EP0715483A2

EP0715483A2 - Electric heaters

Info

Publication number: EP0715483A2
Application number: EP95308639A
Authority: EP
Inventors: Dominic Michael Anthony Oughton
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 1994-11-30
Filing date: 1995-11-30
Publication date: 1996-06-05
Also published as: GB9424173D0; EP0715483A3; GB2296847A; GB2296847B

Abstract

A printed circuit "plate" heater 10 comprises an insulated steel disc having conductive tracks (18 etc.) deposited thereon. Live 11 and neutral 12 terminals are formed in the centre. Towards the circumference of the heater, is an unheated conductive ring 15 connecting eight discrete thermal fuses 16 designed to blow at 180° (ie. serious overheat condition). Conductive track 17 links the fuses to seventy-two heater cells 22 connected in parallel and formed of 1000Ω/square resistivity material or tortuous tracks of 10Ω/square material having a high PTC of resistance. In the event of localised overheating of a heater cell, its resistance will increase, leading to a decrease in its heat output. Serious overheating will result in a flow of heat to one or more fuses 17 which will then below.

Description

This invention relates to electric heaters, and in particular, electric heaters for use in domestic appliances such as electric water heating vessels.
One very common type of domestic electric water heating vessel is the electric kettle or the more recent variant the electric water heating jug. In these devices an immersion heater is located within a water receiving portion formed by the metal or plastics body of the vessel. The heater has an element which is bent into a tortuous shape in order to provide sufficient length within the confines of the jug. Each end of the element is attached to the heater head which is clamped in place through an opening in the side of the vessel, thereby securing the heater in position. On the far side of the head are provided the so-called cold tails which are the electrical connections to the element. Typically, an integrated control, for example one of the type described in GB 2181598 is mounted directly to the back of the head by means of studs or screws. Such a control provides convenient electrical connection to the element and also contains apparatus to cut off the supply of current in the event that overheating occurs, for example as a result of the vessel being allowed to boil dry. Optionally, the control may also have a steam operated switch which may be used to switch of the kettle or jug automatically when it boils.
More recently, it has been proposed to replace the traditional design of immersion heater with one of a generally planar form manufactured by depositing a conductive track on a substrate by means of a thick film circuit printing process. An example of such a heater is illustrated in WO 94/18807 where a stainless steel substrate is first covered with layers of an insulating material such as glass, ceramics or enamel and the surface thus provided then has a resistive heater track printed upon it. Further insulating layers may then be added to complete the device. An alternative arrangement includes a ceramic substrate. Such a heater has the advantage that it may conveniently be mounted adjacent the bottom of the water receiving part of the vessel. This produces a water heating vessel which is both more pleasing aesthetically and also much easier to clean than one with the traditional immersion heater. Mounting the heater in this position also has the advantage that an integrated control may be provided in the base of the vessel. This allows for much greater flexibility in the design of the outward appearance of the vessel and also may simplify manufacture, particularly if it is desired to produce a vessel of the well known "cordless" type in which the vessel mates with a base unit. An example of a control unit which provides these benefits is illustrated in co-pending patent application No. GB 9417243.4.
However, despite the significant advantages of these so-called "plate heaters", they have certain drawbacks. Plate heaters generally have a low thermal inertia as a result of the comparatively small mass of metal which they employ. This has the advantage that they heat up more quickly, but unfortunately it also has the side effect that if, for any reason, overheating occurs, for example as a result of a vessel boiling dry, the overheating may be particularly rapid and this may lead to difficulties in providing sufficiently fast-acting overheat protection devices.
It is standard practice to provide overheat protection devices as a means of disconnecting the electrical supply from the heater of an electric water heating vessel in the event of its overheating. In a traditional kettle or jug, this is provided by means of a Dry Switch On (DSO) thermal actuator (typically a bimetal) inside the control which is held against the heater head. The heater head is in thermal contact with a raised portion of the element called the hot return which ensures that the bimetallic actuator is triggered soon after the element starts to be uncovered. However, as discussed above, simply holding a standard thermal actuator against a region of a plate heater may not always be effective.
