GB2224404A

GB2224404A - Residual current device with missing neutral protection

Info

Publication number: GB2224404A
Application number: GB8908385A
Authority: GB
Inventors: Patrick Ward
Original assignee: Shakira Ltd
Current assignee: Shakira Ltd
Priority date: 1988-10-25
Filing date: 1989-04-13
Publication date: 1990-05-02
Anticipated expiration: 2009-04-13
Also published as: GB8908385D0; ZA892913B; GB2224404B

Abstract

A residual current device (RCD) includes a circuit for disconnecting an AC supply in response to a missing neutral condition on the supply side of the RCD. The circuit includes a bridge rectifier (BR) connected across the live (L) and neutral (N) conductors of the AC supply, and a silicon controlled rectifier (SCR2) whose anode and gate are connected across the DC output side of the bridge rectifier and whose cathode is connected to earth. SCR2 is triggered by the bridge rectifier DC output upon the occurrence of a missing neutral condition to connect the live conductor to earth, and a solenoid, S responsive to the resultant current flowing between the live conductor and earth operates a switch SW to disconnect the AC supply. <IMAGE>

Description

RESIDUAL CURRENT DEVICE This invention relates to a circuit to detect the absence of the neutral connection on the supply side of a residual current device (RCD). Such absence can be caused by, for example, a disconnection or break.

A typical RCD protection circuit known in the prior art is shown in Figure 1. On the load side of the RCD the neutral and live conductors N and L respectively are passed through a current transformer TR. Under normal and balanced load conditions the current IL flowing on the live conductor L from the supply to the load is equal to the current IN flowing on the neutral conductor N back from the load to the supply. There are, therefore, equal and opposite currents flowing through the transformer TR, so that the net current flow through the transformer is zero.

However, if a ground fault occurs on the load side of the RCD, there will be some current flow to ground, the ground fault current IF, leading to an imbalance in the currents IL and IN flowing in the live and neutral conductors L and N.

In this condition, IL - IN + IF.

When IL is not equal to IN, as in the ground fault condition, there will be a resultant differential current flowing through the current transformer TR.

This differential current excites the transformer, resulting in an output signal therefrom. This output is developed across a resistor R2 and measured by the electronic circuitry including integrated circuit ICi.

If the fault current exceeds a predetermined level, the signal developed across R2 will cause a silicon controlled rectifier SCR1 to be triggered (turned on) by IC1.

SCR1 is connected across the output terminals of a bridge rectifier BR whose input terminals are connected on the one hand to the live conductor L via a solenoid S and on the other hand to the neutral conductor N.

When SCR1 is triggered a connection is established across the live and neutral conductors L and N via the solenoid S, the bridge rectifier BR and SCR1. The solenoid S is therefore energised, which removes power from the load by opening switches SW in the live and neutral conductors.

This describes the basic operation of the RCD of Figure 1 in sufficient detail for the purposes of the present invention.

The above RCD circuit suffers from one serious drawback. If the Neutral conductor N is broken or disconnected on the supply side of the RCD circuit, the RCD circuit will not be energised. A ground fault current could still flow from the Live conductor L to ground. Whilst this condition will cause a resultant output signal from the current transformer TR, the electronic circuit will not be able to respond to the fault condition because it will not be powered up.

Thus the solenoid S will not be activated to disconnect the mains supply from the load, and a potentially lethal fault current will be allowed to continue to flow in the circuit.

To overcome this problem, so-called "missing neutral" detection circuits have been developed to detect this condition. However, there are problems associated with existing missing neutral detection circuits. These are mainly: 1. The missing neutral detection circuit responds instantaneously to a missing neutral condition, disconnecting the mains supply from the load.

There can arise situations where there is a momentary break or disconnection in the neutral circuit, eg, where the live and Neutral conductors are connected via a two pole isolating switch or contactor. If there is a slight misalignment between the two poles of the switch such that the live pole is connected momentarily before the neutral pole, it can appear that the Neutral conductor is missing or broken, activating the missing neutral detection circuit.

2. Abnormal line conditions frequently occur between Live, Neutral and Earth conductors. Examples of these conditions are, voltage surges, transients, dips, spikes, brown out, black out, etc. These conditions can be of very short duration, yet still result in activation of the missing neutral detection circuit, because of an apparent missing neutral condition.

3. Some missing neutral detection circuits employ a large number of electronic components to detect the missing neutral condition. This high component count has three drawbacks. These are: a) Substantially increased cost of the RCD.

b) Increased space requirements of the RCD.

c) Reduced reliability of the RCD.

