IES20090113A2 - Multiphase residual current device - Google Patents

Abstract

A residual current device for use with a multiphase AC mains supply comprises an electronic circuit (16) powered from the mains supply for detecting an imbalance in the current supply to a load and providing an output signal (18) when the imbalance meets certain criteria indicative of an earth fault current. An electro-mechanical switch (12) has a set of contacts (SW1) in the mains conductors (P1-P3, N) and is connected to mains power in response to the output signal (18) for opening the contacts. The device further includes means for automatically disconnecting mains power from the EM switch (12) shortly after the set of contacts (SW1) are opened even if the output signal (18) persists. The disconnecting means includes a normally open further contact (SW2) in the series with the EM switch (12) and mechanically independent of the set of contacts (SW1), the further contact (SW2) being closed in response to the output signal (18) and allowed to re-open shortly thereafter. <Figure 6>

Description

This invention relates to a multiphas*e^?§^TOTSTwCTrif«ntdevice.

Residual current devices are used for providing protection against electric shock and dangerous ground fault currents. These devices are known under a range of acronyms such as RCDs, GFIs, GFCIs and ALCIs. Most of these use an electronic circuit to detect an earth fault (residual) current and cause the RCD to trip if the earth fault current exceeds a certain threshold for a certain period of time .

The basic functionality of an RCD will be well known and understood by those familiar with this product area. However, additional information can be found in the article, Demystifying RCDs at www.westernautomation. com, and published in the Irish Electrical Review in 1997.

Figure 1 represents a three phase mains installation comprising phase conductors Pl, P2 and P3 and a neutral conductor N. The neutral conductor is connected to earth E. The construction and operation of such an installation is well known and will only be described briefly here.

In Figure 1, a three phase AC mains supply is fed to· a load 10 via four switch contacts SW1 associated with a solenoid 12, the phase and neutral mains conductors Pl, P2, P3 and N passing through a current transformer (CT) 14 en route to the load. The output of the CT 14 is fed to an RCD integrated circuit (IC) 16. The IC 16 may be a type WA050, BT CL I m oLt 1 13 supplied by Western Automation Research & Development and described in US Patent 7068047.

The IC16 is powered from the mains via diodes Di, D2 and D3 connected respectively to the three phases, and their common point is connected to the IC 16 via dropper resistor Rl. The return path for power to the circuit is connected back to neutral.

Under normal conditions, all of the current IL flowing to the load 10 via one or more of the phase conductors will return to the load via the neutral conductor, i.e. IL = IN. Under a fault condition, a person could come into contact with a live part. Under this condition part of the current, the residual or earth fault current IR, will flow to earth. In such a case there will be a current imbalance in the supply to the load, i.e. IL > IN.

The function of the CT 14 and the IC 16 is to detect a current imbalance in the AC supply to the load, and if such imbalance meets certain criteria indicative of an earth fault current, e.g. it exceeds a certain threshold and/or persists for a certain period of time, it provides a high output voltage on a line 18 sufficient to turn on a normally-off silicon controlled rectifier SCR in series with the solenoid 12. Turning on the SCR allows mains current to flow through the solenoid 12, thereby activating the solenoid and causing the associated switch contacts SW1 to open and disconnect the supply from the load 10.

Due to space and cost constraints, it is generally necessary to make the solenoid 12 in an electronic RCD as small as possible. For this reason, the solenoid cannot be fully rated and can only tolerate the mains voltage being applied across it for a relatively short period, typically <100mS. Continuous energisation or rapid repeat energisation of the solenoid will result in solenoid burn out.

Che main advantage of the arrangement of Figure 1 is its simplicity. However, it suffers from a major drawback m that if the supply neutral is not connected, the RCD cannot operate even if the three phases are connected. For example, it may be desirable to use the RCD on a 3 phase installation which does not have provision for a neutral conductor. It may be desirable to use the RCD on a two phase installation, again without a neutral conductor. In each case, the RCD would not be able to function. The RCD may be connected in a 3 phase + N installation, but if for any reason the supply neutral becomes disconnected, the RCD will not be able to function.

