EP0095315A1

EP0095315A1 - Heat sensitive circuit interrupter

Info

Publication number: EP0095315A1
Application number: EP83302817A
Authority: EP
Inventors: Neil Shaw C/O Heat Trace Limited Malone; Paul Michael Boshell
Original assignee: Heat Trace Ltd
Current assignee: Heat Trace Ltd
Priority date: 1982-05-22
Filing date: 1983-05-18
Publication date: 1983-11-30
Also published as: DE3370559D1; CA1194907A; EP0095315B1; JPS5937625A

Abstract

A heat sensitive circuit interrupter comprising an electrical conductor (6,7) made from a material of predetermined melting temperature and encased in an electrically insulating sheath (1). The sheath is designed to withstand temperatures higher than the melting temperature of the conductor, and the conductor in its molten state flows within the sheath to break the electrical continuity of the conductor. The materials of the conductor and the adjacent portions of the sheath are selected to provide a contact angle for the molten conductor which is sufficiently large for the molten conductor to flow into separated drops. The conductor may be a lead/tin solder incorporating a flux.

Description

The present invention relates to a heat sensitive circuit interrupter.
There are many applications in which a reliable heat sensitive circuit interrupter can be used to advantage. For example an interrupter which operates to interrupt a circuit when exposed to a temperature at or above a predetermined critical temperature can be used to trigger an alarm or any other appropriate response. One possible use of an interrupter is to monitor the temperature of an item of equipment and to shut down that equipment when the critical temperature is detected.
Thermostats of conventional type can perform the function of a circuit interrupter. Thermostats do suffer however from the limitation that they can sense the temperature in their immediate vicinity but cannot detect overheating outside that vicinity. Thus in many circumstances thermostats can only be used if the expense of installing a large number of them can be justified. For example, it is highly desirable to be able to detect overheating of cables whether these cables are themselves used for heating purposes or are simply used to carry power or information signals. Thermostats cannot be used to detect localised overheating in cables at acceptable cost.
Heating cables which are used for example to protect process plant against frost are generally referred to as heating tapes. Such tapes are wrapped around pipework and covered in insulation. It is not possible to detect "hot spots" reliably in the tape by monitoring current supplied or the tape resistance and thus the tapes and the systems in which they are incorporated must be designed to be "fail safe" if they are to be used in hazardous areas. A fail safe design is one in which any predictable fault cannot result in overheating. A failsafe design is expensive because it requires a higher degree of complexity and a higher nominal capacity than would be the case if the design was not required to accommodate a variety of possible fault conditions.
It has been known for many years that the heating of an electrical circuit above a critical temperature can be detected by incorporating in the circuit a circuit interrupter in the form of a wire which melts at the critical temperature. For example British Patent No. 336 270 dated 1929 proposes a heating element energised via a wire which melts to break the supply circuit when the heating element becomes overheated. Such arrangements have not found acceptance however because it is generally necessary to cover the wire in insulation and when the wire melts its insulation often retains the molten metal in its initial position at least for some time, maintaining electrical continuity.
This molten metal retention effect is described in British Patent No. 1 164 238 which proposes to overcome the problem by supporting the meltable wire without insulation inside a stiff insulating tube defining sufficient space internally to allow the molten metal to flow easily away from its initial position. One way of providing this space is to fill the interior of the stiff tube with an insulating substance that is non-flammable and disintegrates or melts at a temperature lower than that at which the meltable wire melts. Examples of such fillers given are silicon grease or a paste flux.
British Patent No. 1 141 234 also refers to molten metal retention, and suggests overcoming the problem by providing a body which is capable of absorbing the molten metal.
Both the above suggested solutions to the problem of molten metal retention are undesirable as they require non-standard extra features which cannot be included in cables at low cost.
More recently, proposals have been made as described in published PCT Application WO 83/01138 to provide a monitoring cable in which a meltable conductor is separated from another conductor by a permeable insulator. When overheating occurs molten conductor diffuses through the insulator and the resultant drop in resistance between the two conductors is detected by suitable monitoring equipment. Accordingly this device relies upon the fact that the molten portion of the meltable conductor remains in electrical contact with the unmelted portion of the meltable conductor leading to the monitoring equipment.
Flux is used in conventional meltable alloys such as solder to help the molten metal "wet" a surface to which it is to adhere. Accordingly it could reasonably be assumed that introducing flux into a sheathed meltable wire would increase the probability of any molten portion of the wire maintaining electrical continuity. Surprisingly it has been discovered however that this is not the case.
It is an object of the present invention to provide an improved circuit interrupter.
According to the present invention, there is provided a heat sensitive circuit interrupter compris- sing an electrical conductor made from a material of predetermined melting temperature and supported by an electrically insulating member which is able to withstand temperatures higher than the said melting temperature, wherein the material of the electrical conductor and the material of the portion of the insulating member with which it is in contact are such that when the electrical conductor is melted the contact angle between the molten conductor and the insulating member is sufficiently large for the molten conductor to flow into separated drops and thereby break the electrical continuity of the conductor.
Preferably, the conductor is made from a solder which incorporates flux. Solders consisting of 60% tin, 40% lead and incorporating longitudinal cores of flux have proved particularly successful, such solders being used conventionally for making electrical connections. The solder may be rolled to form a flat strip.
The present invention is based on the known theory of the behaviour of a liquid when placed on a solid flat surface, which behaviour is dependent upon the contact angle. The contact angle is defined as the angle subtended by the flat surface and a tangent to the liquid surface drawn from the edge of the liquid in a plane perpendicular to the flat surface and the edge of the liquid. If this contact angle is small, the liquid will "wet" the flat surface. If the contact angle is large, the liquid will form drops or bubbles.
The contact angle is the resultant of three thermodynamic forces F₁, F₂ and F₃ that act on each interface in the liquid/solid/surrounding vapour system. These forces are related as follows:

F₁ = F₂+ F₃ cos C
where C = contact angle
F₁ = surface free energy of the solid/vapour interface
F₂- surface free energy of the solid/liquid interface
F₃ = surface free energy of the liquid/vapour interface (surface tension)

The presence of a flux in a molten solder modifies these thermodynamic forces and hence will affect the contact angle. It has been discovered that a melted solder without flux willlie as a liquid film and not contract into separated drops, whereas a melted solder with flux will contract into bubbles.
In conventional solder, the flux is used to help the liquid to wet the surface. In contrast, in the present invention, flux is used to cause the liquid solder to separate into separated drops or bubbles. This contrast can be explained by considering the 3 possible effects that the flux has.

1) It chemically removes (or corrodes) any oxides or imperfections in the flat surface that tend to give the surface a high surface energy.
2) It chemically removes (or corrodes) any oxides of lead and tin that may have formed on the solder surface.
3) Excess flux provides a thin low energy film upon which the liquid solder can move more easily.

In conventional soldering the removal of deposits on the solid surface is the dominant effect and hence the solder will wet the surface, whilst in the present invention the removal of oxide film in the solder is the dominant effect and hence the surface tension of the liquid solder will cause it to flow into low energy forms, that is drops or bubbles.
An embodiment of the present invention will now be described, by way of example, with reference to the accompanhing drawing which is an end view of a heating tape incorporating a heat sensitive circuit interrupter according to the invention.
The illustrated heating tape comprises a sheath 1 within which two copper foils 2, 3 are encased. A woven heating element 4 is positioned beneath the foils but electrically insulated from them by a web 5 of insulating material. A pair of foils 6, 7 of solder are positioned on a support film 8 above the copper foils 2, 3 so as to be separated from the copper foils by a web 9 of insulating material. The support film 8 may be of glass fibre or the plastics marketed as "Kapton". Connections are made between the copper foils 2, 3 and the heating element 4 by inserting rivets through the heating elements and the copper foils at spaced locations along the length of the tape. For example rivets could be placed at one metre intervals along each foil, the rivets on one foil being staggered by 50 cm relative to the rivets on the other foil. A heating tape having a woven heating element and foil conductor structure of this type is described in British Patent No. 1 523 129.
The illustrated heating tape is made up by forming a core comprising the foil conductors 2, 3 embedded in an insulating body including webs 5 and 9. The outline of the core is indicated by a dashed line 10 in the drawing. The solder foils 6, 7 are then adhered to the film 8. The heating element 4 is pressed against one side of the core, and secured by rivets to the foils 2, 3,and the film 8 is pressed against the other side. The resulting assembly is then encased in the sheath. 1 by an extrusion process.
The tape may have any convenient dimensions, e.g. 20 mm wide and 4 mm thick. The solder foils 6, 7 may be formed by rolling out conventional fine multi-core lead/tin solder wire as used for making connections to electronic components to form a strip approximately 4 mm wide. It has been found that using such a solder foil a break of some 10 mm width occurs in the foil as soon as it is heated to its melting point, the molten solder flowing away from the break to thicken the ends of the foil on either side of the break.
Solders can be easily prepared which melt at well defined temperatures over a wide range of temperatures, e.g. 100°C to 300°C. Thus the illustrated tape can be used for a wide variety of purposes.
