected to a load LD via two normally- open pairs of contacts SW1 controlled by the relay RLA. The relay RLA may be constructed as shown in Figure 1 but with two pairs of contacts 22, 26 operated in common by the armature 18, one pair being in series with the live

conductor and the other in series with the neutral

conductor. The circuit is supplied with power via a bridge rectifier XI, and the relay is supplied with a DC current. The live and neutral conductors L, N pass through the toroidal core 50 of a current transformer CT en route to the load. The output of the current transformer is

developed across a secondary winding Wl and is fed to an RCD integrated circuit (IC) 100. In the absence of a residual current fault the currents flowing through the core 50 along the conductors L, N are equal and opposite, so the vector sum of currents flowing through the core 50 is zero and no output is produced at the winding Wl . On the other hand, when the currents in the conductors L, N are not equal and opposite, a differential current (i.e. a non-zero vector sum of currents) will flow through the core 50, leading to an output voltage developed across Wl . The function of the CT and IC 100 is to detect a

differential current flowing through the CT core 50 having a magnitude and/or duration indicative of a residual current, and when such a differential current is detected to provide a high voltage on an output line 40 sufficient to turn on a switching transistor TR1. The construction and operation of such components are well known. The IC 100 may be a type WA050, supplied by Western Automation Research & Development and described in US Patent 7068047. The IC 100 is supplied with current via a resistor R2.

A problem with the arrangement of Figure 2 relates to closure of the contacts SW1. Upon the application of power from the AC supply a capacitor CI in parallel with the relay RLA will charge up via a resistor Rl and the voltage developed across CI, and hence the voltage developed across the relay RLA, will increase. If the voltage across CI increases to a level at least equal to a minimum closing voltage Vc sufficiently high to provide a closing current for the relay, the RLA contacts SW1 will close

automatically as previously described. If the mains supply voltage is removed, the capacitor CI will discharge through the relay. However, the relay RLA will remain closed until the voltage across CI falls below a certain holding voltage Vh below which the current through the relay is less than the holding current. This allows the contacts SW1 to open, under the action of the spring 20 (Figure 1) .

Given that the contacts SW1 can remain closed until the voltage across the relay falls below Vh, any voltage above this level will result in unnecessary power dissipation in the circuit which will contribute to heat and power

dissipation or temperature rise problems. The closing voltage Vc must be achieved at any level of mains supply voltage above the intended minimum operating voltage (MOV) of the RCD, for example, 70% of the rated supply voltage, which in a 230V supply system is about 160V. The lower the intended minimum operating voltage MOV the greater the problem of surplus power dissipation. A Zener diode ZD1 is needed to clamp the voltage across the relay RLA to avoid burn out of the relay coil. As a result, surplus current is conducted though ZD1.

Once the relay contacts SW1 have closed, the RCD will provide protection. In the event of a residual current fault, transistor TR1 will be turned on by the IC 100 and will effectively short out the relay coil. The resultant collapse in RLA voltage will cause the contacts SW1 to open. However, when the fault is cleared, the IC 100 will cease to produce an output and TR1 will turn off again. When the voltage on RLA is restored its contacts SW1 will reclose automatically, and if the residual current fault is still present the circuit will repeat the cycles of opening and closing of the relay contacts, which is a totally unacceptable situation. It is an object of the invention to provide an improved relay circuit for use in residual current devices (RCDs), arc fault detectors (AFDs) and similar products. According to a first aspect the present invention provides a relay circuit comprising:

an electromechanical relay (RLA) having contacts (SW1) in an AC supply to a load (LD) , the relay contacts (SW1) automatically closing when a current greater than a

predetermined closing current is passed through the relay (RLA) and being maintained closed by passing a holding current, less than the closing current, through the relay, the relay contacts (SW1) automatically opening if the current through the relay falls below the holding current, first and second charge storage devices (CI, C2) connected via rectification means (XI or XI, D3) to the AC supply in parallel with the relay (RLA) such that, upon application of power from the AC supply, current flows to the charge storage devices to charge them up, the first charge storage device (CI) being charged to a voltage (Vh) sufficient to provide a holding current for the relay at least when the AC supply is at or above a minimum operating voltage but insufficient to provide a closing current, and the second charge storage device (C2) being charged to a voltage (Vc) sufficient to provide a closing current for the relay at least when the supply is at or above the minimum operating voltage,

a manually operable reset switch (MR) connected between the second charge storage device (C2) and the relay (RLA) to discharge the second charge storage device through the relay, and

means (CT, 100) for detecting a fault in the AC supply to the load (LD) and providing a corresponding output (40), said output causing an interruption in the holding current flow through the relay (RLA) . Preferably the output of the fault detecting means shorts out the relay (RLA) . Most preferably the output of the fault detecting means turns on a normally-off solid state switch (TR1) connected across the relay.

