IES86060Y1 - Leakage current detector - Google Patents

Abstract

ABSTRACT An apparatus for detecting a leakage current flowing between at least one Supply conductor (L or N) of a multi-conductor electricity supply to a load (12) and a non- load current carrying electrical conductor (K) nominally insulated from the at least one supply conductor is disclosed. The apparatus includes a relay (RLA) having a set on contacts (SW) in series with the electricity supply and a movable component (actuator) (100) operable to open and close the contacts (SW). The actuator (100) is resiliently biased towards a first position, wherein the contacts (SW) are open, against the electromagnetic force generated by a first coil (A) which acts in opposition to the resilient bias and whose greater force normally holds the actuator (100) in a second position wherein the contacts (SW) are closed. The actuator (100) is released and moved by the resilient bias to the first position when the holding force of the coil (A) is sufficiently weakened by the electromagnetic force generated by a current flowing in a second coil (B) when a leakage current meeting certain criteria as to magnitude and/or duration flows between the at least one supply conductor (L or N) and the non- load current carrying conductor (K).

Description

This invention relates to a leakage current detector (LCD).

Leakage Current Detector Over recent years there has been a dramatic growth in the use of leakage current detectors fitted on mains plugs, extension cords and appliances. These devices now form a family of products including but not limited to the following: LCDI — leakage current detection interrupter." ALCI — appliance leakage current circuit interrupter.

IDCI — immersion detection circuit interrupter.

Although the applications are slightly different, in all ‘cases the device is arranged to detect the presence of a" dangerous or unwanted leakage current flowing from a load current carrying electrical conductor (e.g. mains supply 'live or neutral) to'a non load current carrying electrical conductor nominally insulated from the load current carrying conductor. The non load current carrying conductor may be a shield, screen or wire, or it may be an electrically conductive chassis, housing or other component containing or associated with electrical equipment, for example, the chassis or body of an electric vehicle. The non load current carrying conductor will be referred to herein as conductor K.

It should also be noted that LCDI type products can also detect arc fault currents in accordance with ULl699, so their application is not strictly limited to leakage current detection.

For convenience, the above devices are referred to in this document simply as leakage current detectors (LCDs).

Figure 1 is an example of a very simple leakage Current detector.

In the arrangement of Figure 1, an electric cable comprises AC mains supply conductors L (live), N (neutral) and E (earth) surrounded by a conductive screen K incorporated within the cable, the screen being a non load current carrying electrical conductor. The cable supplies a load '12 which is contained within an earthed housing, casing or . other conductive exterior component 12A surrounding the load 12. In some cables the individual mains conductors are surrounded by individual screens. .Both arrangements iare well known and common practice in.the field of leakage 2 current detection.

The leakage current detector circuit comprises a solenoid a diode D1, a SOL, a silicon controlled rectifier SCR1, resistor R1 and a capacitor C1 connected as shown. solenoid SOL is coupled to a set of normally closed contacts SW in the mains live L and neutral N conductors, the solenoid SOL and contacts SW being configured as an electromechanical relay. For the purposes of the present specification an electromechanical relay is an electrical switch with mechanical contacts which are operated by a magnetic field produced by current flowing in a coil, usually a solenoid.

In Figure 1 the solenoid SOL may comprise a plunger positioned within a coil and when a current of sufficient magnitude flows through the coil the plunger is moved from a first position to a second position with sufficient force to cause automatic opening of the contacts to which it is coupled. The force required to open the contacts is produced solely by the current flowing through the solenoid coil, and such automatic opening of the contacts is referred to as actuation.

In the event of an insulation breakdown between the live conductor L and the screen K, indicated by the zigzag arrow, a fault current will flow via R1 into the gate of’ the SCR and back to the supply N via diode D1. When this -current exceeds a certain threshold the.SCR1 will-turn on" .and.connect the solenoid SOL across the.live L and neutral- N of the mains supply, thereby activating the solenoid SOL and causing the contacts SW to open and disconnect the supply from the load 12. Any leakage current that exceeds the threshold is deemed to be a fault Current arising from a breakdown in insulation. Such fault currents could result in an electrical fire if not detected and terminated within a certain period of time. Thus the primary role of a leakage current detector is to detect a leakage current above a certain threshold and to terminate the flow of such current within a specified period of time.

The prior art circuit typically includes a test circuit comprising a manually closable switch 14 and series resistor Rt. Pressing the switch 14 simulates a leakage current from the live conductor to the screen 10.

Figure 1 is configured to detect a leakage current between the live conductor L and the screen K. However, the circuit could alternatively detect a leakage current between the live conductor L and the load housing 12A, in “principle of operation of leakage current detection.

.Specifications 6122155, which case the resistor R1 would be connected to the housing 12A rather than to the screen K.

