GB2474245A

GB2474245A - Electrical safety device for electrical supply units

Info

Publication number: GB2474245A
Application number: GB0917515A
Authority: GB
Inventors: Alan Radford; Ian Wedlake
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-10-07
Filing date: 2009-10-07
Publication date: 2011-04-13
Anticipated expiration: 2029-10-07
Also published as: GB0917515D0; GB2474245B; HK1156153A1

Abstract

The safety device is used in association with a unit such as a wall socket or consumer unit which is arranged to deliver electrical power to one or more electrical outlets 11, 12, and having an incoming electrical supply with live, neutral and earth lines protected by a residual current device (RCD). The unit incorporates a temperature-operated latch 16 which responds when the temperature within the housing exceeds a first threshold by activating an audible and/or visual warning device 17, 18. The latch may also disconnect the electrical outlets 11, 12 from the incoming supply. If this does not remove the condition which caused the temperature rise, a thermal switch 19 responds when the temperature exceeds a higher second threshold to produce a current flow into the earth line in order to activate the RCD and thereby electrically isolate the unit from the incoming supply. The electrical supply unit may be used in combination with a message generating device which is configured to send a text message to a remote recipient when the electrical supply unit responds to a temperature rise.

Description

ELECTRICAL SAFETY DEVICE

TECHNICAL FIELD OF THE INVENTION

This invention relates to electrical safety devices for use in electrical supply units such as wall sockets and power outlets, extension sockets, or consumer units.

BACKGROUND

The two main causes of electrical fires are poor connections, which can cause arcing and excessive heat, and overloading of plugs and sockets.

Earlier patent application GB 2 446 186 A describes an electrical safety device which includes a thermal switch arranged to operate a warning device and trigger a thyristor latch that provides a leakage current to earth to activate a remote residual current device (RCD). It has been appreciated that this proposal has a potential drawback in that the remote RCD will often isolate a complete circuit and therefore remove power from appliances which are not at risk, so that refrigerators, freezers and heating systems could cease to operate until power is restored.

The present invention seeks to provide a new and inventive form of safety device which effectively protects an electrical supply unit from overheating and possible fire without unnecessarily disconnecting appliances not at risk.

SUMMARY OF THE INVENTION

The present invention proposes that an electrical supply unit which is arranged to deliver electrical power to one or more electrical outlets and which includes a housing having an incoming electrical supply with live, neutral and earth lines protected by an RCD, in which the unit incorporates a temperature-operated latch which responds when the temperature within the housing exceeds a first threshold by activating a warning device, and a thermal switch which responds when the temperature exceeds a higher second threshold to produce a current flow into the earth line in order to activate the RCD and thereby electrically isolate the unit from the incoming supply.

The electrical safety device therefore operates in two stages: 1. A low temperature latch activates a warning device. The latch may also operate to isolate at least one electrical outlet from the incoming electrical supply.

2. If a higher temperature is reached a switch operates to produce a residual current flow to earth which activates the RCD and removes power from the whole unit.

The temperature-operated latch may be arranged to operate an audible and/or visual warning device, and preferably requires to be manually reset from outside the housing. The unit will therefore continue to provide a warning to identify the source of the fault when power is eventually restored.

The electrical supply unit is preferably used in combination with a message generating device which is configured to send a predetermined text message to a remote recipient via a cellular telephone network when the electrical supply unit responds to a temperature rise.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice.

In the drawings: Figure 1 is a wiring diagram of an electrical wall socket in accordance with the invention; and Figure 2 is a wiring diagram of a consumer unit in accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring firstly to Fig. 1, the electrical wall socket shown by way of example is a double-socket type having a housing 10 which provides sockets 11 and 12 for reception of two electrical plugs. Electrical terminals 13, 14 and 15 are provided within the housing for connection of respective live, neutral and earth lines L, N and E, from an incoming alternating current electrical supply protected by a remote residual current detector device, RCD. In the case of an electrical supply comprising a ring main or fused spur, outgoing live, neutral and earth lines may be connected to the terminals 13-15 as shown, to feed further electrical socket units. The neutral and earth terminals 14 and are connected to the respective neutral and earth connections of the two electrical sockets 11 and 12 by means of conductive metal straps or similar means. The live terminal 13 is connected to the live connections of the sockets 11 and 12 via a thermally-operated latch 16, e.g. a single pole snap-acting change-over switch with mechanical latching and manual reset, mounted within the housing 10. Electrical appliances connected to either of the sockets 11 and 12 will normally receive electrical power from the mains supply via latch 16. However, in the event of an electrical fault which results in an increasing temperature within the housing 10, when the temperature exceeds its design threshold, typically about 50°C, the latch 16 will operate, disconnecting the sockets 11 and 12 and the appliances connected thereto from the live supply. At the same time, the latch 16 connects the incoming live supply to a visual warning light 17 and a sound generator 18, both returned to the neutral terminal 14, in order to produce an audible and visual warning that the socket has overheated. Even if the temperature drops below the design threshold the warnings will continue to operate to alert people to the fact that an alarm condition has occurred until the latch 16 is manually reset by depressing a button accessible from outside the housing 10 This will restore power to the sockets 11 and 12 and end the warning condition.

