WO2013004251A1

WO2013004251A1 - Magnetic insensitive latch actuated relay for electricity meter

Info

Publication number: WO2013004251A1
Application number: PCT/DK2012/050259
Authority: WO
Inventors: Claus Wester; Jan Teilgaard HANSEN
Original assignee: Kamstrup A/S
Priority date: 2011-07-07
Filing date: 2012-07-06
Publication date: 2013-01-10
Also published as: CN103828003B; EP2729949A1; EP2729949A4; CN102867684A; CN103828003A

Abstract

The present invention relates to a relay (1) and to an electricity meter with a relay. The relay comprises a latch actuator (2), possible a linear solenoid-based actuator, and a mechanical latch (3) operable to cyclic switch between a first latch position and a second latch position. A relay actuator (4) is coupled to the latch to follow a movement of the latch, and to a contact spring (6) which closes or interrupts an electric circuit between a first and a second relay terminal (7, 8). The contact spring is part of a conductor path (15-18), where the conductor path comprises at least one bend (17) so that the conductor path comprises a first section (15) and a second section (16) facing each other.

Description

Magnetic insensitive latch actuated relay for electricity meter

FIELD OF THE INVENTION

The present invention relates to a relay and to an electricity meter with a relay.

BACKGROUND OF THE INVENTION

A relay is an electrically operated switch, often using an electromagnet to operate a switching mechanism mechanically, various switching mechanisms are used. One form of relays is the so-called latching relay which has two relaxed states, which are switched electrically, and where the last state is maintained when the electricity is switched off. A known relay, for example as disclosed in GB 2 383 469 Al comprises a contact spring which closes or interrupts the electric circuit between a first and a second relay contact. The relay comprises a latch mechanism or switching mechanism which employs a permanent magnet in the form of an H-type armature. The permanent magnet being pivotally disposed on two yoke legs of a magnet coil. When the poles of the magnet coil are reversed, the permanent magnet is pivoted thereby displacing an actuator causing the contact spring to deflect and either close or disrupt the circuit. The switching state is held by the magnetism of the permanent magnet. The use of a permanent magnet in the latch mechanism renders the latch susceptible to tampering in the form of unauthorized switching by use of an external magnetic field, since a permanent magnet interacts strongly with a magnetic field, and the latch position can be unauthorized changed at a distance.

Relays are used in a number of applications. In a particular application, a relay is used as a circuit breaker inserted into an electricity meter for controlling the supply of electrical power to a consumer.

Relays are generally influenced by magnetic forces. Ideally, a relay only shifts position when the switching mechanism is operated. However, an electromagnetic relay may be influenced by external magnetic fields to tamper with the operation, e.g. forced switching as mention above. But, electromagnetic forces operating on the relay circuit also influence the relay element. In particular, large electromagnetic forces arising in connection with current glitches of an electricity supply net may cause the circuit to break during high current conditions, potentially resulting in fault conditions of the relay afterwards. SUMMARY OF THE INVENTION

It would be advantageous to achieve a relay which is suitable for insertion into an electricity meter, and which is less sensitive to magnetic influence than known relays. Moreover, it is an object of the invention to achieve an alternative relay suitable for insertion in an electricity meter than known relays.

In accordance with a first aspect of the invention a relay is presented which comprises - a latch actuator;

a mechanical latch operable to cyclic switch between a first latch position and a second latch position, by cyclic displacement of a latch element between a first position and a second position, the first latch position and the second latch position being maintained by mechanical forces; a relay actuator coupled to the latch to follow a movement of the latch;

- a contact spring which closes or interrupts an electric circuit between a first and a second relay terminal, one end of the contact spring being conductively connected to the first relay terminal, and a second free end of the contact spring closes the electric circuit in a first contact spring position and interrupts the electric circuit in a second contact spring position; wherein the contact spring is part of a conductor path, where the conductor path comprises at least one bend between the end connected to the first relay terminal and the free end, so that the conductor path comprises a first section and a second section facing each other, wherein the contact spring forms at least the first section; and wherein the relay actuator is coupled to the contact spring so that the electric circuit is closed in the first latch position and interrupted in the second latch position.

