GB2277417A

GB2277417A - AC mains monitoring circuit

Info

Publication number: GB2277417A
Application number: GB9404587A
Authority: GB
Inventors: Patrick Ward
Original assignee: Shakira Ltd
Current assignee: Shakira Ltd
Priority date: 1993-04-19
Filing date: 1994-03-08
Publication date: 1994-10-26
Anticipated expiration: 2014-03-08
Also published as: GB2277417B; GB9404587D0; IE930293A1

Abstract

An AC mains monitoring circuit has a current transformer 10 inductively coupled both to the mains through primary windings T2, T3, and to live and neutral conductors 17, 18 of a load 19 for detecting either a mains fault or an earth current fault. To discriminate between the two types of fault, an indicator such as a neon light 20 is connected across the mains earth and neutral, and responds to missing earth or neutral and to reversal of live and neutral. Detector circuit 14 will also respond to earth fault current causing unbalance. <IMAGE>

Description

AC MAINS MONITORING CIRCUIT This invention relates to an AC mains monitoring circuit.

According to the invention there is provided an AC mains monitoring circuit having means for detecting either a mains fault or an earth current fault, and an indicator which discriminates between detection of the two types of fault.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a circuit diagram of an AC mains monitoring circuit as described in our UK patent application number 9116888; Figure 2 is a circuit diagram of an AC mains monitoring circuit according to the invention; and Figure 3 are circuit diagrams illustrating the operation of the circuit of figure 2.

The AC mains monitoring circuit shown in figure 1 is fully described in our UK patent application number 9116888, whose contents are incorporated herein by this reference.

Briefly, however, by monitoring the current flowing in the secondary winding T1 of a current transformer 10 the circuit can detect a missing earth condition, a missing neutral condition, or a difference between the earth and neutral potentials. This is because in the absence of a mains fault the effects of the currents I2 and I3 flowing in the primary windings T2 and T3 are equal and opposite so that the vector sum of the currents induced into the secondary winding T1 by the currents flowing in these windings is zero (Ir = 0), whereas when any of the above conditions occur one or other of I2 and I3 is missing or different from the other, so that Ir is then not equal to zero. A non-zero Ir is detected in a detecting circuit 14, which provides an alarm and/or disconnects an associated circuit from the mains when the level of Ir rises above a threshold.

In the embodiment of the present invention, figure 2, the monitoring circuit of figure 1 is adapted to detect, in addition to an incorrectly wired mains supply, the presence of an earth current fault. This is achieved by passing through the core 12 of the current transformer 10 the live and neutral conductors 17 and 18 to a load 19.

In the absence of an earth fault current the currents in the live and neutral conductors 17 and 18 will be the same, and since they pass in opposite directions through the core 12 the net effect on the induced current Ir will be zero. In other words, the vector sum of the currents induced into the secondary winding by the live and neutral conductors 17 and 18 is zero. However, an earth fault current will cause an out of balance current in the conductors 17 and 18 which will induce a non-zero current Ir. This can be detected in the detecting circuit 14.

This provides for comprehensive monitoring of the mains by detecting both earth currents in a load and mains wiring faults. However, a potential problem with the arrangement is that although the alarm will be raised and/or the mains disconnected in the case of either fault condition, the user will have no means of determining whether due to an earth current fault or due to a mains wiring fault.

If for example the mains monitoring circuit were provided in the form of a hard wired mains plug which was connected to an appliance, the alarm would be activated as soon as the plug was inserted into the mains socket regardless of which fault condition existed. If the appliance was fitted with an isolation switch, then it might be possible to determine if the fault was due to an earth current in the appliance or due to a mains fault. However, many appliances are not fitted with isolation switches, and those that are usually have single pole isolation switches which could not prevent an earth fault current continuing to flow in the appliance if the mains socket or plug were wired incorrectly.

