GB2091976A

GB2091976A - Electricity supply signalling

Info

Publication number: GB2091976A
Application number: GB8039943A
Authority: GB
Original assignee: ECS ENERGY CONVERSATION SYSTEM
Current assignee: ECS ENERGY CONVERSATION SYSTEM
Priority date: 1980-12-12
Filing date: 1980-12-12
Publication date: 1982-08-04
Also published as: GB2091976B

Abstract

A method of and apparatus for supplying an alternating signal to an electricity supply system wherein the neutral tapping on the supply transformer (T) is connected to earth and alternating signals are induced in the live (L) and neutral (N) conductors of the system by means of a magnetic circuit (M) which encompasses the live and neutral conductors between the supply transformer and the load, the alternating signal being generated by a generator (G) and supplied to a winding (W) on the circuit; the signals cancel one another and are indetectable in the electrical circuit comprising the live and neutral conductors and the load but are detectable by capacitive means between the live or the neutral conductor and earth. The method is also applicable to three-phase supply systems (Fig. 3 not shown). <IMAGE>

Description

SPECIFICATION Electricity supply signalling This invention relates to a method and apparatus for applying a detectable signal to a normal alternating electricity supply system.

It is known to apply a signal between one of the live conductors or the neutral conductor and earth of a normal electricity supply system which may, for example, be used to switch street lighting, or to supply a domestic intercom or baby alarm. However, this known method has the disadvantage that the signal can affect other apparatus connected to the same supply system and electricity supply boards have stringent regulations concerning these signailing methods.

It is an object of the present invention to provide a method of supplying a signal to an electricity supply system that is not detectable by apparatus normally connected to the system but is, nevertheless, readily detectable by suitable means.

According to the present invention, a method of applying a signal to an electricity supply system comprises applying a signal to the live or line conductor of the system and applying the opposite signal to the neutral conductor so that the two signals cancel one another within the system to be indetectable by a detector connected between the live conductor and the neutral conductor whilst the signal is detectable by a detector connected between the live conductor and earth or the neutral conductor and earth.

A preferred method for an alternating signal is to encompass the live conductor and the neutral conductor by a magnetic circuit and apply the alternating signal to a winding around the magnetic circuit, whereby alternating signals are induced in the live and neutral conductors that have opposed phases and cancel one another within the electrical circuit formed by the live and neutral conductors.

The apparatus for the preferred embodiment of this invention is clearly different from that of known earth current leakage circuit breakers. In the latter, a magnetic circuit again encompasses the live and neutral conductors of an electricity supply and the magnetic circuit has a winding connected to the relay of a contactor connected to isolate the supply.

The breakdown to earth of either the live or neutral conductors or apparatus connected thereto causes an imbalance between the live and neutral conductors, setting up a magnetic flux in the magnetic circuit that, in turn, induces a voltage in the winding to trip the conductor. This is clearly different from, and indeed is opposite to, the method of the present invention.

The present invention is illustrated, by way of example, on the drawing, wherein: Figure 1 is a circuit diagram of apparatus in accordance with the invention applied to a single phase electricity supply; and Figure 2 is a similar circuit diagram but for a three phase supply.

As shown by Fig. 1, an electricity supply transformer T has a live or line conductor L and a neutral conductor N connected to a load. The neutral tapping on the transformer is also connected to earth and to an earth conductor that is associated with the load.

The live and neutral conductors L and N only are arranged to pass through a hollow magnetic core, such as an annular soft iron ring, forming a magnetic circuit or magnetic signal injector M. A winding W on the signal injector ring is connected to an alternating signal generator G.

The generator induces, by means of the transformer effect of the signal injector M, a first alternating signal in the live conductor L and a second signal in the neutral conductor N of the same amplitude but opposite phase to that of the first signal. These two signals cancel one another in the secondary winding of the supply transformer T and are thus indetectable on the supply side of the signal injector. The two signals also cancel one another for any load connected between the line and neutral conductors.

However, the first or the second signal is detectable with respect to earth, in the example shown the signal detector is a capacitor C connected between the neutral and earth conductors on the load side of the signal injector M and the alternating signal from the generator is detected by the capacitor C but not by the load. Connections between the live conductor and the earth conductor would result in the signal superimposed on the alternating supply voltage being detected. It thus seems as if the earth conductor is carrying the signal.

Although electricity supply board regulations forbid connections between the neutral and earth conductors, an exception is made for radio interference supression capacitors and it is thought that the use of a capacitor, having a value well within the maximum for a radio interference suppressor, would be acceptable.