One approach to this problem which has been proposed is to form the resistive tracks of a material which increases in resistance as it heats up, ie. one having a positive temperature coefficient of resistance (PTC). (The temperature co-efficient of resistance, multiplied by the temperature rise and the initial resistance gives the increase in resistance of the material.) Thus, when the heater is first switched on, its resistance is comparatively low which results in a high current flow and therefore a high output of heat from the heater (from ${P=V}^{2} /R$
). Subsequently, as the temperature of the element increases, the track resistance increases at a corresponding rate determined by the PTC. Thus, with a suitable choice of component values, the heater will quickly reach its normal operating temperature, but in the event of serious overheating, its resistance will increase to the extent that its power output will reduce and thereby at least reduce the rate of overheating to a manageable level. This arrangement is described in EP 0286217, for example.
However, the applicants have recognised that problems could arise if a heater designed according to the teachings of this document were to be incorporated in a water heating vessel, for example in place of the plate heater in WO 94/18807. Although it would provide protection against overheating in the event that the vessel were to be switched on whilst completely empty, it might not be effective if the element became only partly uncovered, for example as a result of the vessel boiling dry whilst resting on an inclined surface. This is because the exposed part of the heater will increase much more sharply in temperature than the immersed part. Each of the resistive tracks of EP 0286217 is arranged in a tortuous path extending over much of the surface of the element, and only those regions of each track corresponding to the exposed part of the heater will experience a significant temperature rise. Consequently, only those regions of track will experience an increase in resistance. Unless the exposed part of the heater comprises a large part of its total surface area, the overall increase in resistance may be small compared to the total resistance of the tracks. Therefore there may be a relatively small effect on the total resistance of the tracks. Consequently, since the supply voltage will be constant, the current flowing through the heater tracks will not be significantly changed, and neither will the heat output from the major part of the heater. However, as the heat output from an electrical resistor is proportion to its resistance (from ${P=I}^{2} R$
) the amount of heat emitted from the exposed portions of track will actually increase as their temperature, and therefore resistance, increases. This effect is self-enhancing and encourages local overheating. Thus, in these circumstances the positive coefficient of resistivity may lead to extremely rapid local heating which may cause serious damage to the heater or vessel before the heater is disabled by an associated control.
According to a first aspect of the invention there is provided an electric heater comprising a substrate and a plurality of discrete heater cells electrically interconnected in parallel and distributed about the surface of the substrate, each cell having an electrically resistive element with a positive temperature coefficient of resistance.
By dividing the heater into individually heated cells in this manner, overheating of any part of the heater will result in the heating of one or more small elements substantially in their entirety, instead of a small portion of one or more elongate elements, as in EP 0286217. Since the elements are connected in parallel, preferably directly across the electrical supply, the heating of one such element will be along a length corresponding to a large voltage drop (eg. up to 240V in preferred embodiments). Therefore, the increase in resistance resulting from the positive temperature coefficient of the elements will lead to a significant drop in current flowing through overheated elements and therefore to a drop in their heat output. Thus, it will be seen that if a region of the heater is exposed then the heat output of the cells in that region will reduce accordingly.
It will be appreciated that the higher the positive temperature coefficient of the elements, the greater this self limiting effect will be. For the effect to be significant it is preferred that the value of the PTC be at least 0.003 or 0.005/°C and preferably in excess of 0.006/°C. Higher values will produce a more significant effect, but the choice of such materials is presently limited. The maximum practical would be around 0.01/°C - higher values might draw excessive current when cold.
Although it will be seen that the invention will provide advantages with the heater divided into a comparatively small number of heater cells, such as eight or sixteen, larger numbers of smaller cells, such as up to two hundred, would produce a better effect. The cells should be distributed in two dimensions. This is because, in the situation described above, a smaller proportion of the cells will be on the boundary between the normal and the overheated parts of the vessel. Additionally, smaller cells are less likely to have a significant temperature differential across the length of their element which could lead to localised heating in a manner analogous to that described in relation to the prior art. Moreover, any such overheating which did occur for this reason would be limited to a small cell, and so could be accommodated by the substrate acting as a heat sink.
However, there is a practical upper limit to the number of cells resulting from the size constraints of the heater and the need to be able to print each resistor with a certain length and width of track to give the correct heat output and sufficient strength. In practice the maximum number of cells will be limited by the sheet resistivity of the conductive track material and the minimum track width which can be produced. In general, one would wish to minimise track width in order to maximise the number of cells possible for a given track material resistivity. In practice track widths of 0.15-0.4mm may be beneficial. As a result of these considerations, it is thought that more than one hundred elements in a heater designed for use in a domestic water heating jug may be unnecessarily difficult to produce in comparison to the benefits which are to be obtained.
