GB2265014A

GB2265014A - Electricity meters

Info

Publication number: GB2265014A
Application number: GB9205625A
Authority: GB
Inventors: Kenneth Payne
Original assignee: Ampy Automotion Digilog Ltd
Current assignee: Landis and Gyr Ltd
Priority date: 1992-03-14
Filing date: 1992-03-14
Publication date: 1993-09-15
Anticipated expiration: 2012-03-14
Also published as: GB2265016A; GB9219310D0; GB9205625D0; GB2265016B; GB2265014B

Abstract

In an electricity supply meter having a casing with a cover (12) secured to a base (10) by means of brass pillars (26) (Figure 4) and screws (22), in order to prevent build-up of electrostatic voltage on the fastening means (26, 22), e.g. due to tampering for fraudulent purposes, a conductive track (44) on a printed circuit board connects the ground plane (42) of the board with the pillar (26) at a point where the pillar penetrates the board. <IMAGE>

Description

Title: Improvements in and relating to Electricity Metering Field of invention This invention concerns meters for metering the supply of electricity typically to domestic premises and the like.

The invention is particularly appropriate to such meters which incorporate semiconductor circuits for measuring the volatage and the current flow.

Background to the invention Any semiconductor based circuit is susceptible to high voltages such as can occur due to electrostatic effects.

Since it is a requirement that the electrical circuits should be floating and to all intents and purposes insulated from earth, there is always the risk of high voltage build-up in such meters.

Whilst under normal circumstances high static fields are unlikely to occur and result in potential damage and break down, attempts to interfere with the normal operation of such meters have been made using electrostatic discharge devices (such as have been marketed for reducing the static charge which can build up on vynil acetate records and for generating sparks for igniting gas). If such devices are operated in the close proximity of a semiconductor circuit, the electrical field so produced by the discharge can cause the circuit to malfunction and with past designs of meter, it has been known for such meters to fail into a non-recording but conductive condition so that electricity has passed through the meter but not been recorded.

Various proposals have been made to reduce the risk of this particular occurrence but in the event that an attempt is made to interfere with a meter using a high voltage discharge device, high static voltages can still build up and exist on components within the meter where these are not earthed. This is of particular importance where conductive elements (themselves insulated from earth) are exposed at the surface of the meter and are therefore liable to become highly charged in the presence of a high electrostatic field. Cover securing bolts and screw-heads are a typical candidate.It is an object of the present invention to reduce the risk of electrostatic build-up on such items and to render semiconductor power measuring meters less susceptible to any resulting electrical fields such as aforesaid, should an attempt be made to interfere with the meter by discharging an electrostatic charging device close to the meter.

Summary of the invention According to one aspect of the present invention in a meter in which a cover is secured in place by means of conductive fastening devices which themselves communicate with conductive elements which extend into close proximity with a printed circuit board within the meter and which are normally electrically isolated from earth, at least one conductive track is provided on the surface of the printed circuit board communicating between the conductive members in proximity therewith and the ground plane of the printed circuit board.

The conductive track provides an electrical discharge path between the conductive element and the ground plane so that high voltage which could otherwise build up on the conductive element and result in arcing and electrical break-down, cannot build up.

Whilst the conductive path to the ground plane will prevent such conductive element from developing a high potential relative to the rest of the printed circuit board, the conduction to the ground plane will of necessity result in the latter achieving a high potential before the charge can leak away and this may itself cause problems.

According therefore to a further aspect of the invention, a high resistence link is provided between the ground plane of the printed circuit board and an. earth connection or the neutral of the electricity supply if the latter is earthed, so that any charge which does build up on the ground plane and conductive elements connected thereto, can leak away safely to earth either directly or via the neutral of the supply main.

Typically the link is at least of the order of MegOhms.

In order to reduce the cost of the component forming the link, two or more similar valve resistors may be connected in series so that the overall voltage between the opposite ends of the resistor path is divided between the different resistors and the voltage across any one resistor (between its two conductive opposed ends) is only a fraction of that across the whole of the resistence chain.

In a preferred example, two 15MegOhm resistors are connected in series to provide a 30MegOhm resistance path between the ground plane and an earth connection terminal provided on the meter casing well insulated from other conductive items within the meter casing and which when installed, may be connected either directly to earth or if permitted, to the neutral line of the incoming mains supply.

It should be noted that the invention not only reduces the likelihood of electrical arcing which otherwise could occur between high potential components and any neutral potential or earth potential circuit elements within the meter, but also reduces the magnetic field (which is very often the influence which can effect the operation of the meter) due to any current discharging as a result of any static build-up. This arises from the fact that the current leaking through the high resistance link to earth (or supply main neutral) and which effectively prevents any excessive voltages developing, is so small as to not generate any significant magnetic field in its environs.

This should be compared with the alternative in which high electrostatic build-up is permitted to occur and then discharges as a spark between one conductive element and another so that all of the energy is dissipated momentarily in the spark and a relatively high current flows albeit for a very short time. The high current flowing produces a high intensity magnetic field which in turn can induce high EMFs into adjacent circuit elements which in turn cause damage to the sensitive semiconductor components. No such high energy spark can arise where the high voltage is prevented from becoming established in the first place.

