The invention relates to thermostats and particularly, though not exclusively, to thermostats for vessels for containing fluid such as water, typically for controlling the temperature of water or other fluid in a tank.

A typical arrangement for heating fluid (e.g. water) in a tank employs an electric heating element or other type of heater to heat the fluid. The heater may be mounted inside the tank or external to it. A thermostat is provided to control the temperature of the fluid between two limits. The thermostat would and arranged to operate a switch to change its switch state (e.g. open an otherwise closed switch) when the temperature sensed by the thermostat reaches a desired threshold temperature. The switch would typically change to an open switch state as the temperature rises past the threshold temperature and to a closed state as the temperature falls below a lower temperature. The state of the switch would determine the supply of power to the heating element and, ultimately, the temperature of the fluid being heated in the tank regulated by the thermostat.

For safety reasons it is desirable to provide a mechanism whereby the thermostat will operate to cut-off power to the heating element regulated by the thermostat in the event that a second and uppermost threshold temperature is reached. This may occur, for example in the event of a failure of the normal thermostat switch mechanism. This would prevent dangerous overheating of the fluid (e.g. water) and heating element within the heater assembly.

In the past this cut-off provision has been achieved by in a number of ways. One is to provide a separate thermal cut-out mechanism mounted in the heater assembly served by a thermostat, in cases where the heater is an electric immersion heater. A disadvantage with this arrangement is that the immersion heater becomes non-standard and such a thermostat cannot be used to replace a failed thermostat in an existing heater. A second method is to use a separate bi-metal sensor mounted in the head of the thermostat and arranged to act to change the state of a cut-out switch when the sensor reaches a sufficiently high temperature.

A disadvantage of these methods is that the temperature in question is sensed remotely according to such arrangements, and therefore the temperature of the water when the cut-out switch opens can vary widely depending on the arrangement of the tank, heater and thermostat. This means that the setting of the cutout mechanism (to ensure it operates at the desired temperature) has to vary depending on the arrangement. This can cause problems when thermostat assemblies require replacement and the wrong one is fitted.

A third a method is to provide a cut-off sensor which senses the water temperature in much the same location as the temperature sensor of the thermostat switch. This has been achieved in two methods before. First, by using a memory shape metal spring which extends at a set temperature and opens a cut-out switch or, second, by using a low melting point alloy which melts at the cut-out temperature releasing a pre-stressed compression spring which extends and opens the switch.

Both of these latter methods require complex and costly apparatus involving springs and rapidly moving parts prone to malfunction. Furthermore, the second method results in a once- only cut-out switch operation after which a user is required disassemble of the heater and fit a replacement thermostat.

There are two types of thermostat available in the market. The first type has two power terminals, one for power input and one for power output, and is arranged to be wired in series with the heating element of a fluid (e.g. water) heater by the installer .

The second type has four electrical power terminals. Two of these power terminals are electrical power output/ supply terminals and are typically of the "male" push-in type arranged to plug directly into corresponding "female" receiver or socket-type electrical power terminals mounted in connection with the power terminals of the heating element of the heater assembly. The other two of the power terminals of this type of thermostat are power input terminals arranged to connect to the "live" and "neutral" terminals/wires of the power supply.

The second type of thermostat has the advantage that the thermostat can be connected to the supply before installation into the heater and that the overall assembly of heater and thermostat is cheaper to manufacture. However the existence of the first type of thermostat in existing homes and markets is a barrier to introducing the second type into those homes and markets because retailers of heaters also need to stock replacement thermostats. It would be very undesirable (e.g. costly) to have to stock both types of thermostat and there could be confusion about which type is required.

The invention, in its various aspects, aims to provide means which may be used to address some of these problems.

At its most general, the invention (in one aspect), lies in the provision and/or use of a thermostat assembly for controlling the temperature of fluid (e.g. water) in a tank and operated by differential expansion between a first elongate member (e.g. a tube) and a second elongate element (e.g. rod) mounted and secured to one end of the first elongate member (e.g. within the tube) . A first switch mechanism is preferably mounted at the other end of the first elongate member. The first switch is preferably opened at a set temperature by the movement of the free end of the second elongate element (e.g. rod) relative to the end of the first elongate member. This switch is preferably of the type which resets automatically when the temperature drops to a certain level thus controlling the temperature between two limits. This switch may be referred to as the thermostatic switch. There may be a second switch mechanism, preferably in series connection electrically with the first switch mechanism, this may also be operated by the relative movement of the end of the elongate second element (e.g. rod) and the first elongate member. The second switch mechanism may be mounted near to the first switch mechanism. The second switch may be intended to turn off the heater if there is a failure of the thermostatic switch mechanism. This second switch may be referred to as the thermal cut-out switch. It may be of the type requiring manual resetting by the user.

