GB2492401A

GB2492401A - Electrical plug with current and overheat protection

Info

Publication number: GB2492401A
Application number: GB201111222A
Authority: GB
Inventors: Alan Radford; Ian Wedlake
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-07-01
Filing date: 2011-07-01
Publication date: 2013-01-02
Anticipated expiration: 2031-07-01
Also published as: GB201111222D0; GB2492401B

Abstract

An electrical plug includes a replaceable current-operated fuse 12 and a non-replaceable thermal fuse 9. The plug may be reÂused after an over-current condition by replacing the current-operated fuse, which will not generally increase the risk of overheating. However, in the event of an overheat condition (whether or not accompanied by an excessive current flow) the plug will cease to function due to the thermal fuse and must be completely replaced. The plug preferably includes warning means capable of indicating when an overheat condition has resulted in activation of the thermal fuse, such as a neon lamp 26 which is connected across the thermal fuse. The thermal switch may comprise a tension spring (23, Fig 2) held in place by a metal pellet (24, Fig 2).

Description

ELECTRICAL PLUG WITH CURRENT AND OVERHEAT

PROTECTION

TECHNICAL FIELD OF THE INVENTION

This invention relates to electrical plugs.

BACKGROUND

Electrical fittings such as plugs and wall sockets may sometimes be subject to overheating. As such devices age the risk tends to become greater due to an increase in electrical resistance caused by oxidation of electrical contacts, or loss of resilience in spring contacts. Such overheating is even greater in the case of low voltage supplies, since the supply currents, which cause joule heating effects, are generally higher.

US 2007 0 139 842 Al discloses an electrical plug which incorporates a resettable thermal switch which cuts off power to an electrical device and provides a visual indication that an overheat condition has occurred. The switch can be reset once the malfunction no longer appears to exist. However, this uses a mechanical switch which can itself deteriorate with age or exposure to damp atmospheric conditions. Furthermore, the occurrence of any overheat condition can often cause further deterioration in the condition of a plug, which might increase, or even cause, a potential risk of fire.

The present invention seeks to provide a new and inventive form of electrical plug which provides protection against a range of fault conditions without increasing the risk of an overheat condition occurring within the plug itself.

SUMMARY OF THE INVENTION

The present invention proposes an electrical plug which includes a replaceable current-operated fuse and a non-replaceable thermal fuse.

The plug may be re-used after an over-current condition by replacing the current-operated fuse, which will not generally increase the risk of overheating. However, in the event of an overheat condition (whether or not accompanied by an excessive current flow) the plug will cease to function and must be completely replaced.

Preferably the plug includes warning means capable of indicating when an overheat condition has resulted in activation of the thermal fuse. A particularly simple and inexpensive yet effective warning may be provided by means of a light source (e.g. a neon lamp) which is connected across the thermal fuse.

A preferred form of thermal fuse comprises a sealed case containing a spring element which is held across a pair of electrical contacts by a body of meltable material.

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 front view of an electrical plug in accordance with the invention; and Figure 2 is a diagrammatic section through a form of thermal fuse which may be used in the plug.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring firstly to Fig. 1, the electrical plug comprises a moulded housing 1, which is shown with its safety cover removed. The cover can be attached by means of a fixing screw which is inserted into a threaded hole 2. Three pins project from the opposite rear face of the housing, namely an earth pin E, a live supply pin L, and a neutral supply pin N. A flexible three-wire cable 3, connected remotely to an electrical device, is secured to the housing 1 by means of a cable clamp 4. One of the wires 5 is connected to the earth pin E by means of a clamping screw, and a second wire 6 is connected to the neutral pin N using another clamping screw. The third wire 7 is connected to an internal terminal 8 by a further clamping screw.

This terminal 8 is connected to a thermal fuse 9, the opposite end of which is connected to a spring contact ha forming one end of a fuse holder, the spring contact lib at the opposite end of the fuse holder being joined directly to the live pin L. The fuse holder 11 receives a conventional current-operated fuse 12 of the kind which contains a thin wire which melts and breaks the electrical continuity when a predetermined design current is exceeded (e.g. 5 amps). The current-operated fuse is replaceable so that, for example, fuses of different current ratings may be inserted. Furthermore, should a fault occur in the electrical device which causes an excessive current to be drawn the fuse will "blow" in the usual way, preventing the flow of current to the device. When the fault condition has been rectified the fuse 12 can be pulled out of the fuse holder and replaced.

It will be noted that the cable 3 enters the housing at one end of the housing, which will generally be the lower end of the housing when the plug is inserted into a conventional wall socket, The thermal fuse 9 is located opposite the cable entry so that fuse 9 will generally be positioned at the top of the housing in use.

The thermal fuse 9 responds only to excessive temperature, which may be as low as 70 degrees C. When the protection temperature of the fuse is exceeded the fuse will "blow", isolating the device from the live pin L. However, the thermal fuse is non-resettable and non-replaceable, so that if an overheat condition occurs the plug must be discarded and a new plug fitted to the cable 3.

The electrical connections of the thermal fuse 9 are joined to the terminal 8 and contact ha by crimping or another form of permanent mechanical connection. A suitable form of thermal fuse is shown in Fig. 2. The fuse is contained within a sealed case 20 having a pair of electrical connections 21 and 22. One of the connections 21 is joined to a tension spring 23 which is held in contact with the other connection 22 by a pellet 24 of metal which is designed to melt at the required protection temperature. Electrical current will therefore flow through the spring 23 and pellet 24 until the temperature rises sufficiently to melt the pellet, whereupon the spring will retract and permanently break the circuit.

It should be noted that when an excessively high current flows through a thermal fuse this could cause the fuse to heat up and blow. However the current required for this to occur is greatly in excess of the permitted current rating of fuse 12, which will therefore break the circuit long before the thermal fuse 9 overheats. Since the thermal fuse is normally located at the top of the housing it will tend to heat up quickly due to convention currents within the housing.

Returning to Fig. 1, a neon indicator lamp 26 is mounted for registration with a window opening in the safety cover. The lamp is connected in parallel with the thermal fuse 9, between the terminal 8 and fuse contact ha, and in series with a current-limiting resistor 27, typically 100k ohms. When the thermal fuse is intact it will act as a current shunt across the neon lamp which will not therefore illuminate, but once the fuse has blown a sufficient voltage will generally be developed across the neon for it to light up, thereby providing a clear visual indication that the plug has suffered an overheat condition and should no longer be used.

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

<claim-text>CLAIMS1. An electrical plug which includes a replaceable current-operated fuse and a non-replaceable thermal fuse.</claim-text> <claim-text>2. An electrical plug according to Claim 1 which includes warning means capable of indicating when an overheat condition has resulted in activation of the thermal fuse.</claim-text> <claim-text>3. An electrical plug according to Claim 2 in which the warning means comprises a light source.</claim-text> <claim-text>4. An electrical plug according to Claim 3 in which the light source comprises a neon lamp.</claim-text> <claim-text>5. An electrical plug according to Claim 3 or 4 in which the light source is connected across the thermal fuse.</claim-text> <claim-text>6. An electrical plug according to any preceding claim in which the thermal fuse comprises a sealed case containing a spring element which bridges a pair of electrical contacts and is held by a body of meltable material.</claim-text> <claim-text>7. An electrical plug according to any preceding claim which includes a housing having a cable entry, and the thermal fuse is located on the opposite side of the housing from the cable entry.</claim-text> <claim-text>8. An electrical plug which is substantially as described with reference to the drawings.</claim-text>