If disconnection of the sockets 11 and 12 does not remove the fault condition a second thermal snap switch 19 comes into operation. This switch has a higher design temperature threshold than the latch 16, e.g. 80°C, and operates to connect the live supply on terminal 13 to the earth line at terminal 15 via a suitable current limiting resistor 20. The increased current flowing into the earth circuit will be detected by the remote RCD causing the RCD to isolate the complete circuit, removing all electrical power from the electrical socket together with all sockets and devices on the same circuit.

It should be noted that if the switch 19 operates, the latch 16 will still remain in the latched condition even though all power has been removed by the RCD. Thus, when the power is eventually restored the latch will still cause the unit to produce an audible and visual warning to indicate where the fault has occurred. Also, power is not restored to the two socket outlets 11 and 12 until the latch is manually reset.

The two stage safety device therefore *operates to initially generate a warning and disconnect any appliances from the unit, but if this does not remove the overheating condition (e.g. due to faulty contacts in the unit itself) the entire circuit is isolated.

Although in the above example the invention is applied to a double wall socket it will be appreciated that the invention could be applied to other electrical units such as a single wall socket or an extension socket which is connected to a mains supply via a cable and plug.

Fig. 2 also shows how the invention may be used to protect a consumer unit 29 which supplies multiple electrical circuits via individual RCDs 30. The RCDs are each connected to a common bus 31 which is in turn fed from an incoming AC mains supply via a main RCD 32. The incoming neutral supply feeds a neutral bus 33, while the earth connections are all made to an earth bus 34. A first snap-acting thermal latch 35, set to operate at a relatively low temperature, connects the live bus 31 to the neutral bus 33 via audible and visual warning devices 36, so that a warning is generated if the operating temperature within the consumer unit exceeds the first threshold. If the temperature continues to rise to the threshold of a second snap-action switch 37 this connects the live bus to earth via a current limiting resistor 38, causing the main RCD 32 to disconnect the incoming live supply from the live bus 31, thereby isolating all circuits connected to the consumer unit. The two switches 35 and 37 may have a common manually-operated reset button.

The safety device therefore operates to initially generate a warning that a danger condition is imminent, but if this does not result in removal of the overheating condition all circuits are isolated.

The devices described above give a clear audible and visual indication of a possible fault condition so that people in the vicinity are alerted to the heat-related fault condition and can readily identify its location and take suitable action.

It is also possible to raise an alarm at a remote location by using a message generating device which is programmed to send a text message via a cellular telephone network to a predetermined number, or numbers, when the electrical safety device responds to a fault condition. Such a device may operated by the latch 19 or 35 for example. Alternatively, the device may be a plug-in unit which is inserted into a wall socket on a protected circuit and sends the warning message in response to a power failure. The device can also be programmed to send an appropriate text message to the recipient/s when a fault condition no longer exists, e.g. if a reconnection is made after a short power cut.

It should be noted that the second stage switches 19 and 37 are not required to have a latching function since the RCD will have removed power from the unit. Even if the switch resets the power will not be restored until the RCD is reset.

Whilst the above description places emphasis on the areas which are believed to be new and addresses specific problems which have been identified, it is intended that the features disclosed herein may be used in any combination which is capable of providing a new and useful advance in the art.

* * * * * * * *

Claims

CLAIMS1. An electrical supply unit which is arranged to deliver electrical power to one or more electrical outlets and which includes a housing having an incoming electrical supply with live, neutral and earth lines protected by an RCD, in which the unit incorporates a temperature-operated latch which responds when the temperature within the housing exceeds a first threshold by activating a warning device, and a thermal switch which responds when the temperature exceeds a higher second threshold to produce a current flow into the earth line in order to activate the RCD and thereby electrically isolate the unit from the incoming supply.
2. An electrical supply unit according to Claim 1 in which the temperature-operated latch is arranged to operate an audible and/or visual warning device.
3. An electrical supply unit according to Claim 1 or 2 in which the temperature-operated latch requires to be manually reset from outside the housing.
4. An electrical supply unit according to any preceding claim in which the temperature-operated latch is arranged to disconnect at least one such electrical outlet from the incoming electrical supply.

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5. An electrical supply unit according to any preceding claim in combination with a message generating device which is configured to send a predetermined text message to a remote recipient via a cellular telephone network when the electrical supply unit responds to a temperature rise.
6. An electrical supply unit according to Claim 5 in which the message generating device is configured to send the predetermined text message in the event that the electrical supply unit ceases to deliver power to the electrical outlets.
7. An electrical supply unit according to Claim 6 in which the message generating device is configured to send a different predetermined text message to the remote recipient in the event that electrical power is restored to the electrical outlets.
8. An electrical supply unit substantially as described with reference to the drawings.* * * * * * * *