WO2008040054A1

WO2008040054A1 - A control circuit

Info

Publication number: WO2008040054A1
Application number: PCT/AU2007/001470
Authority: WO
Inventors: Wayne Callen; Richard Leslie Brissenden; Robert Keith Henry Smith
Original assignee: Protectelec Pty Ltd
Current assignee: Protectelec Pty Ltd
Priority date: 2006-10-04
Filing date: 2007-10-04
Publication date: 2008-04-10
Anticipated expiration: 2009-04-04
Also published as: AU2006225307A1

Abstract

A control circuit (1) for a generator device in the form of a portable (2) HP petrol- engine powered single-phase generator (2). The generator provides a voltage of about 240 Volts AC between a phase terminal (3) and a neutral terminal (4). Circuit (1) includes two input terminals (5) and (6) for electrically connecting with respective terminals (3) and (4), and two output terminals (7) and (8) for electrically connecting, via an electrical cable (9), with a load in the form of an electric motor (10). A detector (11) provides a fault signal at its output (13) in response to a fault condition at its input (14). A switching device (15) is responsive to the fault signal at output (13) for progressing between a first mode and a second mode wherein: in the first mode terminals (5) and (6) are respectively electrically connected to terminals (7) and (8) for allowing motor (10) to receive the AC voltage from terminals (3) and (4) via switching device (15); and in the second mode terminals (5) and (6) are respectively electrically disconnected to terminals (7) and (8) for preventing terminals (3) and (4) from supplying the AC voltage to motor (10) via switching device (15).

Description

TITLE: A CONTROL CIRCUIT

FIELD OF THE INVENTION

[0001] The present invention relates to a control circuit and a method of control. [0002] The invention has been developed primarily for use with a non-MEN electrical distribution system, and will be described hereinafter with reference to that application. However, the invention is not limited to that particular field of use and is suitable for use in conjunction with an MEN electrical distribution system.

BACKGROUND

[0003] Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

[0004] A known electrical distribution system for the supply of mains power — more typically comprised of AC voltages and currents — is referred to as the Multiple Earth Neutral system, or the MEN system. It is also known to provide in an MEN system a variety of protection devices for detecting and protecting against a fault condition. These devices are generally configured to detect either:

• An over-current condition, in that the current in the active conductor exceeds a predetermined threshold.

• An increase in the current, where the increase is at a rate that suggests a low resistance current path has been established between the active conductor and one or both of the neutral or earth conductors.

[0005] Notwithstanding the use of these protection devices, there are still many electrocutions and electric shocks that occur due to electrical faults, whether those faults arise from the wiring used at a consumption site, the failure of an electrical appliance, an operator's manner of use of the appliance, or a combination of those factors. The most common form of electrocution is due to an operator of a device (or other person) creating a conductive path between the active conductor and the earth. [0006] One possible solution to address the above issues is to use a control circuit as disclosed in Patent Cooperation Treaty Patent Application No. PCT/AU03/00983, the disclosure of which is incorporated herein by way of cross-reference. [0007] Where an MEN distribution system — for example, a public mains utility such as is in common usage worldwide - is not available it is known to use a power source in the form of an electrical generator or an inverter to provide a voltage signal similar to that provided by the mains supply. This allows the use of mains powered appliances notwithstanding the absence of a mains utility. The power from this generator or inverter is distributed, by a distribution system that is isolated from the MEN system, to the one or more electrical loads. While some of these distribution systems are earthed - in that they include at least one earth-stake or earth-pin - in the case of marine and aircraft applications that will not be so. [0008] The isolated power sources include, for example in higher power applications, a diesel engine driven generator and, for example in lower power applications, a DC-to-AC inverter. By way of a particular example, it is known to mount an inverter to a service vehicle, where the inverter uses as an input the 12 Volt DC source of the vehicle. The typical output of these types of inverters is 110 Volts AC or 240 Volts AC at either 50 Hz or 60 Hz respectively. As will be appreciated by those skilled in the art the output of an inverter is usually an approximation of a true sinusoidal signal and, depending upon the inverter, can include considerable noise. Some inverters, particularly those at the lower quality and price, may only provide a square wave, or a minimally smoothed square wave. [0009] Inverters supply relatively high voltages, in that those voltages are above what is considered to be a safe touch-potential of about 50 Volts. This being so, it is necessary, at least in some jurisdictions, for inverters to include an earth-stake that is "grounded" while the inverter is operational. This is an attempt to replicate the earth connection used in the MEN system. However, in practice, this grounding rarely happens due to the inconvenience of doing so, and the fact that in the absence of a fault condition the inverter appears to function normally. This practical impediment is exacerbated in those cases where the inverter is at a building site or mounted to a vehicle or other conveyance that is required to move frequently. In such cases the operators are required to establish a new grounding of the earth stake following each move. Moreover, in some cases the operating environment may not be conducive to allowing an earth stake to establish a good earth. For example, where an inverter is mounted to a truck that is standing on sandy soils, or on a multistory construction site, or in a mine. [0010] And even if the operator of the vehicle or inverter takes the effort to place an earth-stake, the effectiveness of the earth provided by the stake is particularly susceptible to its exact placement and the prevailing conditions in the soil, amongst other things. Hence, the usefulness of the earth-stake for providing a reference is extremely susceptible to operator skill at selecting the earth point, and the available of a good earth point at the location of the vehicle.

[0011] The practices referred to above expose the operators to an increased risk of electric shock or electrocution in the event of a failure or other accident. Similar dangers are also faced by any other persons who come into contact with the inverter, the vehicle, or the equipment being powered by the inverter.

[0012] The total lack of an earth-ground connection or the lack of a good earth- ground connection will render most commonly used MEN protection devices either less effective or, more usually, totally ineffective. For example, a Residual Current Device (RCD) and Ground Fault Unit (GFU) will not function to protect the operators or other individuals if an earth-ground connection is absent or has deteriorated sufficiently.

[0013] Power sources that are isolated from the MEN system are more susceptible than power supply utilities to voltage and frequency fluctuations with time. This is due not only to the requirement for accurate inputs demanded by the regulators of the utilities, but also because of the much smaller size of the isolated systems. Accordingly, the isolated systems are more open to influence from localized factors such as temperature, load, vagaries in the distribution system, and the like. Particularly with solid-state inventers (such as switch-mode inverters) there are often considerable transient voltages generated that can interfere with mains operated devices that are being powered by the inverter. The mains operated devices often include protection circuitry or sensitive control and regulating circuitry, both of which can be either falsely triggered or rendered inoperative. That is, these protection circuits and control circuits are designed for more consistent and less noisy operating voltages such as those provided by a mains utility. Additionally, the use of inverters under fault conditions can render some safety devices inoperable, hence negating the only reason for including the safety device in the first place. This makes it more difficult for conventional protection circuits to adequately protect inventers. SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. [0015] According to a first aspect of the invention there is provided a control circuit for an inverter that provides an AC voltage between at least one phase terminal and a neutral terminal, the control circuit including: at least two input terminals, one for electrically connecting with the phase terminal and another for electrically connecting with the neutral terminal; at least two output terminals, one for electrically connecting with a load terminal of an electrical load and another for electrically connecting the load with the neutral terminal; a detector for providing a fault signal in response to a fault condition; and a switching device that is responsive to the fault signal for progressing between a first mode and a second mode wherein: in the first mode the input and output termirials are electrically connected for allowing the load terminal to receive the AC voltage from the phase terminal via the switching device; and in the second mode the input and output terminals are electrically disconnected for preventing the phase terminal from supplying the AC voltage to the load terminal via the switching device. [0016] It will be appreciated by those skilled in the art that the AC voltage provided by the inverter is usually an approximation of a true sinusoidal signal. Depending upon the inverter the AC voltage can include considerable noise. Some inverters, particularly those at the lower quality and price, may only provide a square wave, or a minimally smoothed square wave. In this specification, unless the context clearly indicates otherwise, the term AC voltage is intended to encompass all such outputs of inverters.