The relay of the present invention is advantageous in that the latch mechanism is based on a cyclic displacement of an element between two positions, where the two positions are maintained by use of mechanical forces, and thereby insensitive to external magnetic forces. Moreover by separating the mechanical latch from the latch actuator, any magnetic sensitive parts can be implemented by the latch actuator which may be shielded from magnetic field intrusion more effective, than e.g. a relay based on an a permanent magnetic H-anchor which constitutes both the latch mechanism and the actuator. Moreover from the construction of the contact spring where at least one section of the contact spring forms part of the conductor path which comprises at least one bend between two opposing sections, the Lorentz force causes these two facing sections to repel each other, thereby assisting in maintaining the electrical contact during a current transient or glitch, and thereby opposing a circuit break during a current glitch due to electromagnetic forces arising in the break area. The switching mechanism and the contact spring construction synergetic provide a relay which as a unit is less sensitive to magnetic influence from known sources.

In a second aspect of the invention, the relay is used as a circuit breaker in an electricity meter arranged for measuring electricity consumption supplied from an AC supply line, the meter comprises:

a measuring circuit arranged to measure the electricity consumption;

a relay according to the first aspect, wherein the relay terminals are arranged between the supply line and a load so that the load is electrically connected to the supply line when the relay is closed, and electrically disconnected from the supply line when the relay is interrupted; and a control circuit for controlling the latch actuator.

The relay is particular suitable for an electricity meter since it is important that an electricity meter is not easily influenced by magnetic fields or effects. By application of a mechanical latch which is not based on a permanent magnet, magnetic tampering with the circuit breaker of the electricity meter may be rendered more difficult or even impossible by use of practical magnets, than with known electricity meters. Moreover, the circuit breaker of the electricity meter is resistant to glitches, which occasionally occur on the public electricity net. In general the two aspects of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1A and IB show schematic illustrations of embodiments of a relay;

Fig. 2 shows a schematic illustration of elements of an electricity meter which comprises relays in accordance with embodiments of the present invention;

Fig. 3 illustrates a schematic example of an embodiment of a switching mechanism in the form of a linearly actuated latch; and

Fig. 4 schematically illustrates an embodiment of the control circuit. DESCRIPTION OF EMBODIMENTS

Schematic illustrations of embodiments of a relay 1 in accordance with the present invention are shown in Figs. 1A and IB. The relay is illustrated in a first, also referred to as a closed, position in Fig. 1A, and in a second, also referred to as an interrupted, position in Fig. IB.

The relay comprises a latch actuator 2, a latch, or latch mechanism, 3 and a relay actuator 4 mechanically coupled to the contact spring 6. The contact spring being arranged between a first 7 and a second 8 relay terminal, and being movable between the first position where the electric circuit between the two relay terminals 7, 8 is closed (Fig. 1A) and the second position where the electric circuit between the two relay terminals 7, 8 is interrupted (Fig IB). In the illustrated embodiment, the contact spring has a first end 9 being connected to a conductor 18 interconnecting the contact spring and one of the relay terminals, and a second free end 5 where a contact rivet is attached. A second contact rivet is attached to an opposing conductor 19 connecting the second relay terminal 8. By moving the free end of the contact spring, the electric circuit between the relay terminals can be closed or interrupted.

The actuating or switching mechanism in accordance with embodiments of the present invention comprises three overall components: a latch actuator 2, a latch 3 and a relay actuator 4, which couples a physical displacement of the latch to the contact spring 6 so that it can be switched between the two positions.

The latch actuator 2 may advantageously be constructed as a solenoid-based electromagnetic linear actuator which exerts a linear force 10 when energized. A solenoid-based electromagnetic linear actuator comprises a number of windings 11 at least partially enveloping a magnetisable plunger element 12, such as an iron plunger, so that when a current is running in the windings, a magnetic force presses the plunger inwards. The use of a solenoid-based electromagnetic linear actuator is advantageous since, even though the actuator is based on a magnetisable plunger and an

electromagnetic coil, the actuator is insensitive to external magnetic fields since it would require an extreme external magnetic field to move the plunger into the coil, which is required to switch the latch. Moreover, the position of the latch actuator with respect to the contact spring is not critical. In an electricity meter the latch actuator can be placed at a position where an external magnetic field cannot easily reach.

