GB2251992A

GB2251992A - Missing earth or neutral mains monitor

Info

Publication number: GB2251992A
Application number: GB9116888A
Authority: GB
Inventors: Patrick Ward
Original assignee: Shakira Ltd
Current assignee: Shakira Ltd
Priority date: 1991-01-17
Filing date: 1991-08-06
Publication date: 1992-07-22
Anticipated expiration: 2011-08-06
Also published as: GB9116888D0; GB2251992B; ZA911358B; IE910159A1; IE64082B1

Abstract

An AC mains monitor comprises a core 12 with counter-acting windings T2, T3 exerting a normally equal mmf. If an unbalance occurs between the L.N and L.E currents I2, I3, due to, for example, a missing earth or neutral. a resultant Ir is induced into the detector 14 to initiate an alarm or supply disconnection. Test switches S2, S3 are normally closed. <IMAGE>

Description

Title: A. C. MAINS MONITORING CIRCUIT P02969GB This invention relates to an A.C. mains monitoring circuit.

According to the invention there is provided an A.C. mains monitoring circuit including a current transformer having at least two primary windings and a secondary winding, the primary windings being connected to the A.C. 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, the circuit further including means for detecting a current induced into the secondary winding.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing whose single figure is a circuit diagram of a mains monitoring circuit.

The circuit comprises a current transformer 10 having a core 12 on which are wound three windings T1, T2 and T3. T2 and T3 are primary windings, and T1 is a secondary winding. One end of T2 is connected to mains live, L, and the other end is connected to mains neutral, N, via a resistor R2. This part of the arrangement is referred to as the Neutral circuit.

One end of T3 is also connected to mains live, L, and the other end is connected to earth, E, via a resistor R3. This part of the arrangement is referred to as the Earth circuit.

Resistors R2 and R3 provide external resistance additional to the resistance of the windings T2 and T3, to control the level of the currents flowing in these windings.

Winding T1 is connected to a monitoring means 14 responsive to any current flowing in winding Ti.

In winding T2, a current I2 will flow from L to N via R2. In the absence of any current flowing in T3, I2 will on its own induce a current Ir into winding T1 which will be detected by the monitoring means 14.

In winding T3, a current I3 will flow from L to E via R3. In the absence of any current flowing in winding T2, I3 will on its own induce a current Ir into winding T1 which will be detected by the monitoring means 14.

If the earth E and neutral N are at the same potential, which they should be under normal mains conditions in accordance with the wiring practices of many European countries such as in T-N installations, I2 will have the same magnitude as I3 provided that the total resistance of R2 and T2 equals that of R3 and T3, since both currents are derived from the same live potential L.

The windings T2 and T3 are wound on the current transformer in opposite directions such that the currents I2 and I3 are in anti-phase relationship. Then, in the absence of a mains fault (I2 = I3), and assuming that there are the same number of turns in each of the primary windings T2 and T3, the vector sum of the currents induced into the secondary winding T1 by the currents I2 and I3 flowing through the windings T2 and T3 is zero.

If the earth E connection is now removed, I3 will cease to flow, and I2 will be the value of net current flowing in windings T2 and T3, and a resultant non-zero current Ir will be induced into winding T1 which will be detected by the monitoring means 14.

Thus, the mains monitoring circuit can detect a missing earth condition.

If the neutral N connection is now removed, but the earth connection remains connected, I2 will cease to flow, and I3 will be the value of net current flowing in windings T2 and T3, and a resultant non-zero current Ir will be induced into winding T1 which will be detected by the monitoring means 14.

Thus, the mains monitoring circuit can detect a missing neutral condition.

Whilst the earth E and neutral N will normally be at the same potential, conditions can arise in the mains where the neutral rises to some potential greater than earth potential. Where the difference in earth and neutral potentials becomes significant, it can be desirable to detect such conditions. If the earth and neutral are not at the same potential, I2 will not equal I3, with the result that the vector sum of the two currents induced in T2 will be greater than zero. The resultant net current Ir can be detected by the monitoring means 14.

Thus, the mains monitoring circuit can detect a difference between earth and neutral potentials.

A normally-closed switch S2 is connected into the neutral circuit as shown. Because this switch is normally closed it has no bearing on the detection of missing earth, missing neutral, or high neutral potential with respect to earth. However, by opening switch S2, I2 can be made to cease to flow, I3 will now be the net current flowing in windings T2 and T3, and a resultant current Ir will flow in winding Ti which will be detected by the monitoring means 14.

Thus, by provision of a test switch 52, the integrity and correct functioning of the earth circuit, windings T3 and T1, and the monitoring means can be checked.

A normally-closed switch S3 is connected into the earth circuit as shown. Because this switch is normally closed it has no bearing on the detection of missing earth, missing neutral, or high neutral potential with respect to earth. However, by opening switch S3, I3 can be made to cease to flow, I2 will now be the net current flowing in windings T2 and T3, and a resultant current Ir will flow in winding T1 which will be detected by the monitoring means 14.

Thus, the mains monitoring circuit incorporates a test facility to check the integrity and correct functioning of the circuit and its associated components.

The monitoring means 14 detects the presence of current Ir induced into winding T1, and is responsive to this current.

Current Ir can be monitored and measured by the monitoring means 14, which can produce an output signal equal or proportional to the magnitude of Ir, thereby providing a means of determining the magnitude of the non-zero vector sum current Ir induced. Thus the mains monitoring circuit can be used to measure the magnitude of an out of balance current which causes Ir to be generated, such out of balance current being due to any of the aforementioned reasons, or due to an additional current conductor being passed through the transformer core 12 for monitoring purposes.

The induced current Ir monitored and measured by the monitoring means 14 can be compared to an internal reference, and if above a predetermined level, an output signal can be generated to activate an alarm and/or a tripping mechanism to cause disconnection of a circuit connected to the mains.

In the foregoing embodiment of the invention it has been assumed that the total resistance in the Earth and Neutral circuits is the same, and also that there are the same number of turns in each of the primary windings T2 and T3. However, it is only necessary that the number of ampere-turns in each of the Earth and Neutral circuits be the same in the absence of a mains fault. Thus the total resistance in the Earth and Neutral circuits may be different provided the number of turns and T2 and T3 are also made non-equal as necessary to make the number of ampere-turns the same. This will still mean that, in the absence of a mains fault, the vector sum of the currents induced into the secondary Ti will be zero.

Claims

CLAIMS:

1. An A.C. mains monitoring circuit including a current transformer having at least two primary windings and a secondary winding, the primary windings being connected to the A.C. 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, the circuit further including means for detecting a current induced into the secondary winding.

2. A circuit as claimed in claim 1, wherein the mains is a single phase A.C. mains with earth, live and neutral, and 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.

3. 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.

4. 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.

5. An A.C. mains monitoring circuit substantially as described with reference to the accompanying drawings.