Various solutions have been used to solve these supply problems. An example of one such solution is shown in Figure 2.

In the arrangement of Figure 2, full wave rectification of the three phase supply is provided by diodes Dl to D8 to provide power to the electronic circuit. In this arrangement, power will be supplied to the circuit in the event of disconnection of any two supply conductors, including the neutral. With this arrangement the RCD could be used on single phase, two phase or three phase installations, thereby mitigating the problems of the arrangement of Figure 1.

II0HH3 The arrangement of Figure 2 requires that the electronic circuit be supplied from the load side of the switch contacts SW1. In Figure 2 this is shown by the full wave rectifier being connected to the mains conductors to the right of the contacts SW1. This is because if the electronic circuit were supplied from the supply side of the contacts SW1, i.e. to the left of the contacts SW1, the SCR would not turn off when the contacts SW1 open because the voltage on the SCR anode would never fall to zero volts with three phases connected. This would almost certainly result in the solenoid burning out as explained earlier, with fatal damage the RCD. This problem compels the RCD designer to ensure that the electronic circuit is supplied from the load side of the contacts, which adds to complexity and cost.

However, even with the electronic circuit powered from the load side of the contacts SW1 as shown in Figure 2, the RCD is vulnerable to severe damage if the RCD is reverse wired, i.e. if the mains supply is connected to the load terminals of the RCD and the load is connected to the supply terminals. In this case the SCR will again be unable to turn off once activated, and the solenoid will burn out. Manufacturers therefore have to go to considerable trouble to clearly mark the RCD supply and load terminals and to warn users against reverse wiring.

Even so, there is another condition that can arise which cannot be overcome by the arrangement of Figure 2 or warnings about reverse wiring. If the arrangement of Figure 2 is used to power up a three phase motor via the RCD and a residual current occurs which causes the RCD to trip, e.g. because of a fault at the load or because of llo Η113 operation of a test circuit (not shown), the solenoid will be activated as before and the contacts will open.

However, the motor will now act as a generator and supply a voltage to the RCD via its load contacts with the result that the SCR will remain turned on and the solenoid will remain energised until the energy in the load has sufficiently dissipated to allow the SCR to turn off. Repeated resetting and tripping of the RCD under such a condition will result in damage to the RCD.

Various solutions have been used to overcome the above problems, an example of which is shown in Figure 3.

In the arrangement of Figure 3, an auxiliary switch contact XI has been placed in the common line of the DC supply to the electronic circuit. This contact is mechanically coupled to the main contacts SW1 so that when the main contacts open, contact XI also opens. When energised, the solenoid 12 causes opening of the main contacts SW1, and these in turn cause opening of XI. It can be seen that auxiliary contact XI is controlled by the main contacts SW1 and mimics their state, and its sole purpose is to remove power from the electronic circuit when the main contacts open. With this arrangement, the solenoid 12 is automatically de-energised when the main contacts open, and thereby burn-out is prevented. The electronic circuit can be supplied from the supply side of the contacts, and can withstand reverse wiring and the effect of motor loads. However, a drawback of the arrangement of figure 4 is the need to mechanically couple contact Xl to the main contacts SW1. This adds considerably to design complexity, cost, and reliability problems. 110 9 01 13 It is an object of the present invention to mitigate the above problems and other problems related to multiphase RCDs.

According to the present invention there is provided a residual current device for use with a multiphase AC mains supply, the device comprising an electronic circuit powered from the mains supply for detecting an imbalance in the current supply to a load and providing an output signal when the imbalance meets certain criteria indicative of an earth fault current, and an electro-mechanical (EM) switch having a set of contacts (SW1) in the mains conductors and which is connected to mains power in response to the output signal for opening the contacts, the device further including means for automatically disconnecting mains power from the EM switch shortly after the set of contacts are opened even if the output signal persists, the disconnecting means including a normally open further contact in series with the EM switch and mechanically independent of the set of contacts, the further contact being closed in response to the output signal and allowed to re-open shortly thereafter.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures 1 to 3, already described, are circuit diagrams of prior art multiphase residual current devices.

