GB2174853A - Safety circuit for electric heater - Google Patents

Abstract

Sensing and control means for an electrical circuit includes a thermosensor incorporating a signal element which has a pair of conductors 10a coaxially or non-coaxially formed with insulation therebetween which is negatively inversely proportional in resistance, impedance, capacitance or any combination thereof to temperature. The thermosensor is mounted in close proximity to a heating element 5 and monitors the heating element operation, the insulation of the thermosensor being arranged so that it can provide for detection of a temperature increase in the heating element, the conductors 10a of the signal element being connected into the same circuit line as an ambient temperature sensor 7 so as to override normal control by the ambient temperature sensor and therefore energisation of the heating element in the event of an overheat on only a portion of the heating element. The heater may be incorporated in a water bed or underfloor heating system. <IMAGE>

Description

SPECIFICATION Safety means for heating circuits This invention relates to a sensing and control means for electrical circuits and in particularto a sensing and control means for assisting in the safe operation of a heating element or other heating apparatus.

In the past, where heating elements are used, for example in environmental heating (such as in buildings or rooms), heating of particular apparatus, (such as the water filled mattress of a water bed, heating blankets), the control of the heating element has, hitherto, essentially been provided by means of an ambient control means, for example a thermostatically controlled switching means which switches on or off, or varies energisation of the heating element dependent on, for example ambient temperature, such as ambient temperature within the room, water temperature of the water bed mattress or the heat at some particular position on the apparatus having the heating element.

It is usual for the thermostatically controlled sensor to be spaced away from the heating element to avoid undue rapid cycling of energisation of the heating element owing to proximity effects of the sensor to the heating element, as distinct from ambient temperature.

Usually, a thermal cut-out is required in addition to the ambient temperature sensing for safety reasons, in the event that there is a short circuit or some other major electrical failure in the heating element. Such cut outs and ambient control means are unable to control overall operation of a heating element in situations where, for example a potentially dangerous hot spot develops in one part of the heating element remote from the temperature sensor, for example, in the case of a water bed damage may occur in an under mattress element which might result in the creation of a hot spot sufficient to melt the sheet plastic forming the bladder for the water filled mattress before any actual breakdown of the element occurs and operates the cut out. This can result in the release of substantial quantities of water or other fluid filling the mattress with disastrous results.Further, in the case of underfloor heating, it has been found that ambient thermostatic controls and cut out controls cannot overcome potential problems which occur where insulative articles; such as some furniture and other items, might be situated over the heated floor area. This can result in significant and potentially damaging overheating of that area of the floor with potential damage to the article, floor coverings, building and the like.

It will be appreciated from the foregoing that a problem exists in that hitherto, control/safety devices for heating elements have generally only operated when a major malfunction of the apparatus occurs, at which time shut down may occur too late to avoid significant damage or flow on effects.

It is an object of this invention to come some way in overcoming the abovementioned problems or at least provide the public with a useful choice.

Other objects of this invention will become apparent from the following description.

It is known to provide control means arranged to respond to a decrease in the resistance of a primary sensing means or circuit consequent upon a temperature change to cut off the power to an energised element or heat source. Conventionally, the sensing circuit includes a thermister located remote from the control means, connected thereto by suitable leads.

It is also known to provide what might be called a "thermosensor" element which, in essence, includes a pair of coaxial wires insulated one from the other by insulation which has a negative temperature coefficient of resistance, which changes its resistivity as the temperature rises. At normal operating temperatures the resistance across the two wires is high but upon the insulation being heated, the resistance drops.

According to one aspect of this invention there is provided a safety means including a pair of conductors insulated one from the other by a material which changes its resistivity, capacity or impedence, or a combination thereof as temperature rises, the conductors being connected into the same circuit line of a control means with which a primary sensing means as hereinbefore defined would normally be connected.

Other aspects of this invention will become apparent from the following description which is given by way of example with reference to preferred embodiments of the invention. Modifications are envisaged and may be incorporated without departing from the scope of the invention as defined in the appended claims.

In this form, the two conductors together constitute the thermosensor in effect a resistance, capacitance, impedence or any combination thereof in parallel with the circuit of the primary sensor. In normal operation the resistance, capacitance, impedence or any combination thereof of the thermosensor is substantially infinitely high when resistive and has minimal or no effect on the primary sensor/ control means arrangement.

Further, capacitance and/or impedence effects can be arranged so that there is little or no effect on the primary sensor/control arrangement.

If heat develops in the heater element circuit or heat source for any reason, the thermosensor is exposed to such heat and the control means will detect a decrease in the resistance and/or a variance in capacitance and/or impedence or any combination thereof and the control means will cause de-energising of the heater element circuit heat source. In the case of a time proportional control means, the decrease in resistance can cause the power to reduce or be shut down, as may be necessary.