[0017] In an embodiment, the inverter is powered by a DC source of a predetermined voltage, and the AC voltage is greater than the predetermined voltage. In preferred embodiments, the predetermined voltage is one of: 12 Volts, 24 Volts, 42 Volts and 48 Volts. In other embodiments, however, alternative predetermined voltages are used. Moreover, in some embodiments, the predetermined voltage changes over time. In the preferred embodiments the AC voltage is at 50 Hz and 240 Volts, while in other embodiments alternative frequencies and voltages are provided, such as 60 Hz and 110 Volts.

[0018] In an embodiment the detector includes a detector relay. Preferably, the detector relay is a low voltage armature relay. More preferably, the detector relay is a small signal (or telecom-type) relay. In some embodiments the detector includes a small signal relay having a typical coil/switching voltage of between about 3 to 6 Volts. The switching voltage is that voltage at which the detector relay will change state: that is, the voltage at which the relay will switch. An example of a suitable small signal relay is sold with the branding "Nashua NEC", and marked with Model number D005-M. For such a relay the typical coil resistance is about 50 Ohms to 300 Ohms.

[0019] In other embodiments the detector includes a solid-state relay. [0020] In an embodiment, the detector relay has a detector relay coil and a detector relay armature that moves between two sets of contacts. Preferably, in the absence of the fault condition, the detector relay coil is not energized and the detector relay armature is engaged with a first of the two sets of contacts. More preferably, once the fault condition appears the detector relay coil is energized and the detector relay armature moves into engagement with the other set of contacts. Even more preferably, once the fault condition is removed the detector relay coil is de-energized and the detector relay armature returns to engage with the first set of contacts.

[0021] In an embodiment the switching device is a switching relay. Preferably, the switching relay is a mains-rated relay. More preferably, the switching relay is a cradle relay rated at 250 Volts and 12 Amps, and has a typical coil voltage of 250 Volts. For example, in some embodiments, the switching relay is a "Finder Type 56.32" which is rated at 250 Volts and 12 Amps. Another suitable switching relay is "Tianbo Model number HJQ/13F/2ZP" which is rated at 240 Volts and 10 Amps. A further example product suitable for use as the switching relay is sold with the branding "Nais HL2-L- AC240" and which is rated at 240 Volts and 10 Amps. A further example product suitable for use as the switching relay is sold by IMO Precision Controls Limited and designated as SRP-1C1N-SL-230VAC and which is rated at 250 VAC and 16 Amps. [0022] In an embodiment, the switching relay has a switching relay coil and a switching relay armature that moves between two sets of contacts. Preferably, in the absence of the fault condition, the switching relay coil is not energized and the switching relay armature is engaged with a first of the two sets of contacts. More preferably, once the fault condition appears the switching relay coil is energized by being placed across the phase terminal and the neutral terminal, wherein the switching relay armature moves into engagement with the other set of contacts. Even more preferably, once the switching relay armature moves into contact with the other set of contacts the switching relay coil remains energized while the detector coil is isolated such that the relay armature returns to engage with the first set of contacts. [0023] According to a second aspect of the invention there is provided a protection device for an inverter, the device including: at least one control circuit of the first aspect; and a housing for encapsulating the control circuit.

[0024] In an embodiment the detector includes a detector relay for providing the fault signal, and the switching device includes a switching relay that is responsive to the fault signal for moving between a first state and a second state that correspond with the first mode and the second mode. In an embodiment, the detector relay includes a detector relay coil and the switching relay includes a switching relay coil both of which, in the absence of a fault, are not energized.

[0025] According to a third aspect of the invention there is provided a control circuit for a generator device that provides an AC voltage between at least one phase terminal and a neutral terminal, the control circuit including: an input terminal for electrically connecting with the phase terminal; an output terminal for electrically connecting with a load terminal of an electrical load wherein, in use, the electrical load is also electrically connected with the neutral terminal; a detector for providing a fault signal in response to a fault condition; and a switching device that is responsive to the fault signal for progressing between a first mode and a second mode wherein: in the first mode the input and output terminals are respectively electrically connected for allowing the load terminal to receive the AC voltage from the phase terminal via the switching device; and in the second mode the input and output terminals are electrically disconnected for preventing the phase terminals from supplying the AC voltage to the load terminal via the switching device. [0026] In an embodiment, the generator device includes one of: an inverter; and a generator. That is, a generating device converts energy of one form to an AC electrical signal. An inverter, for example, converts energy in the form of a DC voltage - typically from an electrochemical cell or cells or from a vehicular alternator - into an AC voltage signal. A generator, on the other hand, converts rotational energy into an AC voltage. In other embodiments other forms of energy are converted to an AC voltage. Moreover, in further embodiments, a combination of types of energy is converted into the AC voltage. It will also be appreciated that the AC signal generated by an inverter will be an approximation of a mains AC electrical signal. [0027] In an embodiment the detector provides the fault signal while the fault condition persists, and the switching device remains in the second mode while the fault signal persists. Preferably, the switching device remains in the second mode until reset. More preferably, the switching device remains in the second mode until manually reset. [0028] In an embodiment the switching device, in progressing from the first mode to the second mode, progresses the detector from an enabled state to a disabled state. Preferably, when the switching device is reset, the detector is progressed to the enabled state. That is, if the fault condition persists when the switching device progresses to the first mode in response to being reset, the detector, being in an enabled state, provides the fault signal and the switching device returns to the second mode.

[0029] In an embodiment, the generator device is mounted to a support frame that is electrically isolated from the terminals, and the detector is electrically connected to the frame. More preferably, the frame defines, at least in part, a housing for the generator device. In some embodiments, the frame and housing are integrally formed. [0030] In an embodiment, the control circuit is mounted to the housing. More preferably, the control circuit is mounted within the housing. Even more preferably, the generator terminal is disposed within the housing and the output terminal is accessible from outside the housing. For example, in some embodiments, the output terminal extends beyond the housing. However, in other embodiments, the output terminal is a female terminal that is accessible for contact with a complementary male terminal. In these embodiments only the output terminal is accessible from outside the housing and not the generator terminal. In other embodiments, the generator terminal is also accessible from outside the housing. [0031] In an embodiment, the neutral terminal is accessible from outside the housing.

[0032] In other embodiments, the frame or housing is conductive and the neutral terminal is electrically connected to the frame or housing, wherein the detector is electrically connected to an element that is electrically isolated from the frame or housing and which, in use, is isolated from the AC voltage.

[0033] In further embodiments, the frame or housing is conductive and the neutral terminal is electrically connected to the frame or housing and the detector is connected to an element that is electrically connected to the housing, wherein the resistance of the element is sufficient to allow a fault voltage to be generated in the presence of a fault current.

