GB2292850A - A switching device - Google Patents

Abstract

In order to switch off electrical power to an appliance in which a fault develops leading to a temperature rise, a temperature sensor 18 is placed in the appliance 12 and is connected to a switch module 16 which is positioned as an interface between the mains and the appliance. When the sensor senses an increased temperature, it causes the switch module to go open circuit and thus to disconnect the appliance from the mains. The switching module 16 comprises a plug and a socket for connection of the appliance to the mains supply. In one embodiment the temperature sensor may consist of a flexible tube 18 made of a thermoplastic which contains a fluid under pressure. Excess heat causes the tube to rupture releasing a pressure switch which cuts off the mains power. In another embodiment the tube 18 may hold a fire extinguishing fluid. In a further embodiment the temperature sensor may also comprise a temperature sensitive conductor including one or more thermal fuses. <IMAGE>

Description

A SWITCHING DEVICE This invention relates to a switching device intended particularly, but not exclusively, for switching on and off a mains electrical power supply.

Electrically powered appliances are always liable to overheat if some part of the circuitry or mechanism develops a fault. Very often, the first manifestation of a fault is local overheating. To avoid the appliance becoming so hot that a fire starts, it is desirable to remove electrical power from the appliance.

According to the present invention, there is provided an electrical switching device comprising a plug, a socket, an electrical circuit connecting the plug and the socket, a switch in the electrical circuit, the switch being operable to open or close the electrical circuit, and an external temperature sensitive sensor connected to the circuit such that generation of a signal by the sensor operates the switch.

The device can be placed between a mains power supply and an appliance to be served by that power supply so that when the switch is open, power is disconnected from the appliance.

The external sensor is preferably connected to the electrical circuit by a flexible lead, which can be a length of pressure tight tubing or a flexible electrical conductor.

When the lead is a length of pressure tight tubing, the switch can be a relay with the tubing terminating in a pressure sensitive switch which is set to send a signal to the relay to open or close the relay when a pressure change occurs within the tubing.

The tubing can be thermoplastic tubing, and can contain a fire extinguishing medium under pressure, for example under a pressure of from 500 to 3,000 kPa.

When the lead is a flexible electrical conductor, preferably at least part of the conductor is temperature sensitive.

The electrical circuit can then include a step-down transformer with the conductor connected across the steppeddown voltage from the transformer. The temperature sensitivity of the conductor can be provided by incorporating one or more thermal fuses.

The plug, the socket and the electrical circuit are preferably combined in a single housing which can be plugged directly into a wall socket. The plug and the socket can be conventional square-pin, 13amp fittings.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a switching device forming a first embodiment of the invention; Figure 2 shows a plug body for use in the first embodiment; Figure 3 shows a plug body for use in a second embodiment of the invention; Figure 4 is a schematic view of a switching device forming a second embodiment of the invention; and Figures 5 and 6 are circuit diagrams for, respectively, the first and second embodiments of the invention.

Figure 1 shows a computer 10 with a monitor 12 protected by a switching device 14. The device 14 has a plug body 46 and an external detector tube 18. Mains power to the computer and to the monitor is provided through mains leads 20 and 22 plugged into an extension lead 24, and the lead 24 is plugged into the plug body 16. If a malfunction resulting in a temperature rise occurs in either the computer 10 or in the monitor 12, a switch in the plug body 16 will open to disconnect the computer and the monitor from the mains, thus preventing any escalation of the malfunction into, for example, a fire.

The plug body 16 has a housing 26 with a plug socket 28 on one side and a set of plug pins 30 on an opposite side. The detector tube 18, which is a tube with thermoplastic walls, extends through a wall of the housing 26 and is connected to a pressure switch 38 (see Figure 5) inside the housing. A 'power on' indicator light 32 is provided on the outside of the housing 26.

Figure 5 shows the circuit which will be fitted within the housing 26. The plug pins 30 are connected to a 13 amp fuse 34 and to a 700C thermal fuse 36 and then to the pressure switch 38. When the pressure switch is closed, a relay 40 is held closed. When the switch 38 is open, the current holding the relay closed falls away and the relay opens to disconnect the plug pins 30 from the socket 28.