It is therefore an object of the invention to provide an RCD with an improved missing neutral detection circuit, in which the aforesaid disadvantages are mitigated or overcome.

Accordingly, the invention provides a residual current device (RCD) including circuit means for disconnecting an AC supply in response to a missing neutral condition on the supply side of the RCD, the circuit means including a bridge rectifier connected across the Live and Neutral conductors of the AC supply, a silicon controlled rectifier (SCR) whose anode and gate are connected across the DC output side of the bridge rectifier and whose cathode is connected to earth, the SCR being triggered by the bridge rectifier DC output upon the occurrence of a missing neutral condition to connect the Live conductor to earth, and means responsive to the resultant current flowing between the Live conductor and earth to disconnect the AC supply.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram, previously described, of a known residual current device (RCD), Figure 2 is a block diagram of an RCD incorporating an embodiment of a missing neutral detection circuit according to the invention, and Figures 3 to 6 are waveform diagrams illustrating the operation of the circuit of Figure 2.

Referring to Figure 2, the new missing neutral detection circuit consists of just four components added to the circuit of Figure 1, as indicated at the top of Figure 2. These are a second silicon controlled rectifier SCR2 whose anode is connected to the +ve terminal of the output of the bridge BR and whose cathode is connected via a diode D to Earth, a resistor R3 connected between the gate of SCR2 and the -ve terminal of the output of the bridge BR, and a capacitor C4 connected between the gate and cathode of SCR2.

It is commonly known that when the Live and Neutral conductors are connected to a bridge rectifier, full wave rectification of the AC mains supply will result at the DC side of the bridge. The voltage output from the bridge will be approximately as shown in Figure 3, for a 240 volt/50 hz AC supply. This signal is as measured across the +ve and -ve terminals of the bridge.

It is also commonly known that the neutral terminal is normally at the same potential as earth, and the live terminal will be at full mains potential with respect to earth.

If we look at the output signals from the bridge +ve and -ve terminals with respect to earth under normal conditions, we get signals as shown in Figure 4.

If the neutral terminal is now disconnected from the bridge rectifier, the signal at the bridge +ve and -ve terminals changes to that shown in Figure 5.

It will be noted that the output from the +ve and -ve terminals of the bridge rectifier are now the same, both being the full AC mains cycle.

Referring back to Figure 2, in order to make the SCR2 conduct, it is essential that the anode and gate are simultaneously positive with respect to the cathode.

As the waveform at the -ve terminal of the bridge rectifier BR comprises both positive and negative half cycles of mains voltage during a missing neutral condition, the positive half cycles can be used to gate the SCR2, provided the cathode of the SCR2 is referenced to earth, as shown. Simultaneously, the positive half cycles at the +ve terminal of the bridge can be used to make the SCR2 anode positive with respect to the cathode. As the peak value of the positive voltage on the bridge -ve terminal can have a value up to 340 volts, it is essential that the SCR2 gate circuit is protected from this voltage level.

This is done by connecting the high ohmic value resistor R3 between the gate and the bridge -ve terminal. Usually, a diode will also be connected in series with the resistor, to prevent the negative half cycles from damaging the gate circuit. However, by making R3 a very high ohmic value, and using the diode inherent in the SCR gate/cathode junction, the need for an external diode can be overcome. The inherent gate/cathode diode will conduct during positive half cycles of the voltage at the bridge -ve terminal, turning on the SCR2. During negative half cycles, the inherent gate/cathode diode will perform a blocking function, preventing turn on of the SCR2.

The purpose of the capacitor C4 is to provide a finite time delay before the SCR2 turns on. If the capacitor was not present in the gate circuit, the SCR2 would turn on almost instantly after its anode and gate went positive with respect to its cathode. This very fast turn on would give rise to nuisance tripping where there was a momentary break in the neutral circuit, or adverse line conditions simulating an apparent break in the neutral circuit. By connecting the capacitor in the gate/cathode circuit, the gate voltage can now only increase in value at a rate determined by the time constant of C4 and R3. The larger the value of C4 the longer the time constant.

The gate voltage level required to turn on the SCR2 is approximately 0.6 volts. The peak value of the positive going voltage signal at the bridge rectifier -ve terminal is approximately +340 volts, at a mains supply of 240 volts.

As the peak voltage of +340 volts is over 550 times greater than the nominal gate voltage required to turn on SCR2, to all intents and purposes the positive going half cycle can be regarded as a positive square wave, of amplitude 340 volts.

The capacitor C4 now sees an aiming voltage of +340 volts, towards which it will charge through R3.

