GB2049205A - Electric potential detector - Google Patents

Abstract

A device for detecting potential in an insulated conductor comprises one or more conductive plates 10, 12 which in use are positioned on the insulator. The plates are connected in series with a display device 26 and a conductive member 28 which the user touches to complete a circuit to earth through his body. The display device is a LCD, neon lamp, or other device operating on a small current. If the conductor is carrying a.c. potential or interrupted d.c. potential, a small current is capacatively induced in the series circuit, and the display device lights up. A modification (not illustrated) employs segmented plates which can be selectively switched, to provide a greater plate area for lower voltages. <IMAGE>

Description

SPECIFICATION Electrical potential detector This invention relates to the detection of electrical potential, and is particularly concerned with detecting whether an electric cable is "live".

ve In domestic, commercial and industrial wiring, it is often desired to test whether a cable is live, in order to examine the integrity of an installation or to locate a fault. At present it is necessary to expose terminals or bare cable ends and to test these with a meter or neon mains tester. This is time consuming and involves some risk to the operator. Moreover, where the fault is a break in the conductor of an insulated and/or sheathed cable, this procedure cannot locate the exact position of the break.

An object of the present invention is to provide a device for detecting electrical potential which allows wiring to be checked in a simple and efficient manner, and which can be made in a cheap and easily portable form.

According to the invention, an electrical testing device comprises a conductive plate for positioning adjacent an insulated cable, a display device operable by a low current and connected in series with said plate, and means for completing said series path to earth.

Preferably, said means comprises a conductive member in series with the display device and positioned for contact by the finger of a user to provide an earth leakage path through the user's body.

The display device may suitably comprise a liquid crystal display or a neon bulb. Where a liquid crystal display is used, it may be provided with rear illumination controlled by a switch; the switch suitably also acts as said conductive member.

In a particularly preferred form of the invention, two electrically-interconnected conductive plates are provided, each being formed as a semi-cylinder, and the plates are mounted on arms arranged for pivotal movement relative to each other and biased to close the plates together.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic illustration of one form of the invention; Figure 2 is a perspective view of a collector arrangement for use in a modification of the embodiment of Fig. 1; Figure 3 is a diagrammatic cross-section of another embodiment; and Figure 4 shows a Unit which may be used in conjunction with the embodiment of Fig. 1.

The tester shown in Fig. 1 comprises a collector unit generally designated 8 and an indicator box 24. The collector unit 8 comprises a pair of semi-cylindrical conductive plates 10 and 12, preferably of copper, which may be covered by a thin insulating coating (not shown) for added safety. Other metals or conductive plastics may be used instead of copper. Each plate 10, 1 2 is carried on one end of a respective plastics arm 14, 1 6. The arms 14, 1 6 are shaped to provide an intermediate fulcrum for mutual pivoting movement, and are biased together by arms 1 8a of a coil spring 18, in the manner of a clothes peg.

The plates 10 and 1 2 are electrically interconnected by a lead 20, and the unit 8 is connected by a cable 22 to the indicator box 24. The box 24 contains a display which in this embodiment is a reflectance LCD 26. The LCD 26 is in circuit between the cable 22 (via a suitable plug and socket) and a metal plate 28 positioned on the box 24 for contact with the finger of a user.

The tester operates on a capacitance effect.

In use, the pates 10, 1 2 are separated by pressing on the lower ends of the arms 14, 1 6 to allow the plates to be manipulated around an insulated cable to be tested. The arms 14, 1 6 are then released to clamp the plates 10, 1 2 around the cable. If one of the conductors in the cable is connected to an alternating potential, a potential is capacitively induced in the plates 10 and 12. The user applies his finger to the metal plate 28 to provide an earth leakage path including the display 26, which thus provides a readout.