In a first of its aspects, the invention may provide a thermostat comprising: a switch assembly attached to a proximal end of an elongate thermal actuator means and comprising two or more separate switches (e.g. electrical switches), the thermal actuator means comprising; a first elongate (e.g. tubular) member along which a second (e.g. inner) elongate member extends (e.g. along the bore of the tube) from a distal part of the first elongate member at which (e.g. to which) it is secured, to the switch assembly where the thermal actuator means is arranged by relative movement of the second elongate member to act upon each of two or more said switches separately to change the switch state thereof in response respectively to a change in temperature of the thermal actuator means in which the second elongate member has a coefficient of thermal expansion different to that of the first elongate member. The two or more separate switches are preferably connected electrically in series, or adapted to be so connected, e.g. such that a common electrical current can in turn flow through each or them. Preferably, the two or more switches are connected such that the opening of any one switch alone breaks the electrical circuit served by, or prevents the flow of electrical current through, the other switch (es) . The thermal actuator means is preferably arranged by said relative movement of the second elongate member to act upon the two or more switches separately with an urging force proportional in strength to the relative movement. The thermal actuator means may be arranged to act either directly or indirectly upon one, some or each of the switches - e.g. directly upon the mechanism of a switch if the switch is mechanical. Consequently, by virtue of the proportionality between the urging force of the actuator means and the degree of movement or displacement of the second elongate member, the degree of heating (or overheating) of the volume served by the thermostat (e.g. a water tank) dictates the strength of the urging force applied by the thermal actuator means to the switches to induce a change of switch state. This provides an additional level of safety when a switch in question is in the form of two electrical contacts closable or separable by the urging force to close or open the switch and which may be vulnerable to "sticking" in an open or a closed state. Such sticking can occur, for example, when two closed contacts become welded/ fused together due to prolonged closure and the heating effects of electrical conduction through the contact points. Such welding/fusing may be strong enough to resist an urging force designed to separate the closed contacts under normal (non-fused) conditions, but may well not be strong enough to resist a larger urging force sufficient in strength to break the weld. Thus, by rendering the urging force in proportion to the displacement of the second elongate member, which itself is proportional to the degree of heating of the first elongate member, a higher heating of the first elongate member may incur a stronger urging force upon the switch (es) via the thermal actuator means. If operation of a switch (e.g. separation of contacts) does not occur at an intended raised temperature then heating may continue thereby generating an increased urging force upon the switch to more urgently induce the switch operate .

The actuator means may be arranged to act upon each of the two or more switches to change the switch state thereof from a closed state to an open state. The two or more switches may be arranged in a series connection formed independently of connection of the thermostat to a load, or may adapted and arranged to be connectable in the series connection when the thermostat is in use (e.g. by action of connecting the thermostat to a load, such as a heating element or the like, served by the thermostat, or by connecting an electrical circuit element [e.g. an adaptor] to complete the series connection) . The switches may be housed within a switch assembly housing containing the switches in permanent series connection therein. Alternatively, two or more of the switches may be electrically connected to terminals adapted to be electrically connected together to place the switches in series electrical connection.

The term "switch" herein is intended to encompass, but not be limited to, a reference to the two separable electrical contacts which, when in contact, permit conduction of electrical current from one contact to the other and, when not in contact, do not permit such conduction of electrical current. Other types of switch, such as would be readily apparent to the skilled person, responsive to an urging force/pressure to change switch state are encompassed, and may or may not be mechanical switches.

Preferably the value of the coefficient of thermal expansion of the first elongate member exceeds the value of the co-efficient thermal expansion of the second elongate member such that heating of the former causes the free end of the latter to be drawn towards it thereby providing the aforementioned relative movement . In this way the movement of the first elongate member alone relative to the switch assembly can serve separately to actuate two (or more) switches within the switch assembly. This avoids the need for separate actuator assemblies or arrangements for actuating second or additional switches. This arrangement is much more efficient in its use of the properties of the thermal actuator means and provides for a reversible and therefore reusable actuator action. That is to say, a change in temperature of thermal actuator means opposite to that which causes a given change in switch state may cause an opposite change in switch state. The switch in question may revert to a prior state in response to the thermal actuator means reverting towards (or to) a prior temperature. Preferably, the end of the second elongate member opposite to that secured to the distal end of the first elongate member, is unbound or unsecured and is free to move axially as a result of movement of the secured end thereof .

Most preferably a free end of the second elongate member is located within or adjacent to the switch assembly and most preferably it is the motion of that free end, or a section or length of the second elongate member terminating at the free end, relative to the switch assembly which serves to act upon the switches separately. Such action may be direct or indirect.

Most preferably the switch assembly is mounted at the proximal end of the thermal actuator means to the first elongate member, but not to the second elongate member.

The thermostat may include urging means arranged to translate the relative movement of the second elongate member into a force to urge the two or more switches to move to change the switch state thereof. The urging means may be coupled to the second elongate member or may be arranged to cooperate with (e.g. be urged by) the relative movement of the second elongate member. For example, an, or each, switch may be a mechanical switch comprising a switch moveable from a first switch state to a second switch state (e.g. from a closed state to an open state) . Examples include a leaf spring switch arranged by a snap-action to change positions between respective switch states. Another simple example includes a switch comprising a pair of switch contacts one of which is mounted upon a resilient switch arm biased to urge one of the switch contacts into or away from contact with the other such that application of an urging force may cause the switch to adopt a switch state opposite to that it is otherwise biased to adopt. Switches of the switch assembly preferably comprise such switches against which the urging means of the thermostat may be adapted to urge at respective pre-determined temperatures by the thermally-induced relative movement of the second elongated member, and particularly the free end region thereof. The urging means may be arranged at pre-determined locations on the second elongate member adjacent the switch mechanisms respectively of the two or more switches such that a pre-determined change in temperature of the thermal actuator means draws urging means into contact with an adjacent switch mechanism of a respective switch such that an increase or decrease in temperature may cause the urging means to urge respective switch mechanisms to change switch state.