Although the arrangement of Figure l is a simplified example, and is offered merely as an example to explain the principle of operation of an LCD and is not meant to imply that such a circuit would be of practical use for such an application, it does serve to demonstrate the basic More ‘sophisticated circuits use a current transformer or integrated circuit or opto coupler or other elaborate circuitry or means to carry out the functions of detection of the leakage current and disconnection of the supply__ within specified limits of fault current and time. Some” :examples of the.prior art are shown in United States Patent ~ , 7359167, 6292337 and.

. US 2010/0020452 discloses a circuit for disconnecting a power source upon the detection of a leakage current from one of a first and a second wire connected to the power source. A sensing conductor is located adjacent to one of the first and second wires for sensing a leakage current from one of the first and second wires. A disconnect switch is interposed within the first and second wires connected to the power source. A disconnect switch control circuit is connected to the sensing conductor for opening the disconnect switch upon the presence of a leakage current from the sensing conductor. The disconnect switch control circuit operates solely from the leakage current from the sensing conductor.

However, the known prior art suffers from problems such as complexity, large component count, large amount of circuitry, and relatively high cost.

It is an object of the invention to provide a leakage current detector which overcomes or mitigates at least some of the problems of the prior art.

According to the present invention there is provided an ‘apparatus for detecting a leakage current flowing between at least one supply conductor (L or N) of a multi—conductor electricity supply to a load (12) and a non load current v.carrying electrical conductor (K) nominally insulated from=_ the at least one-supply.conductor, the apparatus including a relay (RLA) having a set of contacts (SW) in series with; the electricity supply and a movable-component (actuator) (100) operable.to open and close the contacts (SW), the actuator (100) being resiliently biased towards a first? position, wherein the contacts (SW) are open, against the‘ electromagnetic force generated by a first coil (A) which acts in opposition to the resilient bias and whose greater force normally holds the actuator (100) in a second position wherein the contacts (SW) are closed, the actuator (100) being released and moved by the resilient bias to the first position when the holding force of the coil (A) is sufficiently weakened by the electromagnetic force generated by a current flowing in a second coil (B) when a leakage current meeting certain criteria as to magnitude and/or duration flows between the at least one supply conductor (L or N) and the non load current carrying conductor (K).

Preferably the leakage current flows through the second coil (B).

In such a case a capacitor (C2, Figs. 3 and 4) may be connected across the second coil (B) with a zener diode (ZD2) connected in parallel with the capacitor to limit the voltage across the second coil (B) regardless of the magnitude of the leakage current.

Alternatively, the second coil (B) may connected in series between the non load current carrying conductor (K) and at least one other supply conductor (N or L) via an electronic switch (SCR1, Fig. 5), and wherein the apparatus further comprises.a capacitor'(Cl) which is charged up by the leakage.current-and.which turns on the electronic switch when the voltage on the capacitor reaches a certain levelg; "Preferably the first coil (A) is connected across two supply conductors (L, N) on the opposite side of the contacts (SW) to the load (12).

The non load current carrying conductor (K) may comprise a screen incorporated in a multi—conductor electricity supply (12), cable supplying the load the screen surrounding at least one supply conductor of the cable.

Alternatively, the non load current carrying conductor (K) comprises an electrically conductive chassis, housing or other component Containing or associated with electrical equipment.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram illustrating prior art techniques for leakage current detection.

Figure 2 is a circuit diagram of a first embodiment of the invention.

Figure 2a is a schematic diagram of a relay which may be used in the embodiments of the invention.

Figure 3 is a circuit diagram of a second embodiment of the invention.

Figure 4.is a circuit diagram of a third embodiment of thefl “invention. ‘15 Figure 5 is.a circuit diagram of a fourth embodiment of the; invention.

In the embodiment of Figure 2, a detector circuit including a relay RLA is connected between the screen K and the supply conductors L and N. The relay RLA has a set of normally—closed contacts SW in series with the electricity supply conductors L, N and a movable component (actuator), operable to The relay RLA shown schematically as dashed line 100, mechanically open and close the contacts SW. further includes first and second coils A and B respectively, the coil A being connected across the mains supply conductors L, N and the coil B being connected between the screen K and each supply conductor L, N via a respective diode D1, D2.

Figure 2a shows a typical constriction for the relay RLA.

The relay comprises a bobbin 10 fitted over a pole piece I5 .output leads 28, 30. which is fixed in a frame 18, both the pole piece and frame being of ferromagnetic material. The coils A and B are wound on the bobbin 10. A pivoting ferromagnetic armature 100 is fitted to the top of the frame 18 and is biased into a first, open position (as shown in Figure 2a) by a spring . An electrical contact 22 is fitted to the armature 100 via a resiliently flexible arm 24, the Contact 22 being movable into and out of engagement with a fixed electrical Contact 26 by pivoting of the armature 18 towards and away from the top end of the pole piece 16. The pair of contacts 22 and 26, together with a second such pair (not shown), constitute the set of contracts.SW shown in Figure , one of the pair of contacts 22, 26 being inserted in series in the supply line L and the other in series in.the»' supply line N, in each case using respective input and Both pairs of contacts 22, 26 aret operated simultaneously by the armature 100.