The signal injector M is of low impedance and for the range of frequencies currently envisaged, 2 to 10 kilohertz, a large number of capacitive signal detectors could be operated from one signal injector before the sum of the detector and stray capacitances become sufficiently high to produce unacceptable attenuation of the signal.

Fig. 2 shows a similar apparatus except that all three live conductors a, b and c of a three phase supply pass through the signal injector M. Signals of equal amplitude but of such differing phases so as to cancel one another are induced in the live and neutral conductors.

There is one condition needed for the invention to operate and this is that the electrical supply system must be in accordance with l.E.E. regulations which stipulate that the neu tral line must only have a single pick up point and that must be at the supply transformer.

Although the invention has been described with reference to simple alternating signals, any signal that can have an opposite cancelling characteristic could be used; for example a D.C. signal of a given voltage could be added to the live conductor and a D.C. signal of the same voltage but opposite sign could be added to the neutral conductor or A.M., F.M. or pulse signals could be applied.

The special advantages of this signal injector are that signals induced in the conductors running through the injector inherently have opposing phases, there is no direct electrical contact between the signal generator and the conductors and low power signals can be detected anywhere on the load side of the injector provided that there is no intervening transformer.

In electricity supply systems having more than one live conductor, signals may be induced in only one of, some of or all of the live conductors; provided only that the signals induced in the live conductors and the neutral conductor cancel one another.

Claims

1. A method of applying a signal to an electricity supply system comprising applying a signal to the live or line conductor of the system and applying the opposite signal to the neutral conductor so that the two signals cancel one another within the system, to be indetectable by a detector connected between the live conductor and the neutral conductor whilst the signal is detectable by a detector connected between the live conductor and earth or the neutral conductor and earth.

2. A method as claimed in claim 1, and for an alternating signal, wherein the live conductor and the neutral conductor are encompassed in a magnetic circuit and the alternating signal is appled to a winding around the magnetic circuit, whereby alternating signals are induced in the live and neutral conductors that have opposed phases and cancel one another within the electrical circuit formed by the live and neutral conductors.

3. Apparatus for applying a signal to an electricity supply system comprising means to apply the signal from a signal generator to the live conductor of the system, means to obtain the opposite signal from the signal generator and apply it to the neutral conductor and detector means connected between the live conductor and earth or the neutral conductor and earth to detect the signal.

4. Apparatus as claimed in claim 3, wherein a magnetic circuit is provided to encompass the live and neutral conductors and the signal generator is connected to a winding on the magnetic circuit whereby the signal from the signal generator can be induced, with differing cancelling phases in the conductors.

5. Apparatus as claimed in claim 3 or claim 4, wherein the detector means is a capacitor connected between the neutral conductor and earth.

6. A method of applying a signal to an electricity supply system substantially as de scribe.

7. Apparatus for supplying an alternating signal to an electrical supply system substantially as described with reference to or as shown by the Drawings.

7. Apparatus for applying a signal to an electricity supply system substantially as described with reference to or as shown by the drawings.

CLAIMS (9 Dec 1981)

1. A method of applying an alternating signal to an electricity supply system between a supply transformer and a load, the neutral tapping on the transformer being connected to earth comprising encompassing the live conductor and the neutral conductor in a magnetic circuit and applying the alternating signal to a winding around the magnetic circuit whereby signals are induced in the line and neutral conductors that cancel one another to be indetectable within the electrical circuit formed by the live and neutral conductors and the load, whilst the signal is detectable between either the live or the neutral conductor and earth.

2. Apparatus for applying an alternating signal to an electricity supply system between a supply transformer and a load, wherein the neutral tapping on the supply transformer is connected to earth, comprising a magnetic circuit encompassing the live conductor and the neutral conductor, signal generating means connected to a winding on the magnetic circuit and detector means connected between either the live conductor and earth or the neutral conductor and earth to detect the signal generated by the generating means.

3. Apparatus as claimed in claim 2, wherein there are two or more live conductors and all the live conductors and the neutral conductor, are encompassed by the magnetic circuit, the detector means being connected between any of the live conductors and earth or the neutral conductor and earth.

4. Apparatus as claimed in either claim 2 or claim 3, wherein the magnetic circuit is a ferro-magnetic ring, the live and neutral conductors pass axially through the centre of the ring and the signal winding is wound around the ring body.

5. Apparatus as claimed in any of claims 2 to 4, wherein the detector means includes a capacitor.

6. A method of supplying an alternating signal to an electrical supply system substantially as described.