For example, urging means adapted in respect of a given switch may be preferably positioned such that the switching mechanism of the associated switch is located between the associated urging means and the proximal end of the first elongate member of the thermal actuator. The urging means may comprise a nut, lug or block member attached to the free end region of the second elongate member adjacent the switch mechanism of the switch to which it is dedicated. The urging means may include a lever mechanism adapted and arranged to apply said urging force to a respective switch served thereby. The lever mechanism may be positioned between the nut, lug or block and the distal part of the first elongate member. The nut, lug or block may be arranged between a fulcrum of the lever mechanism and the switch served by the lever mechanism, or the part of the lever mechanism arranged to urge against the switch. Consequently, movement of the second elongate member may urge the nut, lug or block against the lever mechanism thereby, in turn, to urge a part of the lever mechanism against the switch assembly to induce a change of switch state.

The separation between the urging means and the associated switch mechanism may be determined according to the values of the coefficients of thermal expansion of the first and second elongate members and the pre-determined temperature (e.g. threshold temperature) at which it is intended to have the urging means interact with the associated switch mechanism to change its switch state. This determines the amount of relative movement, and temperature change, required to cause a change in switch state. It is to be understood that the switch upon which an urging means acts in this way may be such that the urging means may to some extent, deform, bend or depress the part of the switch against which it urges by said relative movement, in order to cause the switch to change state. As a result, the separation in question may be the minimum separation required to begin to induce a switch to change state. A further movement of the urging means may be required to move switch parts to instigate the change in switch state.

The urging means may be arranged to urge a first of the switches in reaction to an aforesaid relative movement of an amount less than that required to urge a second of the switches . For example, an urging means associated with a first switch mechanism may be separated from the switch mechanism by a distance less than the distance separating corresponding urging means from the switch mechanism of a second switch. This means that a greater amount of relative movement is required in the second elongate member in order to draw the urging means into contact with the second switch mechanism than is required to draw the urging means into contact with the first switch mechanism. As a result, the urging means will actuate the first switch mechanism at a temperature which is less than the temperature required for the urging means to actuate the second switch mechanism. In this way, the urging means simply by appropriate separation from an associated switch mechanism may be arranged to actuate the separate switches at different respective temperatures.

The thermostat may be arranged such that the switch state of a first of the two or more switches is changed if the temperature of the thermal actuator means exceeds a first threshold temperature and the switch state of a second of the two or more switches is changed if the temperature of the thermal actuator means exceeds a second threshold temperature greater than the first threshold temperature.

For example, the first (e.g. lower) temperature at which the urging means may be arranged to actuate a first switch mechanism may be the first threshold temperature which the thermostat is designed to not exceed in normal use. The second (e.g. greater) temperature may be a temperature above which operation of the apparatus being served by the thermostat becomes dangerous (e.g. a water heater, such as an immersion heater) . In this way the first switch in conjunction with the urging means may serve to allow the thermostat to operate in normal use so as not to exceed the desired temperature of, for example, water within a water heater tank. The second switch in conjunction with urging means may be adapted in this way to serve as an emergency cut-out mechanism should the first switch fail to be actuated by the urging means thereby to prevent temperature rising beyond the second temperature threshold.

The thermal actuator means may be arranged by said relative movement to act upon two or more of said switches separately to change the switch state thereof in response respectively to a change in temperature of the first elongate member.

The thermal actuator means may be arranged to act on the second switch to change the switch state thereof from a closed state to an open state. The thermal actuator means may be arranged to act on the first switch to change the switch state thereof from a closed state to an open state.

The second elongate member may comprise a metal such as Invar. The first elongate member may comprise a different metal such as brass having a higher coefficient of thermal expansion than that of the inner elongate member. The first elongate member may be an elongate tube along the bore of which the second elongate element extends as an inner elongate member. The first elongate member may define a channel formation along which the second elongate member extends. The first elongate member may be a strip (e.g. flat) adjacent which the second elongate member extends.

The switch assembly may be arranged to form a part of an electrical circuit and the thermal actuator means may be arranged to break the circuit by changing the state of a switch when the thermal actuator means reaches a selective threshold temperature .

At its most general (in another of its aspects) the invention is to provide a connecting link which connects together two (male or female) electrical power terminals of the thermostat of the second type described above. The resulting thermostat then has the same functional characteristics as the first type of thermostat described above.

The link would preferably be insulated e.g. because of the proximity to the conductive (e.g. brass) head of an immersion heater in use. Some thermostats of the second type have extra electrical power terminals to connect a pilot light of a heater in parallel with the heating element of the heater. The invention may be to link these terminals instead of (or in addition to) the two power terminals which connect with the heater element. The invention may be to link the power input terminals of the thermostat intended for connection the power supply and use the two terminals intended to connect in series between to the heating element and a terminal of the power supply in the manner of the thermostat of the first type.