Alternatively, the output of the fault detecting means may interrupt a current flow through the relay (RLA) .

In such a case the output of the fault detecting means preferably turns off a normally-on solid state switch (TR2) connected in series with the relay.

The fault detecting means may comprise a circuit for detecting a current imbalance in the AC supply to the load indicative of a residual current. In such case the electromechanical relay (RLA) is

preferably responsive to said output (40) subsisting for greater than a certain period of time to disconnect the load from the supply by opening the load contacts (SW1), the circuit further comprising a generator (60) of

intermittent test pulses, each test pulse simulating a residual current by causing a differential current to flow in said detecting circuit in the absence of an actual residual current, said detecting circuit providing an output during each said test pulse, the duration of each test pulse being less than said certain period of time, and means (LED1) for providing a visual indication each time the detecting circuit provides an output in response to a test pulse. In an embodiment the first and second charge storage devices (CI, C2) are connected to the AC supply via first and second rectification means (XI, D3) respectively, the relay circuit including a capacitor (C3) at the input to the first rectification means (XI) .

According to a second, independent aspect, the invention provides a fault detection circuit comprising an

electromechanical relay (RLA) having contacts (SW1) in an AC supply to a load (LD) , said relay being responsive to detection of a fault to open the contacts, the relay being powered from the AC supply via a rectification means (XI), the circuit including a capacitor (C3) at the input to the rectification means.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which : Figure 1 (previously described) is a schematic diagram of an electromechanical relay.

Figure 2 (previously described) is a circuit diagram of an RCD incorporating the relay of Figure 1.

Figures 3 to 7 are circuit diagrams of first to fifth embodiments of the invention.

The embodiment of Figure 3 is a modification of Figure 2 which includes the following additional components: diodes Dl and D2, a second capacitor C2, a second Zener diode ZD2, a third resistor R3, and a normally-open manual reset switch MR which can be closed by applying a manual force against a spring bias tending to open the switch. The manner in which these additional components are incorporated in the circuit is evident from Figure 3.

The component values are chosen such that, for supply voltages at or above the minimum operating voltage MOV, the capacitor CI will acquire a voltage which will be at or above the holding voltage Vh but below the closing voltage Vc, with the result that RLA will not be able to close automatically. Nonetheless, a current of sufficient magnitude will flow continuously through the RLA coil to enable the relay contacts SWl to remain closed once they have closed. At the same time, the capacitor C2 will acquire a voltage which will be at or above the closing voltage Vc, although clamped by ZD2 at a safe level. The charge on C2 is supplied via the resistor R3, but this current flow will be limited to a relatively low value so as to minimise power dissipation in R3. In effect, C2 will be trickle charged via R3. When the manual reset switch MR is momentarily closed, the voltage on C2 will be applied to the relay coil and the momentary application of this higher voltage will cause the relay RLA to close its contacts SWl. The voltage applied from C2 will quickly collapse but the RLA contacts SWl will be held closed by the holding current supplied via Rl and CI.

In the event of a residual current fault, transistor TR1 will be turned on by the IC 100 and will effectively short out the relay coil. The resultant collapse in RLA voltage will cause the contacts SWl to open and TR1 will turn off as previously explained. However, with this arrangement, CI will charge up to its previous voltage again but the relay RLA will not automatically reclose until the manual reset button MR is closed again. The current required to hold the relay contacts SW1 closed will depend on its design, but will certainly be

significant, and the overall current flow in the circuit could be as high as 20mA at maximum supply voltage. At 240V, each mA of current equates to 0.24W of power, so at 240V the total power dissipated could be about 5 watts. Resistor Rl would need to be suitably rated to dissipate such a level of power, which would result in size and cost problems as well as heat dissipation problems. Figure 4 shows a modification of the circuit of Figure 3 which provides reduced power dissipation and heat generation by the electronic circuit.

In the arrangement of Figure 4, the relay current is supplied via a capacitor C3 at the input to the bridge rectifier XI. As the current through C3 is out of phase with the voltage across it, the power supplied to the circuit is affectively wattless, so heat dissipation will be reduced. The value of C3 and Rl are selected to ensure that the relay has sufficient holding current throughout the intended operating voltage range of the circuit. The capacitor C2 is charged up from a separate circuit supplied from the mains supply via a diode D3, and overall circuit operation is the same as previously described.

Figure 5 shows an alternative arrangement for opening the relay RLA.

In the arrangement of Figure 5, a transistor TR2 is connected in series with the relay coil. The transistor TR2 is normally held turned on by a current flow via a resistor R4. However, under a fault condition, TR1 turns on, effectively shorting the base of TR2 to ground and causing TR2 to turn off. This interrupts the current flow through the relay RLA and causes automatic opening of the contacts SW1 as before.