A further advantage provided by having a large number of resistors in parallel is that it greatly reduces the chance of a heater having to be rejected during production, since if there is a break in the track in one of the resistors it will have little effect on the overall power output of the heater. Thus, the failure of a small number of heater cells may be tolerated.
The precise arrangement of the cells is not critical to the operation of the device, but preferably they are shaped and arranged such that if the vessel in which the heater is mounted is tipped in any direction, as small a proportion of the cells as possible will be located in positions corresponding to the boundary between the immersed and exposed regions. Thus, significantly elongate cells which extend across a large part of the area of heater should be avoided. One particularly effective arrangement which is appropriate for use with disc shaped heaters, is to arrange the cells in a generally ring-like configuration, for example disposed radially between a pair of concentric conductors. Preferably, more concentric conductors are provided with further cells extending between them. The most preferred arrangement is for the conductive rings to alternate between being connected to the live and neutral supplies.
A heater according to the invention may be provided by mounting thin sheets or strips of metal or resistive material against a suitable insulator on a substrate. An alternative option is to use thick film printed circuit technology. Thus, a steel or stainless steel substrate may be coated with a number of insulating material layers of ceramic, enamel, glass etc. onto which the conductors and elements may be screen printed using conductive and resistive inks. Inks are available in a wide range of resistivities, for example from the order of 1mΩ/square up 10,000Ω/square and higher.
The precise configuration of the resistors forming the elements within each cell is dependent upon the resistivity of the ink employed. For example, if a low resistivity ink is used, it will be necessary to provide a long length of track in each cell by arranging the resistor in a tortuous path. Alternatively, if a high resistivity ink is used, a simple shape, such as a rectangle of ink covering most of the cell may be more convenient.
In practice, it has been found that the highest positive temperature coefficients of inks presently available are found in comparatively low resistance inks (ie 10 Ω/square or lower). Therefore, low resistance inks should be used to provide a significantly self-limiting heater.
In a traditional kettle or jug, the DSO thermal actuator is held against the part of the heater head which is adjacent the hot return part of the element. Since the hot return is the highest part of the element it will generally become uncovered first if the vessel starts to boil dry and so, in such a situation, it is normally the hottest part of the heater. This ensures that the thermal actuator will trip rapidly.
However, as plate heaters are preferably generally planar, it is not desirable to provide a raised portion analogous to the hot return. However, it is still desirable to provide a single region on the heater which is likely to overheat first in order to provide a reliable trigger for the DSO bimetal.
There is, therefore, preferably provided a so-called "hot spot" on the surface of the heater which has a higher output power density, and will therefore experience a faster temperature rise than the remainder of the heater. This may be provided either by using a resistance ink for this portion of the track in series with the rest of the heater, by providing a narrower track, and/or by increasing the density of resistor tracks in the area. In use, the hot spot is arranged adjacent a thermal actuator such as the known type of DSO bimetal in order to provide a rapid control response.
It is believed that this arrangement is in itself a new departure and therefore, from a second aspect, the invention provides a combination of a plate heater and a thermal actuator, the plate heater having a region of track which in use has a higher output power density than the remainder of the plate heater, wherein the thermal actuator is located adjacent said region and the arrangement is such that in the event of the plate heater overheating, the thermal actuator operates switch means to disconnect the supply of electrical current before any part of the plate heater track is ruptured.
In order to improve the control response still further, the hot spot may have a higher PTC than the remainder of the heater. As discussed above, when in series with the heater such a region may enhance localised heating.
Preferably, the heater is designed for use in combination with an integrated control such as that described in our patent application No. GB 9417243.4 which incorporates a thermal actuator. In this event, mounting arrangements such as screw holes or studs would be provided on the heater to cooperate with those on the control. Preferably, therefore, the hot spot is arranged adjacent these mountings to align with the thermal actuator on the control.