[0034] In an embodiment, the generator device is, in use, mounted to a conveyance and the detector is electrically connected to an element of the conveyance that, in use, is electrically isolated from the generator terminal. Preferably, the element is conductive. More preferably, the conveyance is a vehicle having a conductive body and the element is defined by the body. In other embodiments the neutral terminal is referenced to the body, and the detector is electrically connected to a further element of the conveyance. For some embodiments the conductive body is a chassis of the vehicle, while in other embodiments the conductive body is a frame or a panel of the vehicle. Preferably, the vehicle is self-propelled and is, for example, a truck, an SUV, a van or other motor vehicle. In other embodiments, however, the vehicle is not self- propelled and is, for example, a trailer. In further embodiments different vehicles are used, for example, an aircraft, a glider, a boat, a ship, a motorbike, a motor scooter, or the like. [0035] In an embodiment, the conveyance includes a DC power source and the generator device is connected to the DC power source for providing the AC signal. More preferably, the DC power source includes a positive terminal and a negative terminal, wherein the negative terminal and one of the terminals of the generator device are electrically connected to the conveyance. [0036] According to a fourth aspect of the invention there is provided a control circuit including: an input terminal for electrically connecting with a power source that, in use, provides a source voltage; an output terminal for electrically connecting with a load; a detector having: an input that, in use, is connected to the power source; and an output, wherein the detector is responsive to a reference signal being within a predetermined range for electrically connecting the input and the output to apply the source voltage to the output; and a switching device that is electrically connected to the output of the detector and which is responsive to the source voltage being applied to the output for progressing between a first mode and a second mode wherein: in the first mode the input terminal and the output terminal are electrically connected for allowing the load to receive power from the source via the switching device; and in the second mode the input terminal and the output terminal are electrically disconnected for preventing the source from supplying power to the load via the switching device.

[0037] In an embodiment, the detector includes a sensor relay having a sensor coil and a first sensor contact and a second sensor contact, wherein the sensor coil, when energized by the reference signal, connects the first sensor contact to the second sensor contact. More preferably, the first and second sensor contacts respectively define the input and the output.

[0038] In an embodiment, the switching device includes a switching relay having: a switching coil that is electrically connected to the output; and a first switching contact and a second switching contact, wherein the switching coil, when energized by the source voltage at the output, progresses the switching device between the first mode and the second mode.

[0039] According to a fifth aspect of the invention there is provided a control circuit for an electrical appliance having a load element for receiving electrical power from a power source having a floating earth, the appliance having at least one conductive element which is not, in use, electrically connected with the floating earth, the circuit including: at least two input terminals for electrically connecting with the power source; at least two output terminals for electrically connecting with the load element; a detector that is responsive to the voltage of the element for providing a reference signal; and a switching device being interposed between the input and output terminals and being responsive to the reference signal being within a predetermined range for progressing from a first mode to a second mode wherein: in the first mode the input and output terminals are respectively electrically connected for allowing the load element to receive power from the source via the switching device; and in the second mode the input and output terminals are electrically disconnected for preventing the source from supplying power to the load via the switching device. [0040] In an embodiment, the appliance is mounted to a conveyance having a body that is electrically connected with the floating earth. Examples of a conveyance include an automobile, a watercraft, an aircraft, a motorbike, a trailer and the like. In other embodiments, the appliance is mounted to a structure such as a frame, a platform, a housing or other structure. In these embodiments the power source is separate from the mains supply and is preferably a generator device. More preferably, the generator device is a generator — that is, a device that converts rotational energy into electrical energy such as an AC voltage — or an inverter - that is, a device that converts a DC voltage into an AC voltage. [0041] In an embodiment, the power source is mounted to the conveyance or the structure, as the case may be.

[0042] In an embodiment, the appliance is a fluid pump. However, in other embodiments, the appliance is one or a combination of: the fluid pump; an electric drill; an electric motor; an iron; a hair dryer; a consumer white-good such as a refrigerator, a washing machine, a clothes dryer; a computer, be that a laptop computer or a desktop computer; a computer peripheral device or stand alone device such as a printer, modem, facsimile machine or the like; a piece of or combination of domestic hi-fi equipment; a television or associated hardware; a hot water kettle; or the like. [0043] In an embodiment, the control circuit is a protective device for electrically isolating the load from the power source once a fault condition has been detected.

[0044] Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', 'include', 'including', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Embodiments of the invention will now be described, by way of example only and not by way of limitation, in the accompanying drawings in which like references indicate similar elements. It should be noted that references to "an" or

"one" embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one.

[0046] In the accompanying drawings:

[0047] Figure 1 is a schematic representation of a portable petrol-driven generator having a control circuit;

[0048] Figure 2 is a schematic circuit diagram of the control circuit used in the embodiment of Figure 1;

[0049] Figure 3 is a schematic representation of a portable inverter including a control circuit; [0050] Figure 4 is a schematic representation of a further embodiment of the invention including a protection device;

[0051 ] Figure 5 is a schematic representation of a three-phase generator including a control circuit;

[0052] Figure 6 is a schematic side view of a vehicle to which is mounted the inverter of Figure 3; and

[0053] Figure 7 is a schematic representation of a protection device applicable to the Figure 4 embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0054] Referring to Figure 1 there is provided a control circuit 1 for a generator device in the form of a portable 2 HP petrol-engine powered single-phase generator 2.

The generator provides a voltage of about 240 Volts AC between a phase terminal 3 and a neutral terminal 4. Circuit 1 includes two input terminals 5 and 6 for electrically connecting with respective terminals 3 and 4, and two output terminals 7 and 8 for electrically connecting, via an electrical cable 9, with a load in the form of an electric motor 10. A detector 11 provides a fault signal at its output 13 in response to a fault condition at its input 14. A switching device 15 is responsive to the fault signal at output 13 for progressing between a first mode and a second mode wherein: in the first mode terminals 5 and 6 are respectively electrically connected to terminals 7 and 8 for allowing motor 10 to receive the AC voltage from terminals 3 and 4 via switching device 15; and in the second mode terminals 5 and 6 are respectively electrically disconnected to terminals 7 and 8 for preventing terminals 3 and 4 from supplying the AC voltage to motor 10 via switching device 15. [0055] Generator 2 is designed to be portable, and to provide a convenient mains power source for powering electrical appliances or electrical equipment with mains voltages when the normal mains supply is not available. Generators of this type are used commonly on building sites, or carried on service vehicles, or the like. In other embodiments, the generator is powered by an alternative energy source such as a diesel engine, natural gas engine, or the otherwise. In further embodiment, generator 2 is a larger capacity generator for larger electrical loads, and is fixed in a given location. [0056] Terminal 3 is a phase or active terminal, and terminal 4 is a neutral terminal, with the result that in normal use terminal 7 is a phase terminal, and terminal 8 is a neutral terminal. Accordingly, cable 9 includes a phase conductor 17 and a neutral conductor 18. [0057] In other embodiments, generator 2 includes a greater number of phase terminals and a corresponding number of circuits 1 are used in conjunction with those terminals to provide corresponding protection. Alternatively, circuit 1 is replaced by a single protection circuit with the requisite number of input terminals 5. For example, Figure 5 illustrates schematically a fixed location three-phase diesel generator 12, where corresponding features are denoted by corresponding reference numerals. Generator 12 includes three separate phase terminals 3 - one for each of the phases of the three-phase supply being provided - and one neutral terminal 4." Circuit 16 includes three input terminals 5, each for connecting with a respective phase terminal 3, and three output terminals 7 for connecting with respective loads. In other embodiments terminals 7 are all connected to the same three-phase load (not shown). Generator 12 is designed to power larger loads, and is not portable but, rather, fixed in a given location. In other embodiments generator 12 is designed to power devices such as welders or portable lighting towers that are relatively large and difficult to move but portable nonetheless. [0058] Referring again to Figure 1, generator 2 is disposed, together with circuit 1, in a ventilated prismatic sheet metal generator housing 19. The function of housing 19 is primarily to protect generator 2 from the elements, and to reduce the risk of inadvertent or unauthorised access to or contact with generator 2 and the terminals. This housing is electrically isolated from all the terminals and, in the absence of being connected to an earth stake, is floating. It is appreciated that in other embodiments housing 19 will be formed of a conductive material other than sheet metal. [0059] While circuit 1 is contained wholly within housing 19, in this embodiment generator 2 is not. Particularly, housing 19 does not fully encompass the petrol- engine, primarily to allow better management of the heat and gases associated with that engine. In other embodiments housing 19 more fully covers the engine and includes additional ventilation - for example, vents and/or fans — to manage cooling air flows. [0060] Motor 10 is disposed within a prismatic steel motor housing 20 that is spaced apart from housing 19. Housing 20, like housing 19, is electrically isolated from all the terminals and, in the absence of being connected to an earth stake, is floating. It is appreciated that in other embodiments housing 20 will be formed of a conductive material other than steel. [0061] In this embodiment motor 10 and housing 20 are not physically fixed and are portable. In particular, motor 10 is used to drive a condenser of a portable refrigerator (not shown) that is also disposed within housing 20. In other embodiments additional or alternative loads are included. For example, in one embodiment motor 10 is contained within a portable electrical tool such as an electric drill, an electric hammer, a nail gun, and other tools. It will be appreciated by those skilled in the art that the load is able to be any electrical load suitable for use with the voltage and current waveforms provided at terminals 7 and 8.