The latch 3 is a mechanical construction operable to cyclic switch between a first latch position and a second latch position, where the first and second positions are maintained until the latch is switched. The mechanical latch is in an embodiment based on non-permanent magnetic materials, typically in the form of plastic elements and non-permanent magnetic metallic springs. While the latch may comprise magnetisable elements, the keeping of the position is based on mechanical forces and not on magnetic forces. The latch mechanism may be based on cyclic displacing at least two elements linearly with respect to each other. The actuation axis of the latch is advantageously aligned with the actuation axis of the solenoid actuator. An embodiment of the latch is further disclosed below in connection with Fig. 3.

In the illustrated embodiment, the relay comprises a bias spring 14 attached to the housing (not shown) for biasing the contact spring towards the first contact spring position. In general, such biasing may be done by a spring action of the contact spring itself, or it may be done by a biasing action from the relay actuator pressing the contact springs together when the relay is closed. However, the use of a dedicated spring 14 may be preferred for better tailoring of the contact pressure in the closed position. The exerted contact pressure should on one hand ensure a sufficient contact pressure to obtain a low ohmic contact resistance between the two rivets, but on the other hand should not require a high-scale actuation mechanism, since the force exerted by the relay actuator on the contact spring needs to be larger than the bias force of the bias spring. A contact pressure in the range of 1 to 6 Newton per contact rivet is normally appropriate. In the illustrated embodiment, the bias spring is a helical spring, however other types of springs may be used, e.g. leaf springs attached to either the housing (not shown) or to the end of the contact spring.

In a situation where a transient with a current glitch arises, the contact rivets are repelled as the current transient passes by with a risk that the contact is momentarily interrupted by virtue of the repelling force, if the contact is lost during a situation of high currents, sparks will strike (arc) between the two contact rivets dramatically heating the contact rivets, which may cause the contact rivets to weld together, risking that the circuit can no longer be interrupted.

Ensuring that a bias spring force from either the contact spring itself or from the bias spring is sufficiently high to eliminate or limit the risk of sparks during a current glitch requires a high cost actuation mechanism, since it needs to be able to exert sufficient force to contact spring to counteract the necessary bias force.

The inventors of the present invention have realized that by constructing the conductor path between one relay terminal and one contact rivet so that the contact spring is integrated in this path so that at least one section of the contact spring it is positioned after a bend 17, it is ensured that two sections 15, 16 of the conductor path oppose each other. In such construction currents will run anti-parallel along the two opposing sections 15, 16, and in accordance with the Lorentz force, the two sections will repel each other. In particular during an abrupt rise in the current due to a transient, the two sections will repel each other and thereby push the contacts rivets together counteracting the repelling force between the contact rivets, and ensuring at least to a certain degree, that the contact rivets remain in contact during the transient.

The illustrated embodiment shows that the latch actuator exerts force to the contact spring to interrupt the circuit between the two relay terminals (Fig. IB), but does not exert force, or only exerts a very small force, to the free end of the contact spring when closed (Fig. 1A). The illustrated construction of the conductor path, optionally together with spring 14, ensures that the contact remains closed, also during glitch incidents. By only exerting a force to the contact spring during the interrupted state, a simpler construction of the entire switching mechanism can be used.

In the illustrated construction, the contact spring comprises the two opposing sections 15, 16, and is attached to a fixed rigid connector 18 connecting the contact spring to the relay terminal 7. The rigid connector 18 is arranged so that the mean separation distance (100), or characteristic distance e.g. the rest distance in the first latch position, between the first section (15) and the second section (16) is at least five times smaller than a mean separation distance (101) between any of the first and second sections (15, 16) and any section (18, 19) of the conductor path being substantially parallel with the first and/or second sections, here the upper conductor section (18) and the lower conductor section (19). In this manner it can be ensured that any influence for sections with parallel current paths does counteract the repelling action from the Lorentz force between the two sections of the contact spring (15, 16), thereby ensuring that the contact rivet on the contact spring is pushed towards the contact rivet on the opposing conductor 19 during a high-current situation. In an alternative embodiment, the section with reference numeral 16 may be part of the rigid connector where the contact spring is attached, either before or after the bend 17. The dimensioning of the contact spring is normally a compromise between a desired size and desired mechanical properties. For example, the thickness, i.e. the rigidity, of contact spnng, the lengths of the sections 15, 16 and the construction of the bend 17 is adapted so that the contact rivets remain in contact in accordance with a given rating. For example, the construction may be optimized in accordance with ratings specified in accordance with international standards, e.g. the UC3 rating for relays or other ratings. The UC3 rating specifies that the contact needs to be kept during three pulses of 3 kA (kilo- Ampere) over 10 ms (milli-seconds). Fig. 1 A and IB illustrate the contact spring in a schematic side view which indicates that the contact spring is made of two laminate layers bend in shape. In general, the contact spring, or part of the contact spring, may be a single sheet of metal or made of two or more sheets which are laminated. That is the first section 15, the second section 16 and the bend 17 is formed by a single coherent sheet, or each laminate layer is formed by a single coherent sheet. The use of a laminate contact spring brings along a larger flexibility in the obtainable spring properties as compared to a sheet of similar thickness, e.g. due to proper choice of curvature of each laminate of the bend 17. The use of a single sheet (per laminate layer) renders it possible to in a simple manner to form the entire contact spring with the appropriate spring properties.