The thermosensor can alternatively include a pair of conductors disposed non-coaxially adjacent one another, the conductors may include mutually substantially parallel wires or, according to an alternative feature of the invention, two or more laminar conductors arranged mutually substantially parallel.

One advantage of a laminar type of thermosensor is that it can provide uninterrupted temperature sensing over a large area (such as the area occupied by a heating element for a water bed, under floor heating and the like). Another advantage is that the thermosensor may be co-extensive with the heating element or source which may also be laminar rather than elongate. An elongate thermosensor element will, however, operate quite satisfactorily if it routed over a planar heating element or source area.

The safety means also has an advantage in that it can be connected to existing prior art control means for such devices as water beds or electric blankets, under floor heating and the like without modification or affecting normal operation.

An envelope means may be provided to enclose the heating element, to provide electrical insulation and/or a path for the transmission of heat from the heating element; the thermosensor can be situated either inside or outside the envelope, thus providing the option of applying the invention to new or existing devices.

Depending upon requirements of the circuit with which the safety means is used, additional componentry can be provided in parallel therewith, additionally the safety means may be connected in series with a conventional thermistor in suitable cases.

Since the device has little or no effect on the normal operation of the control means, more than one safety means can be provided arranged in series or in parallel with the primary sensor, and can be positioned adjacent or separately. They may also be arranged to operate at different temperatures or between different temperature ranges.

The safety means can also be connected with suitable circuitry in the control device to switch the power on or off or cause the controller to go to a half-wave state operating through a half wave rectifier or any other diode-effect device.

The insulation of the conductors for the thermosensor can be "doped" for example with a conductive carbon or otherwise arranged to provide any desired resistance and/or capacitance, and/or impedence, and/or any combination thereof up to a short-circuit condition (zero resistance) at a specified temperature.

In the case of the laminar thermosensor element, the insulation may be perforated at intervals to enhance its ability to provide short circuit conditions, for example conditions caused by mechanical pressure.

One embodiment of the invention will now be described with reference to the preferred form which is a safety device and heater element "pad" for a water bed to provide "thermosensor" protection of the heater element of the pad and hence the pad environment, that is decking, liners, mattress, underdrawers, carpet and the like and against other factors, for example, improper installation of the heating element such as, blankets tucked underthe mattress and over the pad, wallets or any insulative material accidentally disposed as to negate normal thermal transfer to the water or fluid, causing hot spots or overheated areas, or simply due to a fault developing in the heater element.

The invention is described with reference to drawings in which: Figure 1 shows, substantially diagrammatically the heater pad of the preferred form of the invention in a typical water bed installation.

Figure 2 shows a portion of the heater pad of Figure 1 in substantially plan view.

Figure 3 shows a portion of the heater pad of Figure 1 according to the cross section line AA of Figure 2 with a signal element enlarged "E" above.

Figure 4 shows a typical electronic control circuit for use with the apparatus of this invention showing optional enhancing means outlined by broken lines.

Figure 5shows an alternative embodiment of the invention in enlarged cross section showing the heating element in the form of a flexible sheet and the thermosensor in sheet form positioned thereadjacent.

In the preferred form of the invention, and with reference to Figure 1, the safety device as generally indicated by arrow 1 is provided as a heater pad 2 which positions beneath a fluid filled mattress 3 of the water bed, preferably sandwiched between a base 4 of the water bed and an under surface of said mattress 3.

The heater pad 2 incorporates a heating element 5 which is in the form of a flexible resistance pad and is coupled via a controller 6 with a mains power supply. Operation of the heating element 5 is controlled by a controller including a temperature sensor 7 preferably attached to the mattress 3 to measure "ambient" temperature of the fluid within the mattress 3 (controller circuitry shown typically in Figure 4). Thus the heater pad 1 can be used to maintain the water in the mattress 3 between a temperature of for example 20" to 40" Celcius in the substantially known manner Additionally, the heater pad of this invention incorporates the thermosensor; preferably including a signal elements 10 in the form of a pair of conductors 10a which co-extend with the heating element 5 over at least one surface thereof.With reference to Figures 2 and 3, it will be seen that preferably the signal element 10 conductors are co-axial an separated by insulating material 11, in this form polyvinylchloride "doped", for example with a conductive carbon to have negative temperature coefficient of resistance.

Preferably the insulating material 11 has a resistance of 30 Megohms to infinity at normal operating temperatures of between 20 to 40"C however, the constitution of the insulation is such that it reduces substantially in its resistance to, for example 30,000 to 5,000 Ohms upon encountering temperatures above 50"to 60'Cto in excess of 150"C whereafter melting or breakdown of the insulation 11 occurs causing the conductors to short circuit and provide zero or near zero resistance. The signal element 10 is outwardly insulated by a sheath 15 formed in a non-conductive material such as a suitable plastics material.