Figure 4 is a circuit diagram of a first embodiment of the invention. 110 9 01 13 Figure 5 shows the embodiment of Figure 4 reverse wired.

Figure 6 is a circuit diagram of a second embodiment of the invention.

In the various drawings the same references have been used for the same or equivalent parts.

In the embodiment of Figure 4 the power supply arrangement is similar to that of Figure 3. However, in this embodiment, a relay RLA has been added to the circuit and the SCR has been replaced by the normally open relay contact SW2. It should be noted that there is no mechanical coupling between the main contacts SW1 and relay RLA or its contact SW2. The IC 16 is provided by power from the rectified mains via resistor Rl. This also provides a path for a capacitor Cl to charge up, the voltage on Cl being clamped by a voltage regulator in the WA050.

Under a fault condition, i.e. when the output on line 18 goes high as previously described, a transistor TRI is turned on by the IC 16 and the capacitor Cl discharges through the relay RLA, causing its contact SW2 to close. This in turn connects the solenoid 12 across the rectified voltage of the mains supply, causing activation of the solenoid and opening the main contacts SW1. Once the fault condition is removed, the IC output 18 will go low and TRI will turn off, causing contact SW2 to open again and thereby remove power from the solenoid 12. Likewise, if the voltage on Cl falls below a certain level before the IC output 18 goes low, the relay RLA will de-energise and contact SW2 will open.

SEC S 01 13 It should be noted that components Rl, Cl and RLA are chosen to ensure that the relay RLA can only be energised by the discharge of Cl through its coil, and once the voltage on Ci falls below a certain level the relay RLA will de-energise even if TRI remains turned on. When TRI turns on, the resistor Rl and the coil of the relay RLA will be connected in series, but the impedance of the relay coil is substantially lower than that of Rl with the result that the voltage across the relay coil when TRI is turned on will be insufficient to hold the relay in the closed state.

With this arrangement, like Figure 3, the solenoid 12 is automatically de-energised when the main contacts SW1 open, and thereby burn-out is prevented. However, it avoids the need for an auxiliary contact XI mechanically coupled to the main contacts SW1.

The embodiment of Figure 4 mitigates many of the problems related to the prior art arrangements, and offers other considerable advantages. For example, elimination of the SCR provides considerable additional immunity against nuisance tripping which can be caused by false triggering of the SCR by mains disturbances. However, if the RCD of Figure 4 was reverse wired as shown in Figure 5, i.e. if the mains supply was connected to the load terminals and the load connected to the supply terminals, a problem could arise when a test circuit 20 was operated.

Under this condition, if the test circuit 20 was closed and held closed, the test current would flow continuously. The relay RLA would operate as before and cause opening of the JEO 9 01 13 contacts SW1. However, even though the contacts SWl would have opened, the test current would continue to flow because the RCD was being supplied from the load side. in the arrangement of Fig. 5, the IC 16 is shunted by the relay RLA when TRI turns on. The relay RLA, which is in series with RI, has a relatively low impedance compared to RI with the result that the voltage across the IC 16 collapses to a level causing the IC 16 to power off. This allows TRI to turn off, causing Cl to charge up and restore an operating voltage to the IC 16. The IC output 18 therefore goes high again because of the sustained flow of the test current, and TRI turns on again, discharging Cl through the relay RLA and re-closing SW2. The relay RLA would therefore cycle through closing and opening states indefinitely, and eventually result in the solenoid winding burning out.

This problem is overcome by the embodiment of Figure 6.