In another embodiment, the contact spring, or part of the contact spring, may be bifurcated so that two parallel contact springs each close or interrupts the electric circuit between the first and second relay terminal. Each of the parallel contact springs with one end conductively connected with the first relay terminal, and the second free ends closing or interrupting the electric circuit in the first and second end positions of the contact springs respectively. In an embodiment, the free ends of each contact spring have a contact rivet for contact with either a shared or two opposing contact rivets. The parallel contact springs are actuated in union by coupling the relay actuator to both of the contact springs so that the electric circuit is closed in the first latch position and interrupted in the second latch position, as well as by application of a shared bias spring for biasing both of the contact springs towards the first contact spring position. This may be obtained by using a single centrally pivotally arranged bias spring. The application of a bifurcated contact spring may absorb tolerances as well as assist in maintaining a low ohmic contact resistance between the contact rivets.

Fig. 1A and IB illustrate a single relay circuit for use in connection with a direct current (DC) or one- phase alternating current (AC). However, two or more breaker circuits may be arranged for polyphase AC circuit breaking, an example of a three-phase relay is provided in connection with Fig. 2. In a poly-phase relay, the relay comprises two or more sets of relay terminals each forming an electric circuit comprising a contact spring, wherein the relay actuator is coupled to each of the contact springs so that each of the electric circuits are closed in the first latch position and interrupted in the second latch position.

Fig. 2 schematically illustrates elements of an electricity meter 25 for measuring electricity consumption supplied from an AC supply line with three relays in accordance with embodiments of the present invention. The figure illustrates part of a casing 20 of an electricity meter, the solenoid actuator 2, the mechanical latch 3 arranged to move the relay actuator 4. The meter further comprises breaker circuits 22 comprising the contact springs 6, and terminals 23. The supply line may be connected at one relay terminal 8, and the load at the other relay terminal 7. The electricity meter comprises a measuring circuit (not shown) for measuring the electricity consumption of the load, i.e. the consumption of the consumer. The electricity meter moreover comprises a control unit (not shown) for controlling the latch actuator, so that by controlling the latch actuator, the supply of electricity from the supply line to the load is controlled by virtue of the position of the contact springs. The control unit may be implemented as part of the general circuit of the electricity meter. In the illustrated embodiment, the relay actuator is in the form of a frame 4 moveable to simultaneously interact with each of the three contact springs 6, so that all three phases of a three-phase electricity meter can be interrupted or closed in union. The frame 4 may be biased towards closed position by means of a spring coupling 24, to ensure that the relay actuator does not exert force to the contact springs in the first latch position.

In the illustrated embodiment, the frame 4 couples the solenoid actuator 2 to the latch 3 at the backside of the latch with respect to the solenoid. In this way, when the solenoid is energized and the plunger is moved inwards, the frame pushes onto the latch, which transforms the pushes to the cyclic switching between the first and second latch positions.

In an embodiment, the measured consumption is based on measuring a voltage drop across a shunt resistor 21 with known resistance. Advantageously, the shunt resistor is arranged as a part of the electric circuit between the two relay terminals. Other measuring principles may also be used.

Fig. 3 illustrates a schematic example of an embodiment of a switching mechanism in the form of a linearly actuated mechanical latch 3. The figure illustrates a switching cycle, with the latch in the second interrupted position in Fig. 3A and in the first closed position in Fig. 3D. Fig. 3B, 3C and 3E illustrated intermediate positions between the second and the first positions.

