GB2198299A

GB2198299A - Protected electrical supply arrangement

Info

Publication number: GB2198299A
Application number: GB08726086A
Authority: GB
Inventors: Edward Smith
Original assignee: Fujitsu Services Ltd
Current assignee: Fujitsu Services Ltd
Priority date: 1986-11-27
Filing date: 1987-11-06
Publication date: 1988-06-08
Anticipated expiration: 2007-11-06
Also published as: GB8726086D0; GB8628363D0; GB2198299B

Abstract

A protective arrangement for trailing electrical sockets 13 has a voltage discriminator 20 which monitors the voltage between the earth and neutral conductors. If this rises above a threshold value, it trips an electronic crow-bar switch 21, which then provides a low impedance path between live and neutral, blowing a fuse in the live conductor. The invention protects against the hazard of high leakage currents, accumulated from several items of equipment. Alternatively, the switch 21 may be replaced by an audible warning device. <IMAGE>

Description

ELECTRICAL SUPPLY ARRANGEMENT This invention relates to electrical supply arrangements.

It is becoming common practice for office equipment and data processing equipment to be connected to the main electricity supply by means of a trailing socket arrangement, which allows several items of equipment to be connected to a single wall socket outlet. Indeed, office furniture is often designed with büil." uiLipie sockets which are all connected by a length of cord to a single plug.

The radio frequency interference filters fitted to most office and data processing equipment have a capacitor connected between the phase conductor and the frame of the equipment. This results in a small leakage current flowing from the phase conductor, via the capacitor, into a protective earth conductor.

Experience has shown that a proportion of socket outlets have very poor or no earth continuity, in which case there is no proper return path for this leakage current. As a result, the frame of the equipment rises to about half the mains voltage, and a person touching the equipment is likely to receive an electric shock. For this reason, safety standards limit the leakage current in pluggable equipment to a value which is not likely to be dangerous, e.g. 3.5mA.

There is no particular problem in meeting this requirement with individual items of equipment.

However, the use of multiple trailing sockets can result in the leakage currents from several equipments accumulating on the earth conductor. If the earth continuity at the socket outlet is inadequate, then this accumulated leakage current will be ready to flow through a person who touches any of the equipment connected to any of the trailing socket outlets. If the accumulated current exceeds lOmA, the shock may have serious consequences.

One method of protecting against this hazard would be to provide a voltage sensitive circuit breaker, connected between neutral and earth. Such a device would have to be fitted to each set of trailing socket outputs. However, voltage sensitive circuit breakers are relatively expensive, and are difficult to install retrospectively.

The object of the present invention is to provide a method of protection against the above-mentioned hazard which does not require the use of a voltage sensitive circuit breaker.

Summary of the invention According to the invention, there is provided an electrical supply arrangement having live, neutral and earth conductors, and comprising a plug adapted to connect with an electrical mains supply socket, and a plurality of socket outlets electrically connected with the plug, thereby enabling a plurality of individual items of equipment to be supplied from a single mains supply socket, the arrangement having a fuse in the live conductor, characterised by detection means connected between the earth and neutral conductors, for detecting a rise in voltage between those conductors, and switching means connected between the live and neutral conductors, the switching means being operated when the detection means detects said rise in voltage, to cause a current to flow through the fuse such as to blow the fuse.

Alternatively, the switching means may be replaced by a warning device, e.g. a device giving a loud audible warning, which operates when the rise in potential is detected.

Brief description of the drawings One embodiment of the invention, and modifications thereof, will now be described by wayof example with reference to the accompanying drawings.

Figure 1 shows a trailing socket arrangement.

Figure 2 shows a protective circuit for incorporation in the trailing socket arrangement.

Figure 3 shows a socket extender device.

Description of an embodiment of the invention Referring to Figure 1, this shows an electrical supply arrangement comprising a standard 13A three-pin plug 10, for connection to a mains supply socket outlet (e.g. a wall socket). The plug contains a standard fuse in the phase (live) conductor.

The plug 10 is connected by way of a cable 11 Lu d trailing socket outlet box 12 having, in this example, three socket outlets 13, each of which is adapted to receive a standard 16A German style plug, commonly used in the office furniture industry. The outlet box 12 may for example be built into an office desk.

The outlet box 12 includes a protective circuit as shown in Figure 2.

A voltage discriminator circuit 20 is connected between the protective earth (E) and neutral (N) conductors, and is arranged to produce an output signal if the voltage between these conductors rises above a predetermined threshold value. The discriminator circuit 20 preferably includes an integrator circuit, to avoid nuisance tripping by transient voltages.

This output signal controls an electronic crow-bar switch 21, which comprises a semiconductor device, connected between the live (L) and neutral (N) conductors. When this switch 21 is triggered, it results in a low impedance between live and neutral, which will blow the fuse in the plug 10.

Thus, in the event of a poor earth connection, if the potential of the earth conductor rises above a predetermined value, the fuse will blow, thereby removing the electrical supply from the equipment.

Modifications Instead of being incorporated in the outlet box 12, the protective circuit shown in Figure 2 could alternatively be included in a socket extender device as shown in Figure 3. This is a device which plugs into one of the sockets 13, and has a socket for receiving a 16A plug. Preferably, the socket extender device incorporates some form of locking mechanism to prevent it from being removed from the socket 13, since this would leave the outlet box unprotected.

Alternatively, the protective circuit of Figure 2 could be incorporated in the 13A plug 10. The advantage of this is the close association of the cro-bar and the fuse. However, it would require a double protective earth connection between the plug 10 and the trailing socket outlet box 12, to ensure a proper earth connection.

Another possibility would be to incorporate the protective circuit of Figure 2 into one of the plugs (not shown) that plugs into the outlet box 12. In this case, it is important that this plug should not itself incorporate a fuse, since if the crow-bar switch caused this fuse to blow, rather than the fuse in the plug 10, the other outlets in the box 12 would be unprotected.

Since this plug does not contain a fuse, it is important that it should not be pluggable directly into the wall socket outlet. This is achieved if a 16A plug is used.

Instead of locating the fuse in the plug 10, it could instead be located in the outlet box 12. (This does not apply to the case where the crow-bar switch is in the plug 10; in that case, the fuse must be located in the plug).

In an alternative form of the invention, the circuit of Figure 2 may be modified by replacing the electronic crow-bar switch with a warning device, e.g. a device which when triggered gives a very loud audible alarm. This may be preferable if it is considered undesirable to short-circuit the live and neutral conductors, although it provides less protection.

Claims

CLAIMS:

1. An electrical supply arrangement having live, neutral and earth conductors, and comprising a plug adapted to connect with an electrical mains supply socket, and a plurality of socket outlets electrically connected with the plug, thereby enabling a plurality of individual items of equipment to be supplied from a single mains supply socket, the arrangement having a fuse in the live conductor, characterised by detection means connected between the earth and neutral conductors, for detecting a rise in voltage between those conductors, and switching means connected between the live and neutral conductors, the switching means being operated when the detection means detects said rise in voltage, to cause a current to flow through the fuse such as to blow the fuse.

2. An arrangement according to claim 1 wherein the detection means and switching means are located adjacent to the socket outlets.

3. An arrangement according to claim 1 or 2 wherein said socket outlets are of a kind incompatible with said plug and mains supply socket.

4. An arrangement according to any preceding claim wherein said switching means comprises an electronic crow-bar switch.

5. An arrangement according to any one of claims 1 to 3 wherein said switching means is replaced by a warning device.

6. An arrangement according to claim 5 wherein said warning device is an audible warning device.

7. An electrical supply arrangement substantially as hereinbefore described with reference to the accompanying drawings.