The same thermostat of the second type could be sold together with the link irrespective of the type of heater to which it is intended to fit. If the heater was originally fitted with the first type of thermostat the thermostat of the second type could simply be fitted with the link in place. If the second type of thermostat was originally fitted, the link would be removed before the thermostat was fitted.

The invention may provide a thermostat of the second type fitted with an electrical link between the terminals intended for connection to the heating element allowing it to be used as thermostat of the first type.

The invention may provide a thermostat of the second type fitted with an electrical link between the terminals intended for connection to live and neutral power supply terminals allowing it to be used as a thermostat of the first type with the live and neutral power supply terminals connected to the terminals originally intended to connect to a heating element. The invention may provide a thermostat of the second type fitted with an electrical link between the terminals intended for connection to a pilot light allowing it to be used as a thermostat of the first type.

The invention may provide an electrical link fitted to a thermostat of the second type as described above.

In a second of its aspects, the invention may provide a thermostat adaptor for a thermostat of the type having two electrical power supply terminals each adapted for connection to a respective one of two terminals of a heating element assembly to place a heating element in series connection between the two electrical power supply terminals of the thermostat for supplying electrical power to the heating element to be regulated by the thermostat, the adaptor comprising: two electrical connectors each adapted for physical connection to a respective one of the two electrical power supply terminals thereby each to be placed in electrical connection with a respective electrical power supply terminal; short-circuit means which electrically connecting the two electrical connectors such that a short-circuit is formed between the two electrical power supply terminals when the thermostat adaptor is connected thereto.

In a third of its aspects, the invention may provide a thermostat adaptor for a thermostat of the type having a first two electrical power supply terminals each adapted for connection to a respective one of two terminals of a pilot light assembly to place a pilot light in electrical connection between a second two electrical power supply terminals of the thermostat for supplying electrical power to a heating element to be regulated by the thermostat, the adaptor comprising: two electrical connectors each adapted for physical connection to a respective one of the first two electrical power supply terminals thereby each to be placed in electrical connection with a respective electrical power supply terminal; short- circuit means electrically connecting the two electrical connectors such that a short-circuit is formed between the first two electrical power supply terminals when the thermostat adaptor is connected thereto.

In a fourth of its aspects, the invention may provide a thermostat adaptor for a thermostat of the type having two electrical power input terminals each adapted for connection to a respective one of two terminals of a power supply to place the thermostat in series connection between the two terminals of the power supply to permit the thermostat to supply electrical power to a heating element to be regulated by the thermostat, the adaptor comprising: two electrical connectors each adapted for physical connection to a respective one of the two electrical power input terminals thereby each to be placed in electrical connection with a respective electrical power input terminal; short-circuit means which electrically connecting the two electrical connectors such that a short- circuit is formed between the two electrical power input terminals when the thermostat adaptor is connected thereto.

A short circuit may comprise a deliberate electrical connection of relatively very low or substantially negligible resistance between two points in a circuit.

The short-circuit means may be electrically insulated.

The electrical connectors described above may each be arranged to make direct physical contact to a respective one of the power terminals they are adapted to connect to in use. This physical contact may be via a push-fit, or a gripping and/or abutting action. The adaptor is preferably arranged to "plug- in" to the associated power terminals, or the associated power terminals "plug-in" to the connectors of the adaptor. The thermostat adaptor may be for a thermostat of the type in which one or each of said electrical power supply or power input terminals is a male connector adapted to fit to a corresponding female connector, and one or each of the two electrical connectors is a female connector.

The thermostat adaptor may be for a thermostat of the type in which one or each of said electrical power supply or power input terminals is a female connector adapted to fit to a corresponding male connector, and one or each of the two electrical connectors is a male connector.

In a fifth of its aspects, the invention may provide a thermostat comprising two electrical power supply terminals each adapted for connection to a respective one of two terminals of a heating element assembly to place a heating element in series connection between the two electrical power supply terminals the thermostat for supplying electrical power to the heating element to be regulated by the thermostat, and including the thermostat adaptor described above.

In a sixth of its aspects, the invention may provide a thermostat comprising a first two electrical power supply terminals each adapted for connection to a respective one of two terminals of a pilot light assembly to place a pilot light in series electrical connection between a second two electrical power supply terminals of the thermostat for supplying electrical power to a heating element to be regulated by the thermostat, and including the thermostat adaptor described above .

In a seventh of its aspects, the invention may provide a thermostat comprising two electrical power input terminals each adapted for connection to a respective one of two terminals of an electrical power supply to place the thermostat in series connection between the two terminals of the power supply to permit the thermostat to supply electrical power to a heating element to be regulated by the thermostat, and including the thermostat adaptor described above.

The invention may be sold in component parts or kit form. In an eighth of its aspects, the invention may provide a kit of parts comprising a thermostat and an adaptor and including the thermostat adaptor described above adapted to the thermostat.

The thermostat according to any aspect of the invention may be arranged for use in a water heating system for regulating electrical current supplied to a heating element for heating water .

In a further of its aspects, the invention may provide a water vessel comprising a water heating system including a heating element for heating water within the vessel and including a thermostat as described above for regulating electrical current supplied to a heating element.