The illustrated mechanical latch is a linearly actuated mechanical latch of the so-call ball pen click type. The ball pen click mechanism can take one of two positions, either pushed out for writing or pushed in for protection, continuously shifting between these two positions when the button is pushed. The latch construction is a rather simple mechanical construction employing springs and interacting elements. Such constructions can be made in several ways, as is known to the skilled person in the field of mechanical construction. Fig. 3 illustrates a schematic non-limiting example of elements of the mechanical switching mechanism of the ball pen click mechanism type.

Fig. 3 illustrates three central elements of the mechanical latch 3 : a first cam 30, a second cam 31 and a follower 32. Also a spring force 33 is shown, which in the embodiment of Fig. 2 is the spring designated 24. The figure is illustrated in cross-sectional view, the latch elements are made of circular elements that rotate with respect to each other. In general, the term cam should be construed broadly to mean an element with a shape or profile which influences the movement of the element with respect to other elements. The first cam 30 is coupled to the latch actuator 2, and is movable by the latch actuator. The first cam comprises a tooth-shaped leading edge. The second cam 31 is fixed with respect to the relay. The second cam may be provided by a cylinder enveloping the first cam. In the illustrated embodiment, the second cam is formed by a rib in the housing of the latch mechanism, and is fixed to the housing of the electricity meter 20. The relative displacement 37 between the first and second cam defines the first position (Fig. 3D) and the second position (Fig. 3A). The follower 32 is pushed by the first cam during the actuation. The follower comprises a bi-modal surface with a resting surface 34 and a track 35 arranged in alternation. As is illustrated in Fig. 3B, the actuator by virtue of the first cam pushes the follower 32 until the follower is free from the second cam. By virtue of the bias force 33 and the inclined shapes of the contact surfaces of the first cam and the follower these two elements rotate with respect to each other. In the illustrated embodiment, only the follower 32 rotates whereas the two cams are not rotated. The leading surface 36 of the second cam is maintained in a second resting position in Fig. 3A. The first cam moves the follower by pushing it upwards in Fig. 3B, and as the leading surface 36 becomes free of the notch of the follower in Fig. 3C, the follower is forced to slide into a first resting position of Fig. 3D, where the second cam moves into the track 35 of the follower which locks the rotation of the follower.

In order to shift back to the interrupted position, the actuator by virtue of the first cam pushed the follower again until the follower is free from the second cam (Fig. 3E). Again, by virtue of the inclined shapes of the contact surfaces of the first cam and the follower, these two elements rotate with respect to each other until the rib of the second cam 32 hits the second resting position 34, and the latch resumes the position of Fig. 3A. Thus, by displacement of the latch elements with respect to each other the latch cyclic moves between the first latch position and the second position.

The switching between the first and second latch positions is obtained by a force which is the same or substantially the same in the two situations and which are provided in the same direction in both situations. Thus a simple linear mechanism which does not require a permanent magnet is utilized to obtain a mechanical latching mechanism where the first and second positions are maintained by mechanical forces.

In an advantageous embodiment, the latch actuator is controllably supplied from the AC supply line by the control circuit, which transform the AC supply signal into an appropriate drive signal supplied to the latch actuator when the relay should shift. Fig. 4 schematically illustrates an embodiment of the control circuit in terms of supplying the latch actuator from the AC supply. The specific implementation in the light of the disclosed functionality is within the skills of the skilled person. The control circuit 43 receives one or more phase, V_Ac, at an input side which is input into a rectification stage 40. The rectification stage may in an embodiment be a diode arrangement for selecting the positive part of the signal. The rectification stage transforms the input AC signal into a rectified signal, V_RECT. This rectified signal is input into a selector circuit 41 for selecting a section of the rectified signal, V_SEL- The selected section is supplied to the latch actuator 2 as a drive signal. Upon receipt of the drive signal, the latch actuator pulls the latch so that the relay shifts position.

Fig. 4 further illustrates examples of the voltage in the various stages. These examples are merely shown as illustrative examples. The signals would depend upon a specific implementation. The example is given for using a single phase of a poly-phase signal, also two or three phases may be used.