[0062] Circuit 1, including detector 11 and device 15, is encapsulated in a sealed housing (shown schematically) that is disposed within housing 19. The encapsulation, in this embodiment, takes the form of a polyurethane material that envelops all the components within the sealed housing. The material includes equal parts CCM-80 Parts A and B₅ with a coloring. This material, in the form of a viscous fluid, is poured through a small aperture in the sealed housing to envelope the components and fill any voids between the components and/or the sealed housing. The material is allowed to set to form a barrier that is both water impermeable and electrically insulating. This encapsulation has the added advantage of making circuit 1 more difficult to tamper with or damage. As will be discussed further below, device 1 is able to be encapsulated due to its inherently low power consumption. [0063] In other embodiments alternative encapsulation is used, such as foams. In further embodiments the environmental protection is provided by more direct mechanical arrangements such as a sealed housing that includes interacting O-rings and sealing faces and/or webbing. For those applications, such as military applications, where sealing against dust and moisture is required over a significant temperature range, use is made of both mechanical arrangements and encapsulation. [0064] The sealed housing for circuit 1 is a two-piece construction, where the two pieces are injection molded and sealed against each other. In use, the sealed housing has external dimensions of about 65 mm x 45 mm x 45 mm. It will be appreciated by those skilled in the art that a housing of these external dimensions is relatively small in comparison to the overall dimensions of generator 2 and housing 19. Accordingly, it is relatively easy from a packaging perspective to design a generator housing including circuit 1, or to have circuit 1 retrofitted to an existing generator housing. [0065] In other embodiments the sealed housing has external dimensions other than 65 mm x 45 mm x 45 mm. For example, in embodiments, the external dimensions are greater than 65 mm x 45 mm x 45 mm to provide greater heat protection for circuit 1. [0066] In this embodiment, the sealed housing for circuit 1 is mounted directly to an interior surface of housing 19 adjacent to where terminals 7 and 8 emerge from housing 19. In other embodiments the sealed housing, while being disposed within housing 19, is mounted directly to other than the interior surface of housing 19. In yet other embodiments the sealed housing is integrally formed with housing 19.

[0067] In other embodiments additional components are contained with the sealed housing together with circuit 1.

[0068] In other embodiments, the encapsulation is in the form of a heat-shrink material about the encapsulated components. In further embodiments the sealed housing is encapsulated by heat-shrink material.

[0069] Circuit 1 is effectively passive, in that it consumes little power and dissipates little heat during normal use. During the presence of a fault condition circuit 1 consumes little energy and only operates for a very short time. Accordingly, encapsulation is possible without the danger of thermal runaway. While, in the Figure 1 embodiment, there is not a strong need for encapsulation - as circuit 1 is well shielded from environmental conditions by being disposed within housing 19 — in other embodiments such as marine and aircraft applications the need is greater. [0070] In other embodiments, circuit 1 is disposed in other than housing 19. Examples of such alternate dispositions include: within housing 20; electrically between housing 19 and 20 (such as at one end of cable 9, or intermediate the ends of cable 9); and mounted external to either one of those housings. [0071] Input 14 of detector 11 is electrically connected with a conductor 25 that defines a floating earth for generator 2 and motor 10. Conductor 25 is bundled with conductors 17 and 18 within cable 9, with conductor 25 being defined by what is normally the "earth" conductor in a standard three-conductor mains cable. It will be appreciated that as a floating earth, conductor 25 is not, in fact, connected to earth, and should not be confused with the earth that is available in an MEN system. Rather, conductor 25 ties housing 19 and housing 20 to a common floating voltage. In other embodiments additional components are also joined to conductor 25 such as the metal housing of electrical tools, motor stators, and other conductive components that should, in normal operation, be isolated from the active or phase voltage being provided to the load. To optimize the effect of the Figure 1 embodiment conductor 25 is connected with. any conductive component that is likely to come into physical contact with an operator or other person and which should, under normal operating conditions, be isolated from the phase voltage. As will be described in more detail below, if the potential of any one or more of these components rises beyond a threshold voltage there arises a fault condition. [0072] In other embodiments less than all of the conductive components that should, in normal operation, be isolated from the active or phase voltage are electrically connected to conductor 25. Moreover, in some embodiments, only a single conductive component is electrically connected to conductor 25. For example, in some embodiments only housing 10 is electrically connected to conductor 25 on the basis that should a fault occur this housing poses the greater risk to operator harm.

[0073] Detector 11 monitors the voltage of conductor 25, and hence the potential of housings 19 and 20 and any other conductive component connected to conductor 25. If this voltage moves outside a predetermined threshold from the voltage of terminal 6 (the neutral terminal) a fault condition is defined, and a fault signal is provided by detector 11 at output 13. The switching device 15 is responsive to that fault signal for isolating motor 10 from generator 2. That is, if the housings - or whatever other components are connected with conductor 25 - rise in potential relative to the neutral conductor so as to create a potentially hazardous situation for operators or other personnel, the supply of electrical power to the relevant load or loads is terminated. For the present embodiment that would not necessarily result in generator 2 ceasing to operate, as terminals 3 and 4 are disposed within housing 19 and are not typically subject to causing faults such as those detected by the present embodiment. It is usual for generator 2 to have additional protection circuitry (not shown) for protecting personnel and the generator from short circuits between terminals 3 and 4 or terminals 7 and 8. Circuit 1 is entirely compatible with the existing protection circuitry used by generator 2, while providing additional protection to personnel. [0074] In normal use, both housing 19 and housing 20 are floating at zero voltage or a very small voltage. The lower threshold for a fault voltage is when conductor 25 is about 3 Volts DC greater than the voltage on the neutral terminal. In other embodiments alternative thresholds are used, although preferably a threshold that is less than the safe touch potential. [0075] It will be appreciated that circuit 1 is able to be used instead of or in addition to existing protection circuitry included with generator 2. Circuit 1 does not interfere with that existing circuitry during normal operation and is entirely compatible with it. This allows circuit 1 to be easily and cost-effectively retrofitted to existing generators and other power supplies. [0076] Moreover, it has been found that the operation of circuit 1 is quicker than conventional solid-state protection circuitry by at least about 10 milliseconds, and more typically by about 15 milliseconds. Accordingly, when fitted with conventional protection circuits it is not unusual for a fault condition to be removed by circuit 1 prior to the existing protection circuits acting. [0077] In those embodiments where one or both of housings 19 or 20 are connected to earth via an earth-stake, it is possible for a relatively high resistance to earth to exist. That is, while conductor 25 may be notionally "earthed" by connection to an earth-stake, the effectiveness of the earth connection will vary with time due to changes in climatic conditions, soil conditions, and many other factors. Even with such an earth connection it has been found that circuit 1 will continue to operate effectively, whereas the efficacy of the conventional protection circuitry will be more usually compromised. The greater the resistance to earth the more effective the embodiment of the invention becomes, which is typically the converse of what occurs with conventional protection systems. For example, the embodiment illustrated in Figure 1 makes use of 240 Volts AC which, for conventional protection systems, would provide a hazardous situation once the earth resistance exceeded about 1,000 Ohms. At this point the effectiveness of conventional protection systems - such as RCDs - has considerably, if not fully, degraded. [0078] In the event that conductor 25 was earthed perfectly and there was a fault condition involving a short circuit between the active phase and conductor 25, a conventional solid state protection circuit would typically take about 30 milliseconds to remove the fault condition. Moreover, the peak fault current flowing for a 240 Volts mains appliance could easily be in the order of hundreds of amps before the fault condition is removed. The control circuit 1 of Figure 1 is able to remove the same fault in about 15 to 20 milliseconds, with typical fault currents being an order of magnitude less than the conventional systems. More detail on the operation of circuit 1 is provided below in combination with the description of Figure 2. [0079] This high speed of operation and relatively low peak fault currents allow circuit 1 to act, in practice, as a quasi-circuit breaker. That is, circuit 1 does not require large fault currents to be flowing to detect a fault condition. Rather, circuit 1 is responsive to a voltage and draws little current, and operates to prevent large fault currents from flowing in the first place. The conventional protection circuits, in stark contrast, rely upon the existence of large fault currents to initiate triggering. [0080] While in this embodiment generator 2 provides a 7.5 kW supply (at 240 Volts AC, 32 Amps, and 50 Hz) at terminals 3 and 4, in other embodiments alternative voltages, currents and frequencies are provided.