To use the device, the tube 18 is passed around and placed in proximity to the internal components of an appliance to be protected. Under normal conditions, the interior of the tube 18 will be pressurised, and the pressure will be sufficient to hold the pressure switch (shown at 38 in Figure 5) closed. The tube 18 is however sensitive to above normal temperatures. If the temperature in the vicinity of the tube -rises, the material of the tube wall will soften and at a certain temperature, the pressure inside the tube will cause the tube wall to rupture, thus allowing the tube contents to escape so that the pressure immediately drops, with the result that the pressure sensitive switch 38 opens.

The relay 40 then opens, to remove electrical power from the appliance being protected.

This can re result in power being switched off from the appliance before major damage occurs. If the problem is a particular component overheating, this overheating can be detected and the power switched off before the overheating reaches the stage where other components might be damaged and before fire can break out.

If the tube contains a fire extinguishant, a double action can take place, with the escape of pressure also having a fire extinguishing action.

Figures 3, 4 and 6 refer to a second embodiment in which components which correspond to components described with reference to Figures 1,2 and 5 are designated by the same reference numerals increased by 100.

In place of the tube 18, an electrical lead 118 (Figure 4) can be used. In this case the lead 118 can be permanently wired into a plug housing 116, or can be terminated by a plug 109 which can fit into a socket 111. The lead 118 in the embodiment shown is provided with thermal fuses 113 at intervals along its length, the fuses being wired in series along the length of the lead.

Figure 6 shows the circuit which will be fitted within the housing 126. As in Figure 5, the plug pins 130 are connected to a 13 amp fuse 134 and to a 700C thermal fuse 136. A transformer 115 reduces the voltage from the plug pins 130 to 12v and the power to 0.9va. The socket 111 is then supplied with power at this reduced voltage. A remote switch 113 (in the form of a thermal fuse) is connected to the socket 111 by a plug 109, the plug 19 and the switch 113 being separated by an indefinitely long cable 118.

Whilst the fuse 113 remains in its current-passing state, current will flow to hold a relay 140 in a current-passing state, so that electrical connection exists between the plug 130 and the socket 128. However if any one of the fuses 113 blows, then the circuit will open and the relay 140 will go open circuit and the electrical connection will be broken.

Figure 4 shows the conductor 118 extending into and around components of a computer or other electrical appliance 120.

The figure shows one of the fuses 113a as it blows.

The switching device described is easy to install and provides effective safety controls to remove power from faulty appliances. The illumination/non-illumination of the lamp 32/132 will indicate whether the switch 40/140 has been triggered, and if so the user must investigate inside the appliance to find a fault. The location of the fault should be easy to spot, because it will be at the position where the tube 18 has burst or at the position where one of the fuses 113 has blown.

Thermal fuses are not the only type of 'switch' which can be used in the embodiment of Figures 3,4 and 6. Other types of switch are also possible.

Claims (12)

Claims

1. An electrical switching device comprising a plug, a socket, an electrical circuit connecting the plug and the socket, a switch in the electrical circuit, the switch being operable to open or close the electrical circuit, and an external sensor connected to the circuit such that generation of a signal by the sensor operates the switch.

2. A switching device as claimed in Claim 1, wherein the external sensor is connected to the electrical circuit by a flexible lead.

3. A switching device as claimed in Claim 2, wherein the flexible lead is a length of pressure tight tubing.

4. A switching device as claimed in Claim 3, wherein the switch is a relay, the tubing terminates in a pressure sensitive switch, and the pressure sensitive switch is set to send a signal to the relay to open or close the relay when a pressure change occurs within the tubing.

5. A switching device as claimed in as claimed in Claim 3 or Claim 4, wherein the tubing is thermoplastic tubing containing a fire extinguishing medium under pressure.

6. A switching device as claimed in Claim 5, wherein the fire extinguishing medium is under a pressure of from 500 to 3,000 kPa.

7. A switching device as claimed in Claim 2, wherein the flexible lead is a length of electrical conductor, at least part of which is temperature sensitive.

8. A switching device as claimed in Claim 7, wherein the electrical circuit includes a step-down transformer, and the conductor is connected across the stepped-down voltage from the transformer.

9. A switching device as claimed in Claim 7 or Claim 8, wherein the conductor includes one or more thermal fuses.

10. A switching device as claimed in any preceding claim, wherein the plug, the socket and the electrical circuit are combined in a single housing.

11. A switching device substantially as herein described with reference to Figures 1,2 and 5 of the accompanying drawings.

12. A switching device substantially as herein described with reference to Figures 3,4 and 6 of the accompanying drawings.