If C4 could continue to charge through R3 at its initial charging rate, the time taken for C4 to charge up to +340 volts would be C4 x R3 seconds. For a value of C4 of 4.7uF, and R3 of lMohm, the charging time would be 4.7 seconds. We know that once the gate voltage exceeds approximately 0.6 volts, SCR2 will turn on. Therefore we can calculate the SCR2 turn on time as follows.

SCR2 turn on time 4.7 x 0.6 x 1000 ----------------mSECONDS,- 8.3mSEC.

340 By reducing the value of C4 or R3, a shorter time constant, and hence a shorter SCR2 turn on time, can be achieved. By increasing the value of C4 or R3, a longer turn on time can be achieved. In general, by suitable choice of C4 and R3, the speed of response can be varied from almost instantaneous to several cycles of the main supply, to a missing neutral condition.

This effect is demonstrated in Figure 6, and provides simple but precise control of the minimum turn on time of the SCR2, and thus the minimum speed of response to a missing neutral condition.

Through extensive testing, it was found that by providing a minimum delay of 5 mSec, most conditions of nuisance tripping, due to abnormal conditions in the neutral circuit could be overcome. In practice, there is a 50% probability of a further lOmSec delay, due to the inability of the SCR2 to conduct during alternate negative half cycles of the mains supply. Thus in practice, there will be a delayed response to missing neutral conditions of 5 to 20 mSec.

The purpose of the diode D is to provide a DC reference for the capacitor C4 to charge. When the voltage at the bridge -ve terminal goes positive, the anode of D, and thus the bottom of C4, is held at a maximum voltage of approximately 0.7 volts above earth potential. Therefore, C4 can start to charge up through R3 towards +Vpeak. When the voltage at the -ve terminal of the bridge goes negative, the diode D is reversed biased, and its anode can go negative in harmony with the voltage at the bridge -ve terminal.

Therefore, C4 holds its already acquired charge. When the voltage at the bridge -ve terminal goes positive again, C4 continues to charge positively through R3, until its charge voltage exceeds the SCR2 gate voltage, at which point the SCR2 turns on.

The operation is as follows.

During normal conditions, with both the Live and Neutral conductors L and N connected to the RCD on the supply side, the output of the bridge rectifier +ve and -ve terminals with respect to Earth is as shown in Figure 4. The voltage signal at the -ve terminal does not go sufficiently positive to allow the gate voltage of SCR2 to reach the turn on level. When the Neutral conductor is disconnected, the voltage at the -ve terminal can rise to approximately +340 volts on positive half cycles of the AC mains supply. C4 starts to charge through R3 towards +Vpeak. When Vc exceeds Vg, SCR2 turns on. This effectively connects the solenoid S directly between Live and Earth terminals.

The sudden increase in current flow through the solenoid provides an energising voltage in the solenoid which activates the tripping mechanism, and disconnects the Live and Neutral conductors from the load. Thus the risk of a dangerous fault current flowing to ground during a missing neutral condition is eliminated.

Although the foregoing uses an SCR as the switching means for the missing neutral detection current, a triac or transistor or other suitable gated electronic switch may be used.

Claims

CLAIMS:

1. A residual current device (RCD) including circuit means for disconnecting an AC supply in response to a missing neutral condition on the supply side of the RCD, the circuit means including a bridge rectifier connected across the live and neutral conductors of the AC supply, an electronic switch having input and output terminals and a control terminal for turning said switch on and off, the input and control terminals being connected across the DC output side of the bridge rectifier and the output terminal being connected to earth, the electronic switch being turned on by the bridge rectifier DC output upon the occurrence of a missing neutral condition to connect the live conductor to earth, and means responsive to the resultant current flowing between the live conductor and earth to disconnect the AC supply.

2. A residual current device as claimed in Claim 1, wherein the electronic switch is a semiconductor controlled rectifier whose anode and cathode respectively constitute the said input and output terminals and whose gate constitutes the control terminal.

3. A residual current device as claimed in Claim 1 or 2, wherein the means for disconnecting the AC supply comprises a solenoid-operated switch responsive to the current flowing between the live conductor and earth.

4. A residual current device as claimed in any preceding claim, further including time delay means for controlling the speed of response of the electronic switch.

5. A residual current device as claimed in Claim 4, wherein the time delay means includes a resistor connected between the control terminal of the switch and the bridge rectifier.

6. A residual current device as claimed in Claim 4 or 5, wherein the time delay means includes a capacitor connected between the control and output terminals of the switch.

7. A residual current device as claimed in Claims 1 to 6, further including a diode connected between the output terminal of the switch and earth.

8. A residual current device substantially as described with reference to Figure 2 of the accompanying drawings.