[0081] During normal operation - that is, in the absence of a fault condition - device 15 is in the first mode and the electrical potential of terminals 7 and 8 will be substantially identical to terminals 5 and 6 respectively. While there may be small differences in those potentials due to the impedance of the intervening connections and the quantum of the load current being drawn, it will be appreciated by those skilled in the art that these should be substantially negligible. [0082] It will also be appreciated that in the absence of circuit 1 a fault condition could arise resulting in one or more of housings 19 and 20 being at a raised voltage due to a short between the active conductor and the relevant housing. If use was made of an earth-stake this may not necessarily trigger any conventional protection circuits due to a high resistance to earth. Accordingly, should an operator or other person come into contact with one of housing 19 or 20 and establish a path to earth, there is a danger of electric shock or electrocution. Where circuit 1 is included, the same circumstances give rise to a fault condition, as conductor 25 would be at voltage considerably more than 3 Volts DC in excess of the voltage of the neutral conductor. That is, use of protection circuit 1 allows the fault condition to be removed without having to have an operator coming into contact with the fault and being exposed to the risk of electrocution.

[0083] Reference is now made to Figure 2 where there is illustrated a schematic circuit diagram for control circuit 1, where corresponding features are denoted by corresponding reference numerals. Circuit 1 includes an input terminal 5 for electrically connecting with terminal 3 of generator 2 that, in use and as described above, provides a 240 Volts AC source voltage. An output terminal 7 is electrically connected with a load in the form of motor 10. A detector — that is, detector 11 - has an input 31 that, in use, is connected to terminal 5. The detector is responsive to a reference signal, in the form of the voltage signal on conductor 25, being within a predetermined range for electrically connecting input 31 and output 13 to apply the source voltage to output 13. A switching device — that is, device 15 - is electrically connected to the output 13 and is responsive to the source voltage being applied to output 13 for progressing between the first mode and the second mode. For the sake of completeness, it is mentioned that in the first mode terminal 5 and terminal 7 are electrically connected for allowing motor 10 to receive power from generator 2 via device 15. In the second mode terminal 5 and terminal 7 are electrically disconnected for preventing generator 2 from supplying power to motor 10 via device 15. [0084] Detector 11 includes a detector relay in the form of a low DC voltage relay 33. Relay 33 has a sensor coil 34, a first sensor contact 35 and a second sensor contact 36. Coil 34, when energized by the reference signal on conductor 25, changes the state of relay 33 and connects contact 35 to contact 36. It will be appreciate that contacts 35 and 36 respectively define input 31 and output 13. [0085] Relay 33 is a low voltage armature relay and, more particularly, a small signal relay. Such a relay is, in some jurisdictions, referred to as a telecom-type relay. The switching voltage of the relay is rated at about 5 Volts DC, but there is often considerable variation between notionally like relays. Due to the nature of the fault voltages being of considerably greater magnitude than the switching voltage of the detector relay, the operation of circuit 1 is not adversely affected by the variations in switching voltages. However, regard is had to, amongst other things, the likely range of those switching voltages for a given relay when designing detector 11. More particularly, detector 11 includes a current limiting resistor 37 in series with a diode 38 for limiting the current through coil 34 in the event of a fault condition. The resistance of resistor 37 is selected on the basis of the predetermined fault voltage to be detected, the maximum likely fault voltage to be carried by conductor 25, the notional switching voltage of detector relay 33, and the series resistance of coil 34. It will be appreciated by those skilled in the art that the lesser the resistance of resistor 37 the greater the sensitivity of detector 11, in that a smaller potential on conductor 25 will result in coil 34 being energized and, hence, relay 33 changing states. [0086] In this embodiment, resistor 37 has a resistance of 8.2 kOhms. For a 110/120 Volt load voltage, circuit 1 is the substantively the same, with one exception being resistor 37 has a resistance of 4.7 kOhms. In other embodiments resistor 37 has a resistance of other values.

[0087] Diode 38 is provided to half- wave rectify any fault voltage on conductor 25 to prevent energizing coil 34 with the wrong polarity. Moreover, in line with the fast acting nature of circuit 1, diode 38 is an IN4007 diode, which is a fast acting diode having a minimum rating of 1,000 Volts at 1.5 Amps. Another suitable diode is an IN5408 diode, which is rated at 1,000 Volts at 3 Amps. On the basis of the teaching herein it would be appreciated by those skilled in the art that other diodes would be suitable.