It is generally required that electric water heating vessels to have a further backup protection system which would operate in extremis in the event of the main overheat protector (the DSO) failing. This is to prevent a fire risk in these circumstances. Although these are frequently provided within a control unit, in the present case it is preferred to form a thermal fuse on the heater itself. One way of doing this, namely to provide a narrow part of track which will preferentially rupture is disclosed in WO 94/18807. However, various other possibilities are available. For example, a gap may be provided in a track which may be bridged by an easily meltable conductor, such as solder or solder paste which will melt when it reaches a certain temperature due to the heater overheating, and break the circuit. Alternatively, a leaf spring may be bent into an arc and soldered whilst in tension across the gap. If the temperature becomes sufficiently high for the solder to melt, this will release one end of the spring and thereby break the circuit.
Although a single such thermal fuse may be provided on the heater, it is preferred to have a number of them connected in series and distributed about its surface. As well as providing a backup if one of them fails to operate, such an arrangement is sensitive to different regions of the heater becoming seriously overheated. Preferably, the fuses are arranged around the outside of the heater so that in the event of serious overheating taking place with the heater partially covered by water at least one fuse will blow. The situation of an exposed part becoming dangerously hot after the DSO protection has not operated, whilst the thermal fuse also does not operate because of the adjacent part of the heater being covered by the water, is therefore avoided.
This arrangement is, in itself, believed to be inventive, and therefore from a third aspect the invention provides a plate heater having an overheat protection arrangement provided around its periphery, the arrangement being such that overheating of any substantial part of the periphery of the plate heater will break the supply of current to the plate heater.
The overheat protection arrangement preferably comprises a long single thermal fuse (eg. a narrow portion of track) extending around the heater perifery, or one of the arrangements discussed above.
A heater according to the invention may be mounted in the base of a water heating jug or kettle as a direct replacement for that in WO 94/18807. However, a further application which is thought to be particularly significant is to use a heater according to the invention to produce an example of the European type of water heating vessels known as "wasserkochers". These traditionally have a metal body with a fairly thick base into the outside of which an immersion heater is pressed. Although this design results in less effective transfer of heat from the element to the water within a vessel, it does produce a more attractive vessel since a need for an exposed immersion heater is avoided. This also makes the vessel much easier to clean than a traditional kettle or jug.
The heater of the present invention may be incorporated in such a vessel in a number of ways. For example it may be mounted through the base in a manner similar to that described above in relation to a kettle or water heating jug. Alternatively, it could be mounted on the outside of a vessel, preferably having a comparatively thin base to allow efficient conduction of heat. However, the presently preferred arrangement is to use the heater itself to form a major part of the base of the vessel. This may most conveniently be achieved if the heater is disposed with the side on which the tracks are provided facing downwards. This arrangement avoids any problems regarding the insulation of the heater since the live parts are in a permanently dry region.
A vessel of this type incorporating a heater according to the invention could comprise a generally cylindrical body having a flange at its lower end against which the circumference of a heater could be clamped, welded etc. It is thought that clamping the heater would be most convenient in order to avoid the risk of damage to the heater itself. Preferably, an annular seal is provided to prevent water escaping at the joint between the heater and the vessel.
In order to prevent the heater from damaging the seal, it is preferred to provide an unheated region of the element adjacent the seal, for example in the form of an annulus around the outermost part of the track. The provision of such a region has a further advantage in that it may be used for mounting the thermal fuses so that they are distanced from the heater cells, thereby reducing the risk of their being unnecessarily tripped in the event of slight overheating. However, in the event of serious overheating, the region where they are located will be heated by conduction sufficiently to cause a fuse to blow. Since the fuses are interposed between the cells and the vessel walls, a fuse will blow before sufficient heat reaches the vessel walls to cause serious damage.
It is believed that this arrangement in itself is inventive, and therefore, from a fourth aspect, the invention provides an electric water heating vessel having a body and a plate heater, there being provided an array of overheat protection devices disposed between a heated region of the plate heater and a region of the vessel which is to be protected from overheating.
Conveniently, the overheat protectors are thermal fuses formed integrally with the heater as discussed above, but alternatively, re-settable overheat protectors, such as bimetallic actuators may be used, for example held against the dry side of the plate heater.
If the plate heater is secured to the vessel at its perifery then the overheat protectors are ideally located adjacent the heater perifery. However, if a central part of the heater is sealed to the floor of the vessel, as in WO94/18807, then the overheat protectors may be provided towards the centre of the plate heater, adjacent the seal.