The circuit of figure 2 therefore includes means to overcome this potential problem of fault discrimination in that it provides a means of determining whether the alarm has been activated due to a mains fault or due to an earth current fault. This is achieved by connecting an indicator 20 across the earth and neutral conductors, such indicator being activated in the case of a mains wiring fault condition. As shown, the indicator 20 is a neon light in series with a current limiting resistor Rn, but any device which can be activated in the case of a mains wiring fault condition can be used, such as an LED, a buzzer, or a relay.

Under normal conditions, the earth and neutral are at the same potential, and the neon light 20 will not be activated. However, if the earth or neutral is disconnected, or the live and neutral are reverse wired, a voltage will appear across the neon light 20 causing it to be activated. On the other hand, if the mains is correctly wired but an earth fault current flows in the load, the neon light 20 will not be activated. In either case a non-zero current Ir will be induced in the secondary winding T1, Therefore, the neon light 20 indicates mains wiring fault conditions as distinct from earth fault current conditions.

In the case of a loss of earth or a loss of neutral condition, the neon 20 will light by virtue of the current transformer 10 which provides the basic mains monitoring circuit, even though the neon 20 may appear to be floating under such a condition. The operation of the neon light 20 can be seen more clearly from figure 3, which shows more clearly the current path for the neon light 20 under such conditions.

In the case of a missing earth condition, figure 3a, there exists two current paths from live to neutral, via T2 and R2 and via T3, R3, Rn and the neon light 20. In this case the neon light is lit due to the current path provided by T3.

In the case of the missing neutral condition, figure 3b, there exists two current paths from live to earth, via winding T3 and R3, and via T2, R2, the neon light 20 and Rn. In this case, the neon light is lit due to the current path provided by T2.

The switches 52 and 53 facilitate testing of the overall operation of the device, and the correct functioning of the mains fault indicating device. These switches are normally closed. If either switch is opened it will simulate a mains fault by disconnecting the earth or the neutral. This is turn will cause the mains fault indicator 20 to be activated, and will also cause an out of balance current to flow in the toroid windings with a resultant output from T1.

The detecting circuit 14 includes a device sensitive to the current Ir, such as an RCD, a relay, or a permanent magnet, to provide a means of responding to any of the four fault conditions, that is, the three mains wiring fault conditions (i.e. missing earth, missing neutral and reverse live-neutral) and the earth current condition, by raising an alarm and/or disconnecting a circuit from the mains.

Claims

CLAIMS:

1. An AC mains monitoring circuit having means for detecting either a mains fault or an earth current fault, and an indicator which discriminates between detection of the two types of fault.

2. A monitoring circuit for an AC mains having earth, live and neutral, the circuit including a current transformer having a plurality of primary windings and a secondary winding, wherein the primary windings are connected to the AC mains such that in the absence of a mains fault the vector sum of the currents induced into the secondary winding by the currents flowing through the primary windings is zero, and wherein live and neutral conductors for a load are also inductively coupled to the transformer such that in the absence of an earth current fault the vector sum of the currents induced into the secondary winding by the live and neutral conductors is also zero, the circuit further including means for detecting a mains fault or an earth current fault by detecting a current induced into the secondary winding, and indicator means connected across mains earth and neutral which discriminates between detection of an earth current fault and a mains fault.

3. A circuit as claimed in claim 1, wherein there are two primary windings of which one is connected between earth and live and the other is connected between live and neutral, the two primary windings being connected in antiphase and having the same number of ampere-turns.

4. A circuit as claimed in claim 1 or 2, wherein the detecting means includes means to raise an alarm and/or disconnect a circuit from the A.C. mains when the induced current exceeds a reference level.

5. A circuit as claimed in claim 1, 2 or 3, further including a normally-closed test switch connected in series with at least one of the primary windings.

6. A circuit as claimed in any preceding claim, wherein the indicator is a neon light.

7. An AC mains monitoring circuit substantiallv as described with reference to Figures 2 and 3 of the accompanying drawings.