[0088] For applications of the embodiments to 240 Volt supplies it has been found that a typical coil/switching voltage for the detector relay 33 of between about 3 to 6 Volts DC is suitable for providing the level of speed and sensitivity needed to exceed the performance of conventional systems. In other embodiments similar results are able to be achieved with detector relays having higher switching voltages, but these relays are often more expensive without providing a significant performance gain. And as will be described below, the operation of circuit 1 is to protect relay 33 in the event of a fault condition, so unless the fault voltages are such as to cause jump-over between the contacts of the relay, it is usual to select relay 33 having a lower rather than a higher switching voltage. [0089] For the sake of completeness it is mentioned that the switching voltage is that voltage at which the detector relay will change state: that is, the voltage at which the relay will switch from one state to another in response to a fault condition. Due to the inherent hysteresis characteristics of the relay is not unusual for any given relay to have a switching voltage that is different when progressing from one of the states to the other than vice versa. In the present embodiments the important consideration is the switching voltage to progress the relay from the non-energized state to the energized state. The switching voltage at which the relay progresses from the energized state to the non-energized state is not critical. [0090] The detector relay is left in a non-energized state during normal operation, in that coil 34 is not drawing any current. This provides a relatively consistent operation for a given detector 11, as the initial level of drive of the coil - that is, zero or minimal drive — is relatively consistent over time for a given relay 33 in a given detector 11. It also has the added benefit of ensuring extremely low power consumption during normal operation. [0091] An example of a small signal relay suitable for use within detector 11 is sold with the branding "Nashua NEC", and marked with Model number D005-M. For such a relay the typical coil resistance is about 50 Ohms to 300 Ohms. [0092] Device 15 includes a mains voltage switching relay 41 having: a switching coil 42 that is electrically connected to output 13; and a first switching contact 43 and a second switching contact 44. Coil 42, as illustrated, is not energized and, hence, relay 41 is in a first mode such that contact 43 is electrically connected with both terminal 5 and terminal 7, while contact 44 is electrically connected with both terminal 6 and terminal 8. In other words, in this mode the load is able to be powered by the generator. When a fault signal appears on output 13, coil 42 is energized as the full voltage at terminal 5 is applied across coil 42. That is, one end of coil 42 is connected to output 13, which is now connected to terminal 5, and the other end of coil 42 remains connected to terminal 6. That being so, relay 41 progresses from the first mode to the second mode to change the electrical connection of contacts 43 and 44. Particularly, contact 44 is isolated from output 13 such that coil 34 is isolated from terminal 6. In response, coil 34 is allowed to de-energize. Also, contact 43 is electrically isolated from terminal 7, and electrically connected to maintain the full voltage at terminal 5 across coil 42. In that way, once a fault condition is detected, not only does circuit 1 act quickly to electrically isolate terminals 7 and 8 from live terminals 5 and 6, but relay 41 latches in the second mode as the voltage at terminal 5 is applied to coil 42.

[0093] Relays 33 and 41 are sequentially connected to provide a cascading effect. The operation of cascaded relays — which both include de-energized coils during normal operating conditions — is described in the earlier pending Patent Cooperation Treaty Patent Application No. PCT/AU03/0O983, the disclosure of which is incorporated herein by way of cross-reference. The Figure 2 embodiment includes, in addition, a direct connection between terminal 5 and input 35. This results in circuit 1 being faster acting than the earlier devices as the full voltage at terminal 5 is more quickly applied to energize coil 42 of relay 41. Having input 35 directly connected to terminal 5 also reduces the risk of relay 41 not latching in the presence of a fault condition.

[0094] Based upon manufacturing tolerances the average response times for circuit 1 are 15 milliseconds, with 18 to 20 milliamps of earth leakage current flowing. This is on the basis of 240 Volts AC supply, and relay 33 having a notional 5 Volt DC switching voltage and a 20 Ohm coil resistance. It will be appreciated that only about 5 milliamps will be required through the coil of detector relay 33 to result in that relay changing states. This results in low energy dissipation within the sensor relay during a fault condition. That is, there is very little current required to detect the fault condition, and that current will only flow for about 15 milliseconds, after which coil 34 will be isolated from the fault due to the operation of relay 41. This operation of circuit 1 is to protect relay 33 from the voltages on conductor 25 which can equal or, in the case of a voltage spike, exceed the mains phase voltage provided at terminal 5. Testing has shown that even when the mains voltage is applied to conductor 25 to simulate a fault condition, relay 33 is capable of over 500,000 operations as it is only exposed to the higher voltages for a very short time for each fault condition. Accordingly, it is possible to gain the sensitivity of a low DC voltage relay to detect a fault condition. [0095] The response time is the time period between the fault occurring and the power being disconnected from the load. It will be appreciated for similar conditions a solid state RCD will typically trigger in about 30 milliseconds, although this varies considerably between devices. Typical RCD fault currents for triggering are 30 milliamps ±10%.

[0096] In the embodiment of Figure 2, both relay 33 and 41 are passive during normal operation, and only consume power when working to isolate the load. Moreover, relay 33 will only draw a very small current for a short time during a fault condition. Relay 41, however, may remain active for a number of minutes, hours, days or indefinitely depending upon the circumstances.

[0097] Relay 33 is relatively small physically and electrically - certainly in comparison to relay 41 — and is not designed for use with relatively high voltage such as mains voltages. Notwithstanding, relay 33 and like relays are able to be applied to the present embodiment due to operation of circuit 1 to protect the detector relay once its role in detecting a fault has been performed. Particularly, the cascaded nature of the relays allows relay 41 to gain the benefit of the fast detection and switching of relay 33, and for relay 33 to be quickly thereafter relieved of the electrical stresses of the mains voltages by the operation of relay 41. [0098] As presently understood, the increase speed of progression of circuit 1 between the first mode and the second mode is a result of:

• The voltage at terminal 5 being applied, as the fault signal, to coil 42 of relay 41. That is, a slightly greater voltage is applied than was the case with the earlier circuits.

• The continued application of the voltage at terminal 5 to coil 42, notwithstanding that the progression to the second mode may have commenced. That is, unlike the earlier circuits, coil 42 remains not only energized, but energized by the full supply voltage while such time as terminal 5 is powered.

[0099] Detector 11 also includes a neon lamp 81 across coil 34 for providing additional high voltage protection for that coil. Should the voltage across coil 34 exceed about 90 Volts, the gas within lamp 81 energizes and creates a low impedance conductive path. This provides a failsafe mechanism to divert excessive current that could otherwise damage coil 34 and which is not required to energize the coil. In other embodiments where a shorter operational lifetime for relay 34 is tolerable, or cost is a particularly significant factor, lamp 81 is omitted. Lamp 81, in this embodiment, is a standard helium neon gas lamp having a 90 Volt flashover. In other embodiments alternative lamps or high-voltage protection devices are used. In an alternative embodiment, use is made of a current shunt in parallel with coil 34 to limit the current in that coil.

[0100] Also included within detector 11 are a resistor 82 and a diode 83. These are equivalents to resistor 37 and diode 38 respectively, and are selectively electrically connected in series between coil 34 and terminal 7 to allow testing of the operation of circuit 1. The testing involves an operator manually depressing a normally open switch 84 to apply the active phase - that is, the voltage at terminal 7 - to coil 34 via resistor 82 and diode 83. It will be appreciated that this creates a condition that coil 34 finds an equivalent to a fault condition and, hence, relay 33 operates to provide a fault signal at output 13. In turn, relay 41 will change state to isolate terminal 7 from terminal 5. Accordingly, coil 34 will no longer be subject to the voltage at terminal 5. Once the operator removes manual pressure from switch 84 relay 41 remains latched in the second mode. This latching is removed by resetting circuit 1, which will be described in more detail below. [0101] Device 15 includes a mains rated capacitor 85 in parallel with coil 42. The primary role of capacitor 85 is to minimize the risk of "drop out" of relay 41. That is, once terminal 5 is connected to output 13 by relay 33, there is a risk that in the short time it takes relay 41 to electrically connect terminal 5 with the active side of coil 42 — through the changing in state of contact 43 - that coil 42 will not have been maintained sufficiently energized due to coil 13 being isolated — by the change in state of contact 44. In this embodiment, capacitor 85 is a 0.22 μF mains voltage polyester capacitor. In this embodiment, a single capacitor is used, although in other embodiments this is substituted by a number of capacitors that collectively play the same role in the circuit. In other embodiments alternative capacitors are used, such as a high voltage ceramic capacitor. Moreover, some embodiments are designed with a safety margin - for example, to accommodate possible voltage spikes - and use is made of a larger capacitor. In particular, one alternative embodiment has a capacitor 85 in the form of a 0.33 μF 3 kV rated capacitor. [0102] Circuit 1 also includes a varistor 86 between terminals 5 and 6 to provide additional over- voltage protection for motor 10. It will be appreciated by those skilled in the art that such protection is often provided by a generator, and is not required when circuit 1 is retrofitted to that generator. A varistor is a variable resistor typically including solid state technology. In the present embodiment use is made of a symmetrical varistor.