The invention also extends to a vessel, for example a water heating jug, kettle or wasserkocher incorporating a heater according to the invention.
Certain embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which:-

Figure 1 is a schematic sectional view through a water heating vessel incorporating a heater according to the invention;
Figure 2a is a schematic view of the underside of a heater according to the invention;
Figure 2b is a graph showing the temperature distribution across the heater of Figure 2a in use;
Figure 3 is a view of the underside of a heater according to a first embodiment of the invention;
Figure 4 is a view of the underside of a heater according to a second embodiment of the invention;
Figure 5a is a view of the underside of a heater according to a third embodiment of the invention;
Figure 5b is an enlarged view of part of Fig. 5a;
Figures 6a and 6b are views of part of a heater illustrating a first type of thermal fuse;
Figures 7a and 7b are views of part of a heater illustrating a second type of thermal fuse; and
Figures 8a and 8b are views of part of a heater illustrating a third type of thermal fuse.

Turning first to Figure 1 there is illustrated a water heating vessel 1 having a water receiving portion formed by a generally cylindrical stainless steel body 2 which forms the sides and a disc shaped printed circuit heater 3 which forms the bottom. The heater 3 is clamped to the body 2 and the joint is made water tight by the provision of an annular seal 4. A thermally sensitive control 5, for example of the type described in co-pending patent application No. GB 9417243.4 is clamped against the underside of the heater 3 using screws or by means of studs projecting from the bottom of the heater. The control provides electrical connection to the heater 3 and has means to disconnect the supply of current from the heater in the event of its overheating. The control may also serve to switch off the vessel when water within it boils, in which case a steam channel is provided between the inside of the vessel and a steam operated actuator within the control 5. Finally, the vessel is completed by providing a cup-like housing (not shown) over the bottom of the vessel to cover the electrically live components and provide a suitable base to the vessel. If required, a handle may also be attached to the side of the vessel.
The heater 3 comprises a stainless steel disc on which four or five layers of ceramic material are deposited by printing, spraying or dipping. Alternatively, enamel may be deposited by spraying or dipping. These processes form an insulating layer over the steel.
Next, the conductive and resistive tracks, through which electrical current will flow and which form the heater elements, are printed onto the insulating layer. The configuration of these will be discussed in detail below. After the printing stage the element is fired and then a layer of silicate or low temperature glass is deposited on the heater and fired in order to provide an insulating outer surface. Gaps are provided at certain points in this layer in order to permit electrical connection to the printed tracks. These are all known processes.
The side of the heater 3 on which the tracks are provided faces downwardly (as viewed in Figure 1) so that the other, plain, side forms the visible bottom surface of the water receiving portion. In order to prevent seal 4 from being damaged in use by the heat generated by the heater, an unheated region 6 which has no heater elements is provided around the outside of the heated region 8, as shown in Figure 2a. This prevents the sealing region 7 around the circumference of the heater from becoming as hot as the remainder of the heater so that the seal 4 is not damaged. Figure 2b is a graph showing the approximate temperature distribution in the radial direction X-X across the heater. It will be noted that there is a significant temperature drop across the unheated region 6. This is because steel has a relatively low thermal conductivity and the water in the vessel acts as heat sink.
Figure 3 illustrates the arrangement of tracks on the underside of a first embodiment of heater 10. Towards the centre of the heater are provided exposed areas of conductive track which form the live 11 and neutral 12 terminals respectively. A short piece of conductive track leads from the live terminal 11 to a "hot spot" 13 which is a small region of track arranged in a tortuous path. The track in this region is formed from the same ink as the conductive track, but it is narrower with a result that localised heating occurs when the heater is energised. In fact, the resistance and density of the track forming the hot spot are chosen in order to make this region the hottest part of the heater in use. The hot spot 13 is located against the overheat protection mechanism (bimetallic actuator) (not shown) of the control 5, and this ensures that the electrical supply is disconnected rapidly in the event that, for example, the vessel boils dry. In normal operation, the heater temperature should not greatly exceed 100°C except for the hot spot which should not exceed about 130°C. The bimetallic actuator in the control is arranged to disconnect the electrical supply when the hot spot exceeds its normal temperature during boiling by more than about 20°C.