Exemplary embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 illustrates, in cross section, a thermostat;

Figure 2 illustrates, schematically, the switch assembly and relevant parts of the thermal actuator of a thermostat;

Figure 3 illustrates a cross sectional view of a

thermostat ;

Figure 4 illustrates a top view of relevant parts of the switch assembly of a thermostat; Figure 5 illustrates schematically a thermostat of a first type;

Figure 6 illustrates schematically a thermostat of a second type;

Figure 7 illustrates schematically a variant of a thermostat of a second type; Figure 8 illustrates schematically a thermostat of a second type together with a thermostat adaptor in a first use;

Figure 9 illustrates schematically a thermostat of a second type together with a thermostat adaptor in a second use;

Figure 10 illustrates schematically a variant thermostat of a second type together with a thermostat adaptor in a third use . In the drawings like articles are assigned like reference symbols .

Figure 1 illustrates a thermostat (1) including a switch assembly (2) attached to the proximal end (6) of an elongate thermal actuator (3) . The switch mechanism includes two separate switches comprising a first switch (4) and a second switch (5 ) .

The thermal actuator includes an elongate tubular member (7) along the bore of which extends an inner elongate member (8) . The elongate tubular member is a brass tube having a relatively high coefficient of thermal expansion, and the inner elongate member is an Invar rod having a relatively much lower coefficient of thermal expansion. The Invar rod (8) extends from a distal end (9) of the brass tube (7), at which it is securely fixed through the brass tube to and beyond the proximal end (6) of the brass rod and into the body of the switch assembly (2) at which the free end (18) of the Invar rod terminates . The thermal actuator also includes urging members (20, 21) moveably attached to the free end region (18) of the Invar rod and arranged by relative movement of the free end of the Invar rod with respect to the switch assembly (2) to act upon each of the two switches (4, 5) separately to change the switch state thereof in response respectively to a change in temperature of the brass tube (7) of the thermal actuator (3) .

Each of the first and second switches (4, 5) comprises a leaf spring switch (10, 14) responsive by action of a leaf spring to urge the electrical contacts (11, 15) of the respected switch to a closed state. Each switch further comprises an abutment portion (12, 16) continuing from, and integral with, the leaf- spring (10, 14) and bound between two switch-mounting walls (30) . Each such switch mechanism is arranged such that application of an urging force to the abutment region of a given switch mechanism in the direction of the distal end (9) of the thermal actuator (3) causes the respected switch to react by switching (under action of the associated leaf-spring) to an open state (13, 17) in which the associated contacts (11, 15) of the switch adopt an open state.

Each of the switch mechanisms is formed from a conductive material (e.g. a suitable metal) . The first and second switches are connected in series connection with power input/output terminals (23, 24) of the thermostat via an electrical connection (22) between the respective abutment regions of the first and second switch. In alternative embodiments one or each of the mounting walls (30) may be conductive (e.g. metallic) thus providing electrical connection between the two switches and obviating the need for the electrical connection (22) illustrated in the drawings. In this way, each switch mechanism is placed, in series, between the power input and output terminals (23, 24) of the thermostat (1) . In use, a load (25) , such as the electrical heating element of a water heater tank, is placed in series connection with either one of the power input/output terminals of the thermostat, and electrical power is then supplied between (through) the load and the terminal of the thermostat to which the load is not directly connected. In this way, the thermostat may operate a switching mechanism forming part of a power supply circuit and operable to selectively disconnect power from the load contained in the power circuit (e.g. heating element) thereby to regulate the power supplied to the load in accordance with the temperature experienced by the thermal actuator (3) . The urging members of the thermal actuator include a first urging nut (20) mounted upon the threaded distal end of the Invar rod (8) and positioned thereupon at a linear separation xi from the abutment region (12) of the first switch (4) . The abutment region (12) of the first switch is located between the first urging nut (20) and the distal end (9) of the brass tube (7) to which the other end of the Invar rod is secured.

A manually accessible dial (19) is attached to the first urging nut (20) and is presented through an aperture formed in an outer wall of the switch assembly (2) being manually rotable to cause rotation of the first urging nut (20) about the threaded free end of the Invar rod (18) . In this way, the magnitude of the linear separation x ₁ may be manually varied by rotating the dial (19) to cause linear movement of the first urging nut (20) up or down the threaded free end (18) of the Invar rod.

The brass tube (7) of the thermal actuator (3) possesses a coefficient of thermal expansion Ci whereas the Invar rod (8) within the brass tube possesses a coefficient of thermal expansion C ₂ which is less than the value of Ci . The value of the linear separation x ₁ (equal to or greater than zero) between the first urging nut (20) and the response region (12) of the first switch (4) is related to the thermal properties of the thermal actuator in the following way. xi + δι = (Ci - C ₂ ) (Ti - T ₀ )

Where T ₀ is a reference temperature and Ti is a first threshold temperature at which the first urging nut (20) will actuate the first switch (4) to open thereby to prevent further supply of power to the load (25) responsible for the temperature of the thermal actuator (3) . It is the temperature at which the linear thermal expansion of the brass rod (7) exceeds the linear thermal expansion of the Invar rod (8) to such an extent that the distal end (9) of the brass tube draws away from the switch assembly (2) and pulls the free end (18) of the Invar rod with it through a distance xi sufficient to make contact with and urge against the abutment region (12) of the first switch. An additional relatively small deformation distance (δι) may be traversed by the first urging nut while in contact with the abutment region as the abutment region of the urged switch deforms to an extent as necessary to force the switch to change state .