[0103] Circuit 1 also includes a normally closed switch 87 that is disposed between one end of coil 42 and terminal 6. This switch is in the current return path to the neutral phase and typically remains closed to allow normal operation of circuit 1. Once a fault condition occurs, coil 34 of relay 33 will be energized and relay 33 with change state. Following from this, coil 42 of relay 41 will be energized and relay 41 will change state. As a result, coil 34 will be energized and relay 33 will return to the first state, while coil 42 will remain energized and relay 41 will remain latched in the second state. When an operator manually opens switch 87 coil 42 will de-energize and relay 41 will be unlatched and return to the first state. That is, both relays are now in the first state and ready for normal operation. However, as switch 87 is open there is no return path for current for the load, and the load remains without electrical power. [0104] Once the operator allows switch 87 to return to the normally closed position there are two possibilities. If the fault condition has been removed then the load will be supplied normally via circuit 1. However, if the fault condition persists, coil 34 will be, immediately following closure of switch 87, energized and, in turn, relay 41 will move to the second state so that the load is once again isolated from terminals 5 and 6. [0105] As described above, circuit 1 is included within a sealed housing. In addition, all of the electronic components of circuit 1 are mounted on a single double- sided circuit board having dimensions of about 60 mm x 40 mm. The circuit board is of standard non-flammable UL/EC approved fiberglass construction, and includes tracks that accommodate the relevant current flows. It will be appreciated that switches 84 and 87 include respective manually operable buttons that extend from the housing. It is appreciated that in other embodiments circuit boards other than 60 mm x 40 mm will be used. [0106] Reference is now made to Figure 3 where there is illustrated a further embodiment of the invention in which a control circuit 51 is applied to a portable inverter 52, and where corresponding features from other figures are denoted with corresponding reference numerals.

[0107] Circuit 51 is illustrated with two input terminals 3 and 4, and two output terminals 7 and 8. Similarly to the Figure 1 embodiment, terminal 3 and terminal 7 are, in normal use, electrically connected with the active phase provided by inverter 52. Also, terminals 4 and 8 are, in normal use, electrically connected to the neutral terminal of inverter 52. In other embodiments, the inverter includes more than one phase terminal, and circuit 51 includes a corresponding number of outputs. [0108] Circuit 51 and inverter 52 are collectively mounted internally of housing 19, which in turn is located on a vehicle 53. The vehicle includes a 12 Volt DC automotive supply 54, which supplies power to the electrical systems (not shown) of vehicle 53, and to inverter 52 to allow the generation of a 240 Volt AC supply at terminals 3 and 4. In other embodiments inverter 52, while being mounted to vehicle 53, is supplied power from a source other than supply 54. For example, in another such embodiment inverter 52 is supplied power by an internal combustion engine/generator that is also mounted to the vehicle, and which is able to operate independently of the supply 54. [0109] Vehicle 53 includes a generally conductive chassis 55, and the supply 54 includes a so-called negative earth, where the earth connection is referenced to chassis 55 by way of an earth strap 56. It will be appreciated that earth strap 56 is connected to chassis 55 at only one point. While chassis 55 is generally conductive, it is possible to have quite some resistance between the different points on the chassis. However, neither this factor, nor the absence of an earth-stake, hinders the operation of this embodiment of the invention. These same factors, however, pose significant problems to most prior art protection systems. In an embodiment chassis 55 is formed of metal such as steel. In other embodiments chassis 55 is formed of a conductive material other than steel. [0110] Terminals 7 and 8 extend from housing 19 to define, together with conductor 25, a three-pin electrical mains outlet 90 that is schematically represented. Cable 9 defines at one end a complementary electrical plug that is selectively engaged with outlet 90 to allow the supply voltage to be provided to an electrical plug 91 of a remotely located tool. In this embodiment a tool in the form of an electric drill 57, having an electric motor 58, is attached to the other end of cable 9. In other embodiments different tools or other electrical appliances are supplied power via circuit 51.

[0111] The term "electrical load", as used within this specification, includes a single electrical load or a combination of separate loads that collectively define a single load at a given point in time. For example, inverter 52 is able to supply power via circuit 51 to a single electrical appliance or electrical tool, or to a plurality of such loads, whether that occurs either sequentially or simultaneously. In the event a plurality of loads are supplied simultaneously via a single circuit 51 a fault condition at any one of those loads will disconnect power to all the loads. However, in the event that that same plurality of loads draw power via respective circuits 51, then each load will be protected independently for faults of the kind being detected. [0112] As with the Figure 1 embodiment, conductor 25 is floating, and is connected to the relevant conductive components that, in use: • Should be either floating or at zero voltage during normal operation.

• Are at risk of having some or a portion of the supply voltage applied to them during a fault condition.

• Are likely to be contacted by operators of drill 51 or other personnel. [0113] In other embodiments different or additional components are also in electrical contact with conductor 25.

[0114] Circuit 51 includes switching device 15, which operates in the same manner as described with reference to the Figure 1 embodiment. In the event of a fault condition - that is, if a voltage on conductor 25 is more than about 3 Volts DC above the neutral terminal 6 - circuit 51 progresses to the second mode to prevent the supply of voltage to the drill.

[0115] In this embodiment vehicle 53, when stationary, is able to be earthed by an earth-stake (not shown). However, in practice this is not usual, and even if attempts are made to do so, the effectiveness of the earth is often far less than ideal. Regardless of the existence of an earth connection, or the soundness of that connection, circuit 51 continues to operate in the event of a fault condition, thus reducing the risk of injury to personnel that would not have occurred through reliance upon conventional protection circuitry. [0116] Input 14 of device 15 is connected to chassis 55 at a convenient location based upon the mounting location of housing 19. As best illustrated in Figure 6, in this embodiment vehicle 53 is a light truck having a metal tray 101 mounted to chassis 55 for carrying a physical load. Tray 101 is mounted to chassis 55 by a plurality of spaced apart metal mounting blocks 102 in combination with retaining devices (not shown) such as U-bolts and other fastening means. In other embodiments alternative mounting arrangements are used.

[0117] Tray 101 extends longitudinally, and at one end 103 includes a metal headboard 104 that extends upwardly from the tray to provide a protective barrier between any physical load carried on tray 101 and a cabin (not shown) of vehicle 53. [0118] Housing 19 is fixedly mounted to tray 101 adjacent to headboard 104 and is intended to remain on the tray for an extended period.

[0119] A first 12 Volt DC insulated cable 105 extends from housing 19 and through headboard 104 to connect inverter 52 to supply 54. A further insulated 12 Volt DC cable 106, which defines part of conductor 25, extends from within housing 19 and through tray 101 to terminate in an end that is secured to a metal plate 107. The plate is bolted directly to chassis 55 to electrically connect conductor with chassis 55 at a connection point 108. [0120] Mounted externally to housing 19 are three mains sockets 108 based upon the Australian configuration for receiving respective complementary mains plugs. These sockets are all supplied power by inverter 52 via circuit 51. In other embodiments alternative numbers of sockets 108 are provided. Each socket includes a respective master switch 110 to allow an operator to switch the socket between an active and a non-active configuration. That is, if inverter 52 is operable, each socket 109 will only be able to supply power to the associated load if the respective switch 110 is in the active configuration.

[0121] It will be appreciated that the sockets 109 each include three apertures for receiving complementary pins of an electrical plug. The convention used in Australia is for the lowermost aperture to receive the earth pin of the plug. In this embodiment, however, the pin inserted into the lowermost aperture is electrically connected to conductor 25, not to the earth of the MEN system. More particularly, the pin will be electrically connected to chassis 55 via cable 106. [0122] In those instances where an earth-stake 111 is used to establish an effective link to earth with chassis 55 of vehicle 53, circuit 51 still functions to protect any load and personnel from electrocution or electric shock. For there is typically enough resistance to earth to ensure, in the event of a fault, that detector 11 provides a fault signal. As detector 11 only requires about 3 Volts DC to trigger this allows effective operation even for quite low resistance to earth.