Leading from the hot spot 13 is a further piece of conductive track 14 which leads to an unheated conductive ring 15 located towards the circumference of the heater. The ring 15 connects eight discrete thermal fuses 16 in series so that if any one of them blows, the supply of current to the heater will be cut off. The thermal fuses 16 are located in the unheated region 6 of the heater so that they are not triggered by the normal action of the heated part of the element, rather, they are triggered by heat conducted through the substrate caused by serious overheating. The fuses are designed to blow if they reach about 180°C. Because they are located between the heated part of the element and the seal 4, in the event of serious overheating (ie. if the primary protector of the associated control 5 should fail), the thermal fuses will be heated sufficient to cause them to blow before enough heat reaches the seal to potentially set fire to the seal or vessel or otherwise constitute a serious safety hazard. The thermal fuses are equally radially spaced around the heater so that if the vessel is tipped away from the horizontal whilst energised sufficiently to expose one edge of the heated part of the heater, thereby causing it to overheat, regardless of the direction in which the vessel is tipped, there will always be a fuse sufficiently close to the overheated portion to blow before a safety hazard arises.
A further piece of conductive track 17 links the ring 15 to another almost ring-shaped conductor 18 located within it. Extending radially on either side of this conductor are seventy-two heater cells. These are illustrated as each comprising a rectangle of resistive track of 1000Ω/square resistivity forming an element 22. However, since at present inks with this resistivity are only available with a relatively low PTC, the rectangles may be replaced with a tortuous track of lower resistivity (eg. 10Ω/square) in order to take advantage of the higher PTC inks available with low resistivity. Two further concentric conductors, an outer ring 19 and an inner ring 20 are provided on opposite sides of the ring 18. These form the neutral connections to the elements 22 via a further piece of conductive track 21 which links the outer and inner rings to the neutral terminal 12 of the heater. It will be appreciated that the arrangement of the elements 22 and the conductors 15, 19, 20 etc. is such that all of the resistors 22 are connected in parallel across the terminals 11 and 12.
The resistive ink used to form the elements 22 is chosen to have as high a positive temperature coefficient of resistance as possible. Thus, if an element becomes overheated, for example due to there being no water in contact with the corresponding part of the heater surface, its resistance will increase as the temperature increases. Consequently, the flow of current through the element is reduced and therefore the power produced by it is also reduced. This at least reduces the rate of overheating. Since a large number of discrete resistors are provided, each covering only a small area of the heater, localised heating will cause the resistance of the elements in that area to increase and thereby reduce their output power as discussed above.
A second embodiment of heater 30 is illustrated in Figure 4. This is of generally similar configuration to the first embodiment but does differ in several significant respects which mainly result from its being designed for use with resistive inks having much lower resistivity. The use of such inks is desirable since they are presently available with much larger positive temperature coefficients of resistance than the higher resistivity inks. This embodiment uses ink having a resistivity of 10Ω/square and a temperature coefficient of resistance of approximately 0.007/°C.
The live terminal 31 is connected to a hot spot 32 which is formed from a small area of resistive material having a resistance of about 2Ω. This is connected by a radially extending piece of conductive track to a circumferential thermal fuse 33. This is formed from a low resistance ink having a high PTC. The track is of just sufficient width to carry the normal flow of current. However, if a region of the substrate on which the track lies is heated significantly, eg. in the event of the vessel boiling dry, this will increase the resistance of the track and thereby cause it to heat further. In the event of serious overheating, the track will be heated to such an extent that it will rupture and cut the supply of current to the heater. From the thermal fuse 33 a further conductor leads to the heater cells 40 as in the previous embodiment. However, in this case, there are four rings of cells in place of the two in the previous embodiment. Consequently, there are two conductive rings of track 34 and 35 which are connected to the live terminal via the thermal fuse 33 and three conductive rings 36, 37 and 38 which are connected to the neutral terminal 39. It will be appreciated that in this way the radial dimension of the heater cells 40 is decreased compared to the previous embodiment. This is advantageous because it increases the chance that a portion of the heater corresponding to an entire element will be uncovered if the vessel is tipped sufficiently to partially expose the heater and therefore the decrease in power output in the exposed region will be more significant.