The urging members also include a second urging nut (21) rotatably mounted upon the threaded free end region (18) of the Invar rod between the first urging nut (20) and the proximal end (6) of the brass tube (7) . Between this proximal end (6) and the second urging nut (21) is located the abutment region

(16) of the second switch (5) . That abutment region is separated from the second urging nut by a linear separation x ₂

(equal to or greater than zero) which satisfies the relation; x ₂ + δ ₂ = (d - C ₂ ) (T ₂ - T ₀ ) Where the quantity T ₂ is a second threshold temperature at which differential thermal expansion as between the brass tube and the Invar rod of the thermal actuator is sufficient to draw the second urging nut into contact with the abutment region (16) of the second switch. An additional relatively small deformation distance (δ ₂ ) may be traversed by the second urging nut while in contact with the abutment region as the abutment region of the urged switch deforms to an extent necessary to force the switch to change state thereby to actuate the switch to change from a closed state to an open state (17) to prevent continued supply of power to the load (25) responsible for the temperature of the thermal actuator (3) .

A stop member (31) is located within the switch assembly (2) and is positioned adjacent the leaf spring part (10) of the first switch to serve as an abutment for that switch when the switch mechanism has been actuated and adopts an open state (13) . The positioning of the abutment (31) is such as to limit the separation between the electrical contacts (11) of the first switch (4) when in the open state (13) as illustrated in figure 1. This separation is such that the leaf spring (10) will snap back to a closed state once the first urging nut (20) ceases to urge against the abutment region (12) of the first switch. In this way, the first switch, by virtue of the positioning of the stop member (31) is a fully reversing switch which adopts a closed state when the temperature of the thermal actuator is sufficiently low to separate the first urging nut (20) from the first switch mechanism thereby to allow a supply of power to the load (25) to resume, and to snap open (13) when the thermal actuator reaches a threshold temperature Ti sufficient to actuate the first switch to an open state (13) . In this way, the first switch (4) in conjunction with the thermal actuator (3) serves as a thermostatic switch to regulate the temperature of the environment heated by the load (25) between a nominal lower temperature value at which the actuator disengages from the first switch and a first upper threshold temperature ΊΊ at which the actuator engages with the first switch.

A second abutment (not shown) serving the same purpose in respect of the second switch (5) may be provided in some embodiments, but in the embodiment illustrated here an alternative arrangement is disclosed. The second switch (5) is housed within the body of the switch assembly (2) such that the leaf-spring portion (14) of the second switch mechanism urges the contacts (15) of the second switch apart by such an extent, when in the open state (17), that the leaf spring mechanism (14) is unable to urge the second switch mechanism back into a closed state once the second urging nut (21) disengages from the switch mechanism. In this way, once the second switch has been actuated into an open state, it is not able to self-close in the manner of the first switch. This is a safety feature.

Should the contacts (11) of the first switch mechanism become fused thereby preventing the normal control operation of the first actuator nut (20) of the thermal actuator, a subsequent increase in temperature of the volume sensed by the brass rod (7) of the thermal actuator will, upon reaching an upper threshold temperature T ₂ cause a safety cut-out of the power supply to the load (25) responsible for that heating in a way which is not reversible by subsequent lowering of temperature. Reversal of the switch state of the second switch in such circumstances would require manual operation by a user to close the second switch (5) from the open state (17) to the closed state illustrated in figure 1.

This may be achieved by simply pushing the leaf-spring portion of the second switch into a closed state using a push pin (33) inserted through a specifically positioned and adapted reset aperture (32) formed within the wall of the switch assembly towards which the leaf spring part of the second switch moves when adopting the open state. Most preferably this reset aperture (32) is formed in a wall of the switch assembly (2) which is not readily accessible in normal use of the thermostat. For example, the reset aperture may be formed in the face of the switch unit from which the brass tube (7) extends such that when the brass tube is inserted into a volume to be monitored by the thermostat (e.g. a water tank), the reset aperture (32) is rendered inaccessible. This positioning effectively forces a user to fully disengage the thermostat from the water tank in order to reset the second switch a closed state and therefore discourages the casual repeated resetting of the second switch in the event that the first switch has welded closed.

Figure 2 schematically illustrates, in cross section, an alternative embodiment of the invention in which the urging means of the thermal actuator comprise pivot assembly (40, 45) arranged to translate the relative motion of the free end (18) of the Invar rod (8) into an urging force upon the switch assemblies of the first and second switches respectively.