It will be appreciated that the current flowing is extremely small, of the order of microamperes, and thus there is no danger to the operator. The display 26 could take any other form which will give an indication nonsuch a small current, for example a neon bulb. A neon bulb similar to that used in a conventional mains tester may be used; the high series resistance normally used in mains testers for current limiting may be omitted since the current is already small in the device of the present invention. I prefer, however, to use a LCD since it has been found that a neon bulb gives a low response which requires either larger collector plates or viewing in subdued light.

Instead of using a reflectance type illuminated by ambient light, a back-lit LCD and internal light source may be used. In this case the tester is suitably provided with a pushbutton switch for the light source, the button also acting as connection between the user and the electrical circuit.

The sensitivity of the tester is a function of the area of the plates. With a construction of the type illustrated in the drawings, it has been found convenient to have the plates dimensioned to form, when closed, a tube of about 10mum diameter and about 80mm length. This gives sufficient sensitivity for detecting the condition of, for example, domes tic wiring while providing a tester which is simple and of a convenient size. The collector plates may conveniently be dimensioned to give, for a cable size and voltage of interest, a current to earth of about 1 pA to y3 A.

In a modification (not shown), the collector plates may be segmented and the tester provided with switching to connect 1, 2 or 4 segments, e.g. 1 segment in circuit for 500V in the cable, 2 segments for 250V, and 4 segments for 110V.

The collector arrangement 10-20 of Fig. 1 can be replaced by a plug-in box 30 as shown in Fig. 2, which plugs directly into the box 24 of Fig. 1 by means of a plug 32. The box 32 comprises a plastics housing which carries a half-cylindrical metal plate 34. A cable 36 interconnects the plate 34 and plug 32.

A further embodiment is shown in Fig. 3, in which the collector plate is a flat plate 40 housed entirely within a plastics box 42, which also houses the display LCD 26 and plate 28. The box 42 is suitably of matchbox size.

The indicator box 24 of Fig. 1 may be used as a dual-purpose tester, by implugging the unit 8 and cable 22 and plugging in the unit shown in Fig. 4. This comprises an insulating plastics sleeve 50 of any suitable length, having an exposed metal contact 52 at one end and a plug 54 at the other. These are interconnected via a 3pF capacitor 56 and a 7.5 megohm resistor 58. The combination of the units 24 and 50 may be used as a mains tester on exposed conductors.

The tester can rapidly and easily locate breaks in the line conductor of a cable and in any equipment connected in series such as lamp bulbs and fuses. Where a break is in the neutral conductor, the exact location can be found by reversing polarity at the supply.

In the case of-a d.c. installation, e.g. in car wiring, the tester can be used in conjunction with a device for repetitively interrupting the d.c. supply to give capacitive coupling.

Claims (9)

1. An electrical testing device comprising a conductive plate for positioning adjacent an insulated cable, a display device operable by a low current and connected in series with said plate, and means for completing said series path to earth.

2. The device of claim 1, in which said means comprises a conductive member in series with the display device and positioned for contact by the finger of a user to provide an earth leakage path through the user's body.

3. The device of claim 1 or claim 2, in which the display device is a neon bulb.

4. The device of claim 1 or claim 2, in which the display device is a liquid crystal display (LCD).

5. The device of claim 4, in which the LCD is a reflectance-type LCD.

6. The device of claim 4 when dependent on claim 2, in which the LCD is a transmission LCD, and the device includes illumination means for the LCD and a switch for actuating said illumination means, the switch also acting as said conductive member.

7. The device of any preceding claim, which comprises a collector unit and a display unit, the display unit housing the display device and said path completing means.

8. The device of claim 7, in which two electrically-interconnected conductive plates are provided in the collector unit, each plate being formed as a semi-cylinder, and the plates being mounted on arms arranged for pivotal movement relative to each other and biased to close the plates together.

9. The device of claim 7 or claim 8, in which a conductor testing unit is also provided which may be connected to the display unit; the conductor testing unit comprising an insulated cover, a test contact protruding from the cover for contacting with a conductor, and a capacitor and a resistor in series between the test contact and the display unit.

1 0. An electrical testing device substantially as herein described with reference to the accompanying drawings.