The heater element 5, via leads 12 and the signal element 10 via connecting leads 13 are both connected with the controller 6 preferably via "ganged" or common terminals 14. The conductors of leads 13 can be encased as one cable.

In operation, the heater element 5 in conjunction with the temperature sensor/control means will cause the fluid in the mattress 3 to attain and maintain a desired temperature and whilst the temperature of the heater element 5 remains below the operative or actuating level for the thermosensor, the resistance of such a safety means will be such that it will have little or no effect whatever on the normal operation of the heater element 5 via sensor 7 arrangement of the controller 6.

In the event that a portion of the heater element 5 overheats for any reason, the insulation 11 between the conductors 10a of the signal element 10 will decrease in resistance as the heat increases to override the temperature sensor resistance and cause the control means 6 to shut down or proportionately reduce the power output to the heater element 5. In the event of a significant overheat, where the insulation 11 breaks down and a short circuit results between the conductors 10a, a perma next shut down occurs until such time as the cause for the overheat is rectified.

Whilst the invention is described with reference to the preferred form where the operation of the safety device is dependent on resistance between the conductors of the thermosensor, in alternative embodiments of the invention, it is to be appreciated that an alternative control device may be provided whereby instead of resistance between conductors 10a being measured, capacitance, impedence or any combination of capacitance and impedence and their variation according to the temperature encountered by the signal element 10 may be provided.

Referring now additionally to Figure 4, the Figure shows a typical electronic control unit 6, the invention is not however limited to such a control unit and is merely provided for the purposes of example.

In operation the conductors 10a of the signal element 10 are connected in parallel with an existing thermistor primary sensor 7 which switches off on a resistance reduction of what is in this preferred form an electronic solid state controller 6 to power output terminals of the element 5.

In this form of the invention, one safety device 1 is depicted however, it is to be appreciated that a number of safety devices 1 may be connected in the line or across the primary sensor 7 of the controller.

The sensor is connected with the power supply via discreet or integrated control circuitry 21 and adjustment means in the form of, for example a triac 22 and with mains source at terminals 23 and 24. In the preferred form, preferably a neon indicator 25 is provided so as to indicate operation of the device as being on or offor cycling (fault condition).

Where the alternative means of control for the safety device is provided utilising capacitance, impedence or resistance of the signal element 10 preferably enhancing circuitry powered from the controller or independently is provided between the sensor 7 contacts and the signal element 10 so to enhance the signal from the signal element 10.

In the preferred form, the enhancing circuitry is positioned as shown by numeral 26 and preferably includes a short curcuiting device such as a switched transistor to provide a short across the sensor 7 and enhance the shut down effect, in some cases before melting of the signal element 10 takes place.

Referring to operation of the apparatus as described with reference to Figures 1,2 and 3, at ambient temperature, the resistance between the inner and outer conductor of the signal wire is extremely high and as a result has little or no effect on the resistance of the sensor 7. In normal operation of the heating pad, the resistance of the signal element 10 remains very high and has minimal effect on the sensor 7 and so allows heating to take place until the set temperature is reached. At this point, the sensor 7 causes the controller to switch off thus de-energising the heating element 5.

If a fault condition occurs in the heater pad 1, or other conditions result in overheating of the heater pad, or small sections thereof, temperatures in the faulted area will rise rapidly, typically from 50 to 60"C at normal to well in excess of 150"Cwhere, under normal conditions destruction of the insulation 11 will take place to provide a short between the conductors 10a.

The temperature rise causes the insulation between the conductors 10a of the signal element 10 to decrease in resistance and thus override the sensor 7 to have the effect of causing the controller 6 to de-energise the heater element 5, that is, cause the controllerto behave as though it had received signals from the temperature sensor 7 on the mattress 3.

The heater pad then starts to cool and so causes the resistance of the insulation 11 to increase. This in turn reduces the effect of the signal element 10 on the controller 6 which re-energises the element 5.

The cycle becomes continuous, holding the faulted area or heating area of the pad 1 at a non-destructive temperature until either the fault is rectified or the normal operating mattress temperature is reached, whereupon the sensor 7 will, via the controller 6 de-energise the heater element 5 until the sensor7 signals the controller 6 to turn "on" again, that is, the sensor 7 overrides the signal element 10. However, once the sensor 7 signals the controller 6 to reenergise the heater element 5, the signal element 10 will recommence its cycling so long as the fault exists.