In Figure 6, the capacitor Cl is supplied from the rectified mains supply via resistor RI as before, but the resistor RI is now placed in series with the relay RLA as a shunt across the IC 16 which is now supplied with power via a separate resistor R2. As a result, the IC 16 remains powered up whenever the mains supply is present since RI is chosen to have a value which prevents the voltage across the IC 16 collapsing to a level causing the IC 16 to power off when TRI is on. If the test button 20 is operated and held closed, the IC output 18 will go high as before and turn on TRI which in turn will cause the relay RLA to be energised to close the contact SW2. When Cl has sufficiently discharged, the relay RLA will be deenergised. However, the IC 16 will continue to be powered ICO 9 01 1 3 ilss 0 UJ up and its output will remain high and hold TRI in the on state with the result that Cl will be unable to charge up to a level sufficient to energise the relay RLA until such time as TRI is allowed to rurn off. In this arrangement, the solenoid 12 will operate only once regardless of how long the test current flows, thereby preventing burnout of the solenoid.

Refinements can be added to the embodiment of Figure 6.

The voltage across Cl can be clamped by a zener diode to prevent it rising to an excessively high level. Also, the duration for which the relay RLA1 remains closed could be controlled by suitable component selection or by provision of simple time delay circuitry. The test circuit 20 can be arranged to be fully rated so as to withstand continuous operation if held in the closed state, etc. Furthermore, given that the neutral N is normally at earth potential it is not always necessary for the neutral to be switched. In such cases an RCD with a solid neutral conductor may be used, thereby obviating the need and costs associated with providing a switched neutral pole.

In summary, Figure 6 shows an arrangement for powering up an electronic RCD for use in multiphase installations which mitigates many of the problems associated with conventional arrangements and which provides for reverse wiring of the RCD and is simple to implement, reliable in operation and cost effective.

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 (5)

1. A residual current device for use with a multiphase AC mains supply, the device comprising an electronic circuit (16) powered from the mains supply for detecting an imbalance m the current supply to a load and providing an output signal (18) when the imbalance meets certain criteria indicative of an earth fault current, and an electro-mechanical (EM) switch (12) having a set of contacts (SWl) in the mains conductors and which is connected to mains power in response to the output signal (18) for opening the contacts, the device further including means for automatically disconnecting mains power from the EM switch (12) shortly after the set of contacts (SWl) are opened even if the output signal (18) persists, the disconnecting means including a normally open further contact (SW2) in series with the EM switch (12) and mechanically independent of the set of contacts (SWl), the further contact (SW2) being closed in response to the output signal (18) and allowed to re-open shortly thereafter.

2. A device as claimed in claim 1, wherein the further contact (SW2) is the contact of a further EM switch (RLA) which is energised in response to the output signal (18) and automatically de-energised shortly thereafter even if the output signal (18) persists.

3. A device as claimed in claim 2, wherein the disconnecting means includes a capacitor (Cl) which is charged up by the mains and discharges through the further EM switch (RLA) when an electronic switch (TRI) in series with the further EM device (RLA) is closed in response to «Hflf f j the output signal (18), discharge of the capacitor (Cl) energising the further EM switch (RLA) to close the further contact (SW2), the further EM switch (RLA) being deenergised upon discharge of the capacitor (Cl) to allow the 5 further contact (SW2) to re-open.

4. A device as claimed in claim 3, wherein the electronic switch (TRI), the further EM switch (RLA) and an impedance (Rl) are connected in series across the power supply (Dl10 D8) to the electronic circuit (16), the voltage drop across the impedance (Rl) ensuring that the electronic circuit (16) remains powered when the electronic switcn (TRI) is closed and the capacitor (Cl) discharged.

5. A device as claimed in any preceding claim, wherein the power supply to the electronic circuit (16) comprises a full wave rectifier (D1-D8). Hom π Fis 1 Λ* Ή. & >* Ή c3 GO cr f ' ’ rf Cl· Ph ft CO ft ft Ol ο !1O 9 0113 SWl Test