The electromagnetic field established by the coil A acts in opposition to the spring 20 and, provided the mains supply voltage is above a certain level, the coil A exerts a force on the actuator 100 which is slightly greater than that of the spring 20, so that, once closed, the actuator 100 is normally held in a second position closed against the top of the pole piece 16. In this second position of the actuator 100 the contacts 22 engage the respective contacts the actuator the contacts SW are closed. However, , i.e. 100 is released and moved by the spring 20 to the first, contact—opening, position when the holding force of the coil A is sufficiently weakened by a current of appropriate magnitude and direction flowing in the coil B. The actuator 100 may be initially closed against the top of the pole piece 16 manually. _15. w-during that half cycle. . supply, -the net.magnetic holding force will be reduced or weakened In operation, the AC mains supply current flows through coil A, and when the supply voltage is above a certain level the relay contacts SW may be closed by manual or automatic means and will be held closed as described above.

In the event of an insulation breakdown at point X, a current will flow from the supply neutral N through the screen K back to supply live L via the coil B and the diode D1. The current flowing through coil A.is AC, but the current flowing through coil B will be half wave rectified AC. Coil B is arranged such that current flow through it ' will be in the opposite direction to the AC supply-current flowing.through coil A during.alternate half cycles.of the . either positive or negative, with the result that When the leakage current exceeds a certain threshold, and persists for longer than the response time of the relay RLA, the holding force of the relay will be sufficiently reduced so as to cause automatic opening of the contacts SW.

The circuit will behave in like manner in response to an insulation breakdown at point Y, with the leakage current flowing through diode D2. In the event of the relay RLA opening due to an insulation fault, it will remain open.

However, the relay contacts can be reclosed by manual means or by temporary removal of the mains supply to facilitate automatic reclosing when the supply is restored.

Figure 3 shows a refinement of the circuit of Figure 2. a diode D3 rectifies the AC The In the arrangement of Figure 3, supply to develop a DC voltage across a capacitor C1. relay coil A is connected to the mains supply via a resistor R1 and a diode D3, and the contacts SW may be closed by manual or automatic means. Once closed, the DC voltage across Cl will keep the relay RLA in the closed state under normal conditions. A zener diode ZDl clamps the DC voltage to a certain level such that the DC supply voltage to the relay coil A remains relatively constant over a wide supply voltage range.

‘Under'a fault condition a leakage current will flow via either diode D1 or D2 to the screen K via a resistor R2 and relay coil B. This current will initially oharge_up a :capacitor C2 and the voltage developed across C2 will be ‘clamped by a zener diode ZD2 such that the voltage across v.coil B will not increase above a certain level-regardless of the magnitude of the leakage current.- This prevents the relay being inadvertently closed or held closed by-the leakage current. Coils A and B are arranged such that the current flow in A is always of opposite polarity to that in B. When the fault current reaches a certain level the net holding force on the actuator 100 will be sufficiently reduced so as to cause automatic opening of the contacts SW. The charge on C1 ensures that the relay will not open in the event of a momentary interruption in the AC supply.

The values of the components R2 and C2 can be chosen to set the leakage current threshold for opening of the relay, and also for calibration of the response time of the circuit to a fault condition.

Figure 4 shows an embodiment illustrating the application of the invention to a DC supply system.

Figure 4 shows a DC supply system representative of a .and.the:~ve supply conductor via a resistor R3, conventional DC supply system or that of a PV solar panel or an electric vehicle EV). In general such systems are isolated from earth. In this arrangement, the non load carrying conductor K may be the frame or chassis of an electric vehicle.

The relay RLA includes first and second coils A and B as before, the coil A being connected across the mains +ve and ~ve supply conductors and the coil B being connected between the conductor K and the +ve Supply conductor via a resistor R2. A zener diode ZD2 and a Capacitor C2 associated with the coil B operate essentially as described for Figure 3, except in a DC context. The relay also comprises a third coil C connected.between the conductor K the coil also having an associated zener diode ZD3 and a capacitor C3. The circuitry associated with the coil C is essentially a duplicate of the circuitry associated with the coil B except that it is connected to the -ve supply conductor rather than the +ve supply conductor. The relay in this embodiment may be constructed as described for Figure 2a, but with a third coil C wound on the bobbin 10.

The relay contacts SW are held closed by a sufficient current through coil A, as previously described. In the case of an insulation breakdown F1 between the -ve side of the supply and the conductor K, a leakage current will flow from the +ve side of the supply through R2 and coil A, and through the conductor K back to the —ve side of the supply.