It is possible to dispense with the copper foils 2, 3 and use the solder foils 6, 7 to supply energy to the heating element. In some circumstances this might not be so advantageous however as if power is supplied via the solder sparks might occur when it melts and breaks. In contrast in the illustrated arrangement a low voltage monitoring circuit could be connected between the foils 6, 7 at one end of the tape, the other ends of the foils 6, 7 being connected together. With a low voltage monitoring circuit there is no risk of sparking.
It will be appreciated that one of the solder foils 6, 7 could be replaced by a non- fusible conductor of for example copper.
It will be appreciated that the invention has applications not related to heating tapes. For example a monitoring tape could be produced having only one or two solder conductors within it and no heating element or separate supply conductors. The monitoring tape could then be placed in areas where it is desired to detect excessive temperatures, e.g. in electrical cable conduit, or in the ceiling of a warehouse, and connected to a simple circuit adapted to sound an alarm if the solder conductor breaks. The monitoring tape could also be incorporated in equipment, e.g. the windings of electric motors, to automatically shut the equipment down in the event of overheating.
The illustrated embodiment shows the solder conductors in the form of thin foils. It will however be appreciated that the solder may be in other forms to suit particular applications providing that once molten it is capable of flowing to form a break.
Experiments have shown that both single and multi-core fluxed solder work satisfactorily although multi-core solder is particularly good as it flows more freely to form separate balls of molten metal. Simple unfluxed solder generally does not work as it melts but does not flow easily to form a break. Unfluxed solder lying in flux powder will also not work effectively if the flux powder is allowed to oxidise.
The described embodiment of the invention utilizes a solder in which flux is provided in the form of cores. The solder could however be externally coated with flux.
The term "solder" is used herein to mean any electrically conductive fusible material. Generally solder will be in the form of a low melting point fusible alloy. The flux can be of any suitable type, but care must be taken to ensure that the flux is stable at the normal temperatures to which it is in use exposed.

Claims

1. A heat sensitive circuit interrupter comprising an electrical conductor made from a material of predetermined melting temperature and supported by an electrically insulating member which is able to withstand temperatures higher than the said melting temperature, wherein the material of the electrical conductor and the material of the portion of the insulating member with which it is in contact are such that when the electrical conductor is melted the contact angle between the molten conductor and the insulating member is sufficiently large for the molten conductor to flow into separated drops and thereby break the electrical continuity of the conductor.

2. A heat sensitive circuit interrupter according to claim 1, wherein the electrical conductor is made from a solder which incorporates flux.

3. A heat sensitive circuit interrupter according to claim 1 or 2 wherein the conductor is in the form of a flattened strip.

4. A heat sensitive circuit interrupter according to claim 2, wherein the conductor is in the form of a flattened strip formed by rolling flat a cylindrical solder wire incorporating a plurality of cores of flux.

5. A heat sensitive circuit interrupter according to any preceding claim, comprising two conductors arranged in parallel at least one of which is the said conductor made from a material of predetermined melting temperature, the conductors being connected together at one end such that their electrical continuity can be monitored from the other end.

6. A heat sensitive circuit interrupter according to any preceding claim, incorporated in an electrical heating tape.

7. A heat sensitive circuit interrupter according to claim 6, wherein the said conductor made from a material of predetermined melting temperature constitutes a heating element of the heating tape.

8. A heat sensitive circuit interrupter according to any preceding claim, wherein the support member is in the form of a sheath which encases the or each electrical conductor.