A first urging pivot comprises a first pivot arm (40) pivotingly mounted at a pivot end (42) to a first fulcrum socket member (43) and terminating at an opposite end of the pivot arm with a first urging lug (41) . The function of the first urging lug (41) is the same as the function of the first urging nut (20) described above with reference to figure 1 - namely to apply an urging force to the abutment region (12) of the switch mechanism of the first switch (4) to change the switch state thereof. The first pivot arm (40) possesses a through-hole (44) through which the free end region (18) of the Invar rod (8) extends and is freely moveable. The first pivot arm (40) is located between the first urging nut (20) and the brass tube (7) of the thermal actuator. The first urging nut (20) is separated from the opposing surface of the first pivot arm (40) adjacent the through-opening (44), by a linear separation xj. Differential thermal expansion as between the brass tube (7) and the Invar rod (8) serves to draw the first urging nut (20) against the opposing surface of the first pivot arm (40) when the first threshold temperature ΊΊ is achieved at the brass tube (7) . As a result, the urging force applied to the first pivot arm by the first locking nut (20) is translated by the first pivot arm (40) into an urging force applied by the first urging lug (41) to the abutment region (12) of the switch mechanism of the first switch (4) . The through-opening (44) is located within the first pivot arm (40) between the first urging lug (41) and the first fulcrum socket (43) of the assembly. As a result, the lever-action of the first pivot arm enables a greater sensitivity in the urging apparatus in the sense that a greater linear movement may be produced at the urging lug (41) in response to a relatively smaller linear movement of the first urging nut (20) relative to the switch assembly (2 ) .

A second urging pivot arm (45) is pivotingly mounted at a pivot end (47) to a second fulcrum socket member (48) and terminates at an opposite end of the pivot arm with a second urging lug (46) . The function of the second urging lug (46) is the same as the function of the second urging nut (21) described above with reference to figure 1 - namely to apply an urging force to the abutment region (16) of the switch mechanism of the second switch (5) to change the switch state thereof. The second pivot arm (45) possess a through-hole (49) through which the free end region (18) of the Invar rod (8) extends and is freely moveable. The second pivot arm (45) is thereby located between the second urging nut (21) and the brass tube (7) of the thermal actuator. The second urging nut (21) is separated from the opposing surface of the second lever arm (45) adjacent the through-opening therein (49), by a linear separation x ₂ . Differential thermal expansion as between the brass tube (7) and the Invar rod (8) serves to draw the second urging nut (21) against the opposing surface of the second pivot arm (45) when the second threshold temperature T ₂ is achieved at the brass tube (7) . As a result, the urging force applied by the second urging nut (21) is translated by the second pivot arm (45) into an urging force applied by the second urging lug (46) to the abutment region (16) of the switch mechanism of the second switch (5) . The through-opening is located within the second pivot arm between the second urging lug (46) and the second fulcrum socket (48) of the assembly. As a result, the lever- action of the second pivot arm enables a greater sensitivity in the urging apparatus in the sense that a greater linear movement may be produced at the second urging lug in abutment to a relatively smaller linear movement of the second urging nut (21) at the free end (18) of the Invar rod relative to the switch assembly (2) . Figure 3 illustrates a cross sectional view of the thermostat described above with reference to figures 1 and 2, and figure 4 illustrates a top view of the contents of the switch unit (2) like articles are referenced by like reference numerals. Figure 5 schematically illustrates a first type of thermostat assembly (40) having two power supply terminals (41, 44) in series electrical connection and between which is a first thermostat switch (47) actuable by a bi-metal rod (48) to adopt an open switch state when the temperature of the bi-metal rod reaches a pre-determined first threshold temperature, and a cut-out switch (45) served by a cut-out mechanism (46) operable to open the cut-out switch when the temperature sensed by the thermostat (40) reaches a second threshold temperature exceeding the first threshold temperature. A first terminal (49) of a heating element (42), such as for an immersion heater (not shown), is connected to one terminal (41) of the two power terminals (41, 44) of the thermostat. In this arrangement, a power supply is to be connected between the other of the power terminals (44) of the thermostat and the free terminal (43) of the heating element (42) . Typically, the "neutral" power terminal of a power supply would be connected to the free terminal (43) of the heating element (42), whilst the "live" terminal of a power supply is connected to the free terminal (44) of the thermostat.

Figure 6 schematically illustrates an alternative type of thermostat (50) having two electrical power input terminals

(51, 52) each adapted for connection to a respective one of two terminals (not shown) of a power supply to place the thermostat

(50) in series connection between the two terminals of the power supply to permit the thermostat to supply electrical power to a heating element (62) to be regulated by the thermostat. The thermostat also includes two electrical power supply terminals (53, 54), each adapted for connection to a respective one of two terminals (60, 61) of a heating element

(62) to place the heating element in series connection with the two electrical power supply terminals (53, 54) of the thermostat for supplying the electrical power to the heating element to be regulated by the thermostat. Typically, the "neutral" terminal of a power supply (not shown) would be connected to a first (51) of the power input terminals of the thermostat (50) whilst the "live" terminal of the power supply

(not shown) would be connected to the other (52) of the two power input terminals of the thermostat. The thermostat includes a first switch (58) actuable by a bi-metal actuator

(59) to adapt an open switch state when the temperature of the actuator reaches a first threshold temperature Τχ . A second and third cut-out switches (55, 57) is each served by a common cutout unit operable to force the second and third switches into an open state when a second threshold temperature T ₂ is sensed, being greater than the first threshold temperature Τχ . Each of the first and second cut-out switches is placed in series connection with a respective one of the two power supply terminals (53, 54) of the thermostat.