The circuit may advantageously be used in RCDs, AFDs and similar products.

Figure 6 shows a further embodiment of the invention which is based on that of Figure 3 but further includes a manual test switch. The test switch comprises a manually operable test button TS which, when pressed, bridges normally-open contacts SW2. Pressing the test button TS diverts a portion of the supply current through a winding W2 on the core 50, via a resistor Rt . The current diverted through the core 50 will produce a differential current flowing through the CT core 50, and the magnitude of the diverted current is selected such that the differential current so produced simulates a residual current. Accordingly, provided the RCD is operating correctly, the CT winding Wl will produce an output which will be detected by the IC 100. The IC 100 will, in turn, produce an output on line 40 to turn on TR1 and effectively short out the relay coil just as in the case of an actual residual current.

Windings Wl and W2 may be separate windings or formed from a bifilar winding.

Verification of the correct operation of the RCD requires the user to operate the test button, but there could be intervals of several months between such testing, and if the RCD becomes inoperable in the interval, the user will have no way of knowing that the RCD is no longer capable of providing protection. Figure 7 shows an arrangement for automatic self-testing of the RCD on a continuous basis, and means to alert the user to the possible failure of the RCD. Figure 7 includes automatic self-test circuitry comprising pulse generator 60, winding W2, switching transistor TR2, resistor R5 and light emitting diode LED1. R5 and LED1 form a first circuit branch in series with the relay RLA, and TR2 forms a second circuit branch also in series with the relay and in parallel with the first circuit branch.

The pulse generator 60 generates a continuous stream of relatively short duration test pulses at regular intervals, and during the period of each pulse a current will flow through the CT winding W2, causing a corresponding short duration differential current to flow through the core 50 of the current transformer. The amplitude of the test pulses is sufficiently high that the differential current caused thereby has a magnitude sufficient for detection by the IC 100 as a residual current and consequent generation of an output on line 40. This will cause TRl to turn on for the duration of each test pulse, as before. Turning TRl on will in turn cause TR2 (which is normally on) to turn off. The current flow through RLA will then be diverted through LED1 and R5, causing LED1 to light up. On expiration of the test pulse TRl will turn off and TR2 will turn on again, restoring the full current flow through RLA. When TR2 is turned off, the resultant current flow through RLA will be less than its holding current, so opening of RLA will be initiated. However, due to the inherent magnetic hysteresis in the relay RLA its contacts SW1 will not open immediately when TR2 is turned off but only after a certain response time. By ensuring that the duration of the test pulses is shorter than the response time of the relay RLA, the period during which RLA holding current is reduced will be correspondingly short such that the

contacts SW1 will not be opened and no interruption in the supply to the load LD will occur. However in the event of a sustained output on the line 40 from the IC 100, due to a sustained residual current fault or operation of the manual test circuit, TR2 will remain turned off for a much longer period, longer than the response time of the relay RLA, and the load contacts SW1 will open.

If the RCD fails at any time the user will notice that the diode LED1 is not flashing at regular intervals as normal and will be inclined to operate the test button and verify the non functioning of the RCD.

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

Claims :

1. A relay circuit comprising:

means (CT, 100) for detecting a fault in the AC supply to the load (LD) and providing a corresponding output (40), said output causing an interruption in the holding current flow through the relay (RLA) .

2. A relay circuit as claimed in claim 1, wherein the output of the fault detecting means shorts out the relay (RLA) .

3. A relay circuit as claimed in claim 2, wherein the output of the fault detecting means turns on a normally-off solid state switch (TR1) connected across the relay.

4. A relay circuit as claimed in claim 2, wherein the output of the fault detecting means interrupts a current flow through the relay (RLA) .

5. A relay circuit as claimed in claim 4, wherein the output of the fault detecting means turns off a normally-on solid state switch (TR2) connected in series with the relay .

6. A relay circuit as claimed in claim 1 wherein the fault detecting means comprises a circuit for detecting a current imbalance in the AC supply to the load indicative of a residual current.

7. A relay circuit as claimed in claim 6 wherein the electromechanical relay (RLA) is responsive to said output (40) subsisting for greater than a certain period of time to disconnect the load from the supply by opening the load contacts (SW1), the circuit further comprising a generator (60) of intermittent test pulses, each test pulse

simulating a residual current by causing a differential current to flow in said detecting circuit in the absence of an actual residual current, said detecting circuit

providing an output during each said test pulse, the duration of each test pulse being less than said certain period of time, and means (LED1) for providing a visual indication each time the detecting circuit provides an output in response to a test pulse.

8. A relay circuit as claimed in claim 1, wherein the first and second charge storage devices (CI, C2) are connected to the AC supply via first and second

rectification means (XI, D3) respectively, the relay circuit including a capacitor (C3) at the input to the first rectification means (XI) .

9. A fault detection circuit comprising an