The invention will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 is a general view of an electricity meter which will supply electricity in response to the insertion of an appropriate pre-paid card, Figure 2 is a plan view of the inside of the meter with the top cover removed, Figure 3 is a side view on the arrow X of Figure 2, and Figure 4 is a perspective view of part of the assembly of Figures 2 and 3 showing how a conductive pillar secures and supports printed circuit boards within the meter.

Detailed description of drawings In Figure 1 the meter is shown as a two part housing comprising a base 10 and a cover 12. A second cover 14 is provided below the main cover 12 to protect the terminals to which the cables are connected.

In the main cover 12 is a window 16 through which an LCD display can be viewed for example to enable a user to determine the number of electricity units which have been credited on the insertion of a ca-rd such as 18 into the slot 20.

The cover 12 is secured in position by means of two screws 22 and 24 which threadedly engage into two brass pillars shown at 26 and 28 in Figure 2 and the left hand one of which 26 can be seen in Figure 3.

The upper end of each pillar 26 (28) is in fact formed with a shoulder and an upper region beyond, of reduced diameter 30 (32) respectively is adapted to protrude through appropriately apertured holes in an upper printed circuit board 34 which carries the display assembly which can be viewed through the window 16.

At its lower end each of the pillars is formed with a similarly reduced diameter section which is at least in part threaded for engagement in an appropriately threaded aperture in a pillar moulded integrally with the wall of the base 10. Plastics washers 36 and 38 are sandwiched between the lower shoulders of the pillars 26 and 28 and a lower printed circuit board shown in dotted outline at 40 in Figure 3, thereby securing the latter in position on top of the two integrally formed pillars 26, 28 in the base 10.

The casing is typically formed from an electrically insulating material such as a polycarbonate and consequently the conductive pillars such as 26 and 28 and the conductive screws 22 and 24 engaging the pillars, can be very substantiallyabove earth potential before electrical breakdown will occur. In the presence of an intense electrostatic field, the conductive elements concerned can assume very high potentials at which electrical break-down will eventually occur between the conductive pillars and adjoining conductive elements The construction of the pillar and securing screws etc. is more clearly shown in Figure 4 in which the pillar 26 is shown together with its screw 22 in an exploded perspective view containing the base 10, lower printed circuit board 40 and upper printed circuit board 34.The ground plane 42 on the lower printed circuit board is shown connected by a finger track 44 so as to engage the brass pillar 26 where it penetrates the printed circuit board 40 and thereby provide an electrical connection between the pillar 26 and the ground plane 42. The pillars therefore cannot attain a potential which is different from that of the ground plane and although this means that the printed circuit board may itself attain similar high potentials to those to which the pillars may become charged, at least there is no potential difference between the pillars and the printed circuit boards causing electrical discharge to occur therebetween.

In accordance with a further aspect of the invention, the ground plane is connected to an ear thing terminal shown diagrammatically at 46 by means of two series connected resistors 48 and 50 which may for example be mounted on the printed circuit board 40 and a conductor 52. The two resistors 48 and 50 are typically high resistance and of the order of 15MegOhms each and if tey are in fact mounted on the printed circuit board 40, a position must be chosen thereon which is well clear of the ground plane or any other conductive elements on the board so that there is little tendency for high voltages at the two resistors to arc across to an adjacent conductive element.

The invention is not limited to the use of two resistors and if appropriate a single resistor of appropriate size or more than two resistors may be used to create the conductive leak path to earth.

Although the example shown in Figure 4 provides for the resistive path to be connected to earth via earthing terminal 46, it is to be understood that the lead 52 could alternatively go to the terminal in the lower part of the case 10 to which the neutral of the incoming supply main is connected in the lower part of the housing below the coverplate 14.

Claims

1. An electricity supply meter in which a cover is secured in place by means of conductive fastening devices which themselves communicate with conductive elements which extend into close proximity with a printed circuit board within the meter and which are normally electrically isolated from earth, wherein at least one conductive track is provided on the surface of the printed circuit board communicating between the conductive elements in proximity therewith and the ground plane of the printed circuit board.

2. A meter according to claim 1, wherein a high resistance link is provided between the ground plane of the printed circuit board and an earth connection or the neutral of the electricity supply if the latter is earthed.

3. A meter according to claim 2, wherein the resistance link is at least of the order of MegOhms.

4. A meter according to claim 3, wherein the resistance link comprises two or more similar value resistors connected in series.

5. A meter according to claim 3 or claim 4, wherein the resistance link has a value of the order of 30 MegOhms.

6. A meter according to claim 5, wherein the resistance link comprises two 15 MegOhm resistors connected in series.

7. A meter according to any of claims 2 to 6, wherein the resistance link is connected between the ground plane and an earth connection terminal provided on the meter casing insulated from other conductive items within the meter casing.

8. A meter according to claim 7, which, when installed, has the earth terminal connected either directly to earth or to the earthed neutral line of the incoming mains supply.

9. A meter according to any of claims 1 to 8, having an insulating base to which an insulating cover is secured by means of screws which engage into electrically conductive pillars also employed to secure the printed circuit board in position, said screws and/or pillars constituting the said conductive elements extending into close proximity with the board.

10. A meter according to claim 9, wherein the pillars extend through the circuit board and the said conductive track comprises conductive fingers extending from the ground plane of the board to the pillars at the positions where the pillars penetrate the board.

11. An electricity meter as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.