The control circuit may comprise additional elements, such as an element for scaling the amplitude of the drive signal in accordance with the specification of the latch actuator. It is advantageous to use a control circuit for generating a drive signal from the AC supply line, since the drive signal is not limited by the DC supply of the meter, moreover relay power is ensured as long as the supply line is connected irrespective of the DC supply of the meter.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The invention can be implemented by any suitable means; and the scope of the present invention is to be interpreted in the light of the accompanying claim set. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A relay comprising - a latch actuator (2);

a mechanical latch (3) operable to cyclic switch between a first latch position and a second latch position, by cyclic displacement of a latch element (38) between a first position and a second position, the first latch position and the second latch position being maintained by mechanical forces;

- a relay actuator (4) coupled to the latch to follow a movement of the latch;

a contact spring (6) which closes or interrupts an electric circuit between a first and a second relay terminal (7, 8), one end of the contact spring being conductively connected to the first relay terminal, and a second free end of the contact spring closes the electric circuit in a first contact spring position and interrupts the electric circuit in a second contact spring position; wherein the contact spring is part of a conductor path (15-18), where the conductor path comprises at least one bend (17) between the end connected to the first relay terminal (7) and the free end, so that the conductor path comprises a first section (15) and a second section (16) facing each other, wherein the contact spring forms at least the first section; and wherein the relay actuator (4) is coupled to the contact spring (6) so that the electric circuit is closed in the first latch position and interrupted in the second latch position.

2. The relay according to 1 further comprising a bias spring (14) for biasing the contact spring towards the first contact spring position.

3. The relay according to any of the preceding claims, wherein the relay actuator in the second latch position pushes the contact spring towards the second contact spring position.

4. The relay according to any of the preceding claims, wherein the relay actuator in the first latch position does not exert force to the contact spring.

5. The relay according to any of the preceding claims, wherein the first section, the second section and the at least one bend are formed by one or more sheets, where each sheet is form by a single coherent piece.

6. The relay according to any of the preceding claims, wherein the switching between the first and second latch positions is provided by a force in the same direction.

7. The relay according to any of the preceding claims, wherein the latch actuator (2) is a solenoid- based electromagnetic linear actuator which exerts a linear force causing the latch to switch between the first and second latch position when the latch actuator is energized.

8. The relay according to any of the preceding claims, wherein all elements of the mechanical latch (3) are made from a non-permanent magnetic material.

9. The relay according to any of the preceding claims, wherein a mean separation distance (100) between the first section (15) and the second section (16) is at least five times smaller than a mean separation distance (101) between any of the first and second sections (15, 16) and any other section (18, 19) of the conductor path being substantially parallel with the first and/or second sections

10. The relay according to any of the preceding claims, wherein the mechanical latch comprises latch elements in the form of a first cam (30), a second cam (31) and a follower (32).

11. The relay according claim 10, wherein the first and the second position are obtained by a relative displacement (37) between the first cam and the second cam.

12. The relay according to any of the preceding claims, comprising two parallel contact springs which each close or interrupts the electric circuit between the first and second relay terminal, each contact spring having one end conductively connected with the first relay terminal, and the second free ends close or interrupt the electric circuit in the first and second end positions of the contact springs respectively, wherein the relay actuator is coupled to both of the contact springs so that the electric circuit is closed in the first latch position and interrupted in the second latch position.

13. The relay according to any of the preceding claims, wherein the relay comprises two or more sets of relay terminals each forming an electric circuit comprising a contact spring, wherein the relay actuator is coupled to each of the contact springs so that each of the electric circuits are closed in the first latch position and interrupted in the second latch position.

14. An electricity meter (25) arranged for measuring electricity consumption supplied from an AC supply line, the meter comprises:

a measuring circuit arranged to measure the electricity consumption;

- a relay according to any of the preceding claims, wherein the relay terminals are arranged

between the supply line and a load so that the load is electrically connected to the supply line when the relay is closed, and electrically disconnected from the supply line when the relay is interrupted; and

a control circuit for controlling the latch actuator.

15. The electricity meter according to claim 13, wherein the latch actuator is controllably supplied from the AC supply line by the control circuit.

16. The electricity meter according to claim 15, wherein the control circuit comprises a rectification stage for selecting the positive part of one or more phases of the AC supply line to generate a rectified signal, and a selector circuit for selecting a section of the rectified signal, where the selected section is supplied to the latch actuator as a drive signal.