In the case of an insulation breakdown F2 between the +ve side of the supply and the conductor K a leakage current will flow from the —ve side of the supply through R3 and coil C, and through the conductor K back to the +ve side of v‘ will be reduced as described previously, _in the case of Figure 4, Lcurrent charges-up a capacitor.Cl, the supply.

The electromagnetic force generated by each of the coils B and C when a leakage current flows through it is in opposition to the electromagnetic force generated by the coil A, so that when a leakage current flows in either coil B or C the net holding force on the contact actuator 100 and when the leakage current exceeds a certain level for a certain time ‘ the contacts SW will automatically open._ Figure 5 shows a.modification of.Figure 2. The primary difference is that in Figure 2, as well as Figures 3 and 4, Ithe.leakage current flows directly through the coil B and,_H coil C. In.Figure 5 the leakage -current from the capacitor which flows though the coil B Under normal conditions no current flows in the screen K.

In the event of insulation breakdown as shown at X or Y, a‘ current path will be established to the screen. For example, a breakdown at point X from the neutral conductor to the screen will result in a current path from live L through diode D1, resistor R1, capacitor C1 and the screen to neutral N, and a breakdown at point Y from the live conductor to the screen will result in a current path from neutral N through D2, resistor R1, capacitor C1 and the screen 10 to live L. The fault or leakage current will cause the capacitor C1 to charge up in each case, so in effect the leakage current will be stored in the capacitor.

When the charge on C1 exceeds the breakover voltage of a zener diode ZDl, an SCR1 in series with coil B will turn on and cause C1 to discharge via coil B. The component and it is the dischargev values are chosen so that the electromagnetic force resulting from the current flow through coil B, being in opposition to the electromagnetic force from coil A, will be sufficient to cause the contacts SW to open for a given level of leakage current.

The value of resistor R1 is chosen to control the rate at which Cl charges up so that the response time of the circuit can be calibrated. Resistor Rb is a bleed resistor which discharges Cl at a certain rate such that if the leakage current is below a certain level IA, Cl will not acquire sufficient charge to exceed ZDl breakover voltage - and cause the SCR to turn on., Through suitable_selection‘ of component values for R1, Rb, Cl and ZDl, the circuit can »be calibrated in terms of its operating leakage current. ’threshold and response time for given levels of leakage . current.

It can be seen from the aforementioned examples_that the leakage current detector circuit may be used on AC or DC supply systems. It may also be used on multiphase systems.

It is important to note that in the embodiments of the invention described herein, the fault or leakage current alone is used to cause automatic opening of the contacts and no additional energy is required from the mains supply.

It should be noted that relay contacts SW can only be closed when its coil A is located on the supply side of the contacts, and this feature prevents mis~wiring of the supply and the load. The contacts will open automatically in response to a loss of one or more supply conductors, and thereby provide protection under this condition. The contacts will also open automatically in the event of the supply voltage falling below a certain level insufficient to hold the relay in the closed state, and thereby provide protection against an undervoltage condition.

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)

Claims:

1. An apparatus for detecting a leakage current flowing (L or N) (12) between at least one supply conductor of a multi- conductor electricity supply to a load and a non load current carrying electrical conductor (K) nominally insulated from the at least one supply conductor, the apparatus including a relay (RLA) having a set of contacts (SW) in series with the electricity supply and a movable (100) operable to open and close the (100) component (actuator) being resiliently biased (SW) contacts (SW), the actuator towards a first position, wherein the contacts are open, against the electromagnetic force generated by a first coil (A) which acts in opposition to the resilient bias and whose greater force normally holds the actuator (100) (SW) closed, in a second position wherein the contacts are the actuator (100) being released and moved by the resilient bias to the first position when the holding force of the coil (A) is sufficiently weakened by the electromagnetic force generated by a current flowing in a second coil (B) when a leakage current meeting certain criteria as to magnitude and/or duration flows between the at least one supply conductor (L or N) and the non load current carrying conductor (K).

2. An apparatus as claimed in claim 1 wherein the leakage current flows through the second coil (B).

3. An apparatus as claimed in claim 2 wherein a capacitor (C2) is diode (ZD2) connected in parallel with the capacitor to (B), connected across the second coil (B) with a zener limit the Voltage across the second coil regardless of the magnitude of the leakage current.

4. An apparatus according to claim 2 wherein the second coil (B) is connected in series between the non load current carrying conductor (K) and at least one other supply conductor (N or L) via an electronic switch (SCRl), and wherein the apparatus further comprises a capacitor (C1) which is charged up by the leakage current and which turns on the electronic switch when the voltage on the capacitor reaches a certain level.

5. An apparatus as claimed in claim 1 wherein the first coil (A) is connected across two supply conductors (L, N) on the opposite side of the contacts (SW) to the load (12).