Since the resistivity of the ink used in this heater is low it is necessary for each cell 40 to comprise an element having a tortuous path, rather than a single block of resistive material.
A third embodiment of heater 45 is illustrated in Figures 5a and 5b. It will be noted that this is generally similar to the previously described embodiment, but there are certain differences. An additional ring of elements 40 has been provided (reference numerals correspond to those used in the second embodiment), together with an additional conductive ring 46 to provide electrical connection to the live terminal. The elements 40 are shown schematically in Fig. 5a. In fact, each of these is a tortuous track of low resistivity material similar to those of Figure 4. Fig. 5b illustrates one such element 40.
It will be appreciated that, for the reasons explained previously, by increasing further the number of elements, this embodiment provides still greater protection against overheating caused by a partially uncovered element.
This heater also differs from the second embodiment in that it has the thermal fuse arrangement of the first embodiment, ie. an array of discrete fuses 33'. In addition, the discrete tracks connected to each side of the hot spot have been dispensed with. Instead the hot spot directly abuts terminal 39 and track 36.
Although the second embodiment incorporates a different type of thermal fuse from the other embodiments, these fuses are, in fact, interchangeable. With regard to the discrete thermal fuses, various methods of producing these are possible, as illustrated in Figures 6a to 8b. In general, they comprise a break 50 in the conductive track 51 which is bridged by some form of heat sensitive material. An alternative form of fuse which is not illustrated in these figures is to provide a short narrow region of track.
In Figure 5a, the gap is bridged by means of solder paste 52 which has been applied and partially cured without reflowing. In normal operation, the current will flow through this paste, but in the event that the fuse overheats, the paste 52 will melt and tend to form spheres 53 around each end of the track 51 as a result of surface tension. This will result in the bridge being broken as illustrated in Figure 5b.
A similar method is illustrated in Figure 6a in which the solder paste is replaced by solder wire 54. The wire may be attached by means of soldering, compression or discharge welding or using conductive epoxy. Again, overheating of the fuse leads to the wire melting at which time surface tension again results in the solder accumulating around the conductors at opposite sides of the break in the track, again breaking the bridge between the two pieces of track (Fig. 6b).
Finally, in Figure 7a a leaf spring 55 is bent into an arc and each end is soldered to the conductor across the break in the track. Since the spring is under tension, if the solder connecting it to the conductor at each end were to melt as a result of overheating, the spring will tend to straighten which will pull it away from the conductor, thereby breaking the circuit as illustrated in Figure 7b.

Claims

A plate heater having an overheat protection arrangement provided around its periphery, the arrangement being such that overheating of any substantial part of the periphery of the plate heater will break the supply of current to the plate heater.
A plate heater as claimed in claim 1 wherein the overheat protection arrangement comprises one or more re-settable overheat protectors.
A plate heater as claimed in claim 1, wherein the overheat protection arrangement comprises a thermal fuse.
A plate heater as claimed in claim 3, wherein the thermal fuse is a narrow part of the track of the plate heater, the narrow part being arranged to preferentially rupture.
A plate heater as claimed in claim 3, wherein the fuse comprises a break in a track of the heater bridged by an easily meltable conductor.
A plate heater as claimed in claim 3, wherein the fuse comprises a break in a track of the heater bridged by a leaf spring.
A plate heater as claimed in any of claims 3 to 5 having a plurality of thermal fuses connected in series and distributed about its periphery.
A plate heater as claimed in claim 4, wherein the narrow part of the track extends substantially around the heater.
An electric water heating vessel having a body and a plate heater, there being provided an array of overheat protection devices disposed between a heated region of the plate heater and a region of the vessel which is to be protected from overheating.
An electric water heating vessel as claimed in claim 9, wherein the plate heater is as claimed in any of claims 1 to 8.
An electric heater comprising a substrate and a plurality of discrete heater cells electrically interconnected in parallel and distributed about the surface of the substrate, each cell having an electrically resistive element with a positive temperature coefficient of resistance.
A combination of a plate heater and a thermal actuator, the plate heater having a region of track which in use has a higher output power density than the remainder of the plate heater, wherein the thermal actuator is located adjacent said region and the arrangement is such that in the event of the plate heater overheating, the thermal actuator operates switch means to disconnect the supply of electrical current before any part of the plate heater track is ruptured.