In thermostats of the type illustrated in figure 6, one or each of the two electrical power supply terminals (53, 54) is of the "male" or prong-type connector, or is of the "female" or socket-type electrical connector. Accordingly, the thermostat (50) may be placed in electrical connection with the two terminals (60, 61) of a heating element (52) by a simple push- fit in a plug-and-socket arrangement.

Figure 7 schematically illustrates a common modification of the thermostat of the type illustrated in figure 6 further including a second pair of power supply terminals (63, 64) adapted for a push-fit connection to respective terminals (65, 66) of a pilot light assembly (67) as is often provided in water heater units (e.g. emersion heaters) . Each of the second power supply terminals (63, 64) is connected in parallel to a respective one of the first two power supply terminals (53, 54) of the thermostat adapted for connection to the corresponding heating element (62) of the same heating apparatus.

Figures 8 and 9 each schematically illustrate a thermostat (50) of the type described with reference to figure 6. Figure 10 schematically illustrates a thermostat (50) of the type described with reference to figure 7.

A thermostat adaptor (70) comprises two electrical connectors (71, 72) each adapted for physical connection to a respective one of the two power supply terminals (53, 54) of the thermostat thereby each to be placed in electrical connection with a respective electrical power supply terminal. A short- circuit conductive element (73) electrically connects the two electrical connectors (71, 72) such that a short-circuit is formed between the electrical power supply terminals (53, 54) of the thermostat when the thermostat adaptor is connected thereto. The short-circuit element (73) is encapsulated within an electrically insulating material (74) such that only the two electrical connectors (71, 72) are accessible for electrical connection. The result of connecting the thermostat adaptor (70) to the thermostat (50) is to produce a short-circuit between the power supply terminals (53, 54) of the thermostat thereby to render the thermostat - with adaptor - analogous to a thermostat of the type illustrated in figure 5. Accordingly, a heating element (42) may be connected to one of the two power input terminals (51, 52) of the thermostat in the manner described above with reference to figure 5, such that a "live" terminal (not shown) of a power supply may be connected to the three power input terminal (52) of the thermostat (50) and the "neutral" terminal (not shown) of the power supply connected to the free terminal (43) of the heating element (42) .

Electrical insulation (74) is provided in the thermostat adaptor to prevent unwanted electrical connection to the short circuit element (73) by any conductive surfaces of the heating apparatus served by the thermostat (50) in use. An example would be the conductive metal body of a water tank containing a heating element (42) which the thermostat (50) is intended to regulate. The short-circuit element may be a wire, trip or rod of conductive material (e.g. metal, alloy etc) having a relatively very low resistance (e.g. minimal)

Figure 9 schematically illustrates an alternative use of the thermostat adaptor (70) being adapted for connection to a thermostat of the type illustrated in figure 6 having two electrical power input terminals (51, 52) each adapted for connection to a respective one of two terminals (not shown) of a power supply (not shown) to place the thermostat (50) in series connection between the two terminals of the power supply to permit the thermostat to supply electrical power to a heating element (42) to be regulated by the thermostat. In this way, the thermostat adaptor (70) may form a short circuit between the power input terminals of the thermostat thereby to render the thermostat (50) functionally equivalent to the thermostat of the type illustrated and described above with reference to figure 5. Accordingly, a heating element (42) of a heating apparatus (e.g. an emersion heater) may be connected to one (52) of the two power supply terminals (53, 54) such that an electrical power supply (now shown) may be connected between the other of the two power supply terminals (54) of the thermostat, and the free terminal (43) of the heating element

(42) . For example, the "neutral" terminal (not shown) of the power supply (not shown) may be connected to the free terminal

(43) of the heating element, and the "live" terminal (not shown) of the power supply may be connected to the free power supply terminal (34) of the thermostat (50) . In this way, the function of the power input and power supply terminals (51, 52, 53, 54) of the thermostat (50) is effectively reversed. Figure 10 schematically illustrates a thermostat of the type described above with reference to figure 7 comprising a first two electrical power supply terminals (63, 64) each adapted for connection to a respective one of two terminals of a pilot light assembly (67) to place the pilot light in electrical connection between a second two electrical power supply terminals (53, 54) of the thermostat for supplying electrical power to a heating element to be regulated by a thermostat. The thermostat adaptor (70) is adapted for connected between the first two electrical power supply terminals (63, 64) in place of a pilot light assembly thereby to form a short-circuit between the first two electrical power supply terminals. Being in parallel connection with the second two electrical power supply terminals (53, 54) the thermostat adaptor also renders those two terminals short-circuited. Accordingly, the thermostat (50) is functionally rendered equivalent to the thermostat (50) of the type described above with reference to figure 5. A heating element (42) to be regulated by the thermostat (50) may be connected to one (51) of the two power input terminals (51, 52) of the thermostat such that a power supply (not shown) may be connected between the free terminal of the heating element (43) and the free power input terminal (52) of the thermostat (50) in the manner described above with reference to figure 8 or figure 5.

It is to be understood that the exemplary embodiments described above are not intended to be limiting and that modifications or variations thereof, such as would be readily apparent to the skilled person, are intended to be encompassed within the scope of the invention, such as is defined by the claims.