If the case should arise that a rapid, for example, fault caused temperature rise reaches the insulation breakdown or melt temperature of the signal wire, the two conductors 10a ofthe signal element 10 will short together causing the contoller 6 to turn off completely (and probably permanently) and so require not only the removal ofthe fault condition but also the repair or replacement of the heater pad 1 to restore operation of the signal element 10.

In the preferred embodiment of the invention, the signal element 10 is calculated so as to provide a pad surface temperature of 55" to 65"C at normal conditions over the whole pad area and a progressive control to 80" to 100"C in local hot spots of about 7.5 centimetres square. A breakdown or short condition at the destruct temperature of the signal element insulation 11 which is usually 100" to 150"C. The sensor 7 provides a water temperature control of 20 to 40"C.

It will be appreciated that the safety device of this invention does not necessarily require any modification to existing electronic solid state circuitry as it operates in parallel with normal sensor 7 operations and simply alters the normal sensor 7 characteristics in a positive manner i.e. drives the circuitry to "off" when subjected to fault conditions and to stay off if the fault if more serious.

Now turning to Figure 5, an alternative embodiment of the invention is provided where the heater element 5 is provided as a resistance sheet for example a flexible sheet as is used in under floor heating, heating of water beds and the like. In this form of the invention preferably the signal element 10 is provided in a non coaxial manner, conductors 1 0a preferably in the form of a pair of sheets or strips of conductive material insulated outwardly by insulation material 15 and mounted to one side of the element 5, and separated from one another via the insulating material 11.

It will be appreciated that in this form of the invention the apparatus is particularly applicable to controlling the operation of large area heater elements and preventing hot spots occurring even in small areas thereon down to virtual pin points, for example, should mechanical damage occur to the sheet element or alternatively, the placement of articles over such a heating element, for example in under floor heating installations. It will be appreciated that upon a part of the sheet or strip signal element 10 becoming overheated, the resistance of the insulation 11 in that area will drop thus causing the sensor 7 to cycle to the off condition, deenergising the heater element 5 either temporarily in a cycling operation as described herein before or permanently, depending on the extent of the overheat.

Thus by this invention there is provided a safety device for heating elements.

Claims (21)

1. A safety means comprising a pair of conductors, insulated one from the other, the conductors being connected into the same circuit line to which the sensing means would normally be connected.

2. Apparatus as claimed in claim 1 wherein the conductors are connected one to each of the connections to which the sensing means would normally be connected.

3. Apparatus as claimed in claim 1 or claim 2 wherein the insulation between said conductors is arranged to alter a resistance/capacitance, impedence or any combination thereof upon heating of said insulation to cause operation of said control means to de-energise said energised heat source independently of an ambient temperature sensor.

4. Apparatus as claimed in any one of the preceding claims wherein the insulation of said conductors is negatively inversely proportional at least over a partial range to temperature.

5. Wherein the safety means is positioned predeterminedly adjacent at least a portion of said heat source.

6. Apparatus as claimed in claim 5 wherein said safety means incorporates the conductors which co-extend adjacent said heat source.

7. Wherein said conductors of said safety means are arranged co-axially one with the other.

8. Apparatus as claimed in any one of the preceding claims 1 to 7 wherein the conductors are non-coaxially arranged.

9. Apparatus as claimed in claim 8 wherein the conductors are provided in planar form as a strip or sheet.

10. Apparatus as claimed in any one of the preceding claims 1 to 9 wherein the safety means is provided for use with a planar heat source and is of a shape and size so as to lie adjacent a majority of said element area.

11. Apparatus as claimed in any one of the preceding claims wherein the safety means is positioned within an envelope.

12. Apparatus as claimed in claim 11 wherein said envelope includes in addition the heat source.

13. Apparatus as claimed in claim 11 or claim 12 wherein the heat source is positioned on either an inner wall or an outer wall of said envelope.

14. Safety means as claimed in any one of the preceding claims incorporating a plurality of separated thermosensors each mounted adjacent a heat source and each coupled to one control means.

15. Apparatus as claimed in any one of the preceding claims wherein said safety means is provided as a laminate structure.

16. Apparatus as claimed in claim 15 wherein the insulation between the conductors is perforated or otherwise apertured.

17. Apparatus as claimed in any one of the preceding claims wherein said insulation is in the form of a plastics material "doped" to provide a predetermined resistance capacitance, impedence or a combination thereof, which has a negative temperature coefficient of resistance, capacitance, impedence or a combination thereof.

18. Apparatus as claimed in claim 17 wherein said insulation is doped with conductive carbon.

19. Apparatus as hereinbefore described with reference to Figures 1, 2 and 3 of the accompanying drawings.

20. Apparatus as hereinbefore described with reference to Figures 1,2,3 and 4 of the accompanying drawings.

21. Apparatus as hereinbefore described with reference to Figure 5 of the accompanying drawings.