[0123] Even if the resistance to earth is minimal, circuit 1 continues to operate in the embodiment provided in Figure 6. This arises from the more typical arrangement with vehicles for conductor 25 to be connected to chassis 55 at a location somewhat spaced apart from the actual earth point for the supply 54. In the Figure 6 embodiment stake 111 is connected to chassis 55 by an insulated cable 112. This cable is, at one end, electrically connected to stake 111, and at the other end electrically connected to metal plate 113 that is bolted to chassis 55 at location 114. That is, location 114 is spaced apart from location 108. This spacing factor results in some resistance (referred to as the chassis resistance) between conductor 25 and location 114. The chassis resistance is usually sufficient in the event of a fault condition to generate a voltage at input 14 to trigger detector 11. In this embodiment, location 114 is intentionally spaced apart from location 108 to ensure that even if the resistance to earth is low, that the chassis resistance is sufficient to allow effective operation of circuit 51. [0124] In the event that earth-stake 111 is not used, and chassis 55 remains floating, the chassis resistance is not relevant and circuit 51 continues to operate to protect drill 20 and any other load.

[0125] A further embodiment is illustrated in Figure 4 where corresponding features are denoted by corresponding reference numerals. Inverter 52 is connected similarly to inverter 51 to supply a load (not shown in this embodiment) via a control circuit 121. Circuit 122 is similar to circuit 51 of Figure 3 but includes, in addition, a protection device 122 for more comprehensively isolating terminals 7 and 8 from terminals 5 and 6 when there is no power applied to terminals 5 and 6 by inverter 52. Device 122 interfaces, on the one hand, with inverter 52 via terminals 5 and 6 and, on the other hand, with device 15 via conductors 123 and 124. Device 122 moves between a first state where terminals 5 and 6 are electrically connected with conductors 123 and 124 respectively, and a second state where terminals 5 and 6 are electrically isolated from conductors 123 and 124 respectively. [0126] Reference is now made to Figure 7 where there is illustrated an embodiment of a protection device 122. This device includes a mains-rated armature relay 125 with two contacts 127 and 128 that are normally open (as shown) and a relay coil 129 that is connected between terminals 5 and 6. When inverter 52 provides an electrical potential to those terminals, coil 129 will be energized and contacts 127 and 128 will both close. This causes terminal 5 to electrically connect to conductor 123, and terminal 6 to conductor 124. Conversely, when inverter 52 does not provide an electrical potential between terminals 5 and 6, coil 129 will not be energized and contacts 127 and 128 will remain (or move to) the normally open state. [0127] The embodiments described above are applicable primarily to installations where there is no earth reference used, or one is not readily available. This makes those embodiments particularly suited to use with inverters and generators (and other generating devices) that are not part of an MEN system. Examples include inverters or generators mounted to: • Field service vehicles such as automotive repair service vehicles.

• Recreational vehicles and SUVs.

• Vehicles carrying TV relay units.

• Emergency vehicles such as ambulances, fire brigade and law enforcement vehicles. • Vehicles used by power distribution repair crews.

• Military vehicles.

[0128] The embodiments are also suitable for hybrid vehicles, although it will be appreciated that these vehicles typically use higher voltages of up to about 48 Volts DC as the source voltage. [0129] The embodiments are also well suited to marine applications in general, and in particular to: leisure craft; rescue marine; police other services; and tug boats. This allows the safe use of more common mains electrical equipment in the marine environment rather than having to rely upon custom voltage appliances for the specific vessel (which are typically DC voltage appliances). This is extremely advantageous, for example, for a cruise ship where passengers which to bring and use their own 240 Volt equipment, but in an environment where it is safe to do so. [0130] In those embodiments where the marine craft is a ship with a steel hull, that hull provides a conductive superstructure and, hence, a common earth. However, that hull will also have a resistance - analogous to the chassis resistance referred to above. Accordingly, use is made of control circuits of the preferred embodiments that are responsive to the voltage between the hull, at a given point, and the neutral conductor of an electrical outlet near that point. If the hull voltage is greater than a certain threshold, the outlet will be disabled by the control circuit. This is able to occur in addition to the use of a control circuit for each appliance.

[0131] For those marine craft having a non-conductive hull, the control circuit of the embodiments is used to protect the electrical appliance (or other electrical load) per se.

[0132] The embodiments are also well suited for use in aircraft, where they allow the safe operation of, typically, personal electronic equipment.

[0133] In broad terms, the embodiments are advantageously used with an inverter or generator that provides an AC (or other voltage) that is not considered to be a safe touch-potential. That is, where the AC voltage is typically greater than about 50 Volts. Increasingly electrical appliances and electrical tools make use of the higher voltages for improved performance - greater torque, greater efficiency - or due to the manufacturing being less expensive. Accordingly, the embodiments described above facilitate the proliferation of such higher voltage equipment by offering a safer environment in which to operate.

[0134] A neutral conductor is typically provided in an electrical distribution system (for example a MEN system) to act as a return current path for the one or more active phases in that system. For an MEN system the neutral conductor is commonly bonded to earth at multiple points. For an inverter and generator the "neutral terminal" is either bonded to a conductive superstructure, or left floating. While conventional protection circuitry can be left wanting in such circumstances, the embodiments described above are operable to protect the load in either case. [0135] In general terms, there are two types of inverters: pure sine wave inverters; and modified sine wave (or stepped) inverters. Pure sine wine inverters are more desirable for generating supply voltages for electronic equipment due to the more consistent and clean voltage supply they provide. Modified sine wave inverters are generally less expensive and feed through more interference patterns on the output voltage. The embodiments described above function with both these types of inverters.

[0136] In those embodiments making use of an electrochemical battery (or other energy store such as a fuel cell) to provide power to an inverter, it is usual for the voltage provided by these devices (and the inverter) to fall away, particularly toward the end of the available runtime. The above-described embodiments of the control circuit will continue to operate effectively while the load voltage is sufficient to energize the coil of the switching relay 41.

[0137] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0138] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination as would be understood by a skilled addressee given the benefit of the teaching herein. [0139] Although the invention has been described with reference to specific examples it will be appreciated by those skilled in the art that it may be embodied in many other forms.

Claims

1. A control circuit for an inverter that provides an AC voltage between at least one phase terminal and a neutral terminal, the control circuit including: at least two input terminals, one for electrically connecting with the phase terminal and another for electrically connecting with the neutral terminal; at least two output terminals, one for electrically connecting with a load terminal of an electrical load and another for electrically connecting the load with the neutral terminal; a detector for providing a fault signal in response to a fault condition; and a switching device that is responsive to the fault signal for progressing between a first mode and a second mode wherein: in the first mode the input and output terminals are electrically connected for allowing the load terminal to receive the AC voltage from the phase terminal via the switching device; and in the second mode the input and output terminals are electrically disconnected for preventing the phase terminal from supplying the AC voltage to the load terminal via the switching device.

2. A control circuit according to claim 1 wherein the inverter is powered by a DC source of a predetermined voltage.

3. A control circuit according to claim 1 or claim 2 wherein the detector includes a detector relay.

4. A control circuit according to claim 3 wherein the detector relay is a low voltage armature relay.

5. A control circuit according to claim 3 or claim 4 wherein the detector relay is a small signal (or telecom-type) relay.

6. A protection device for an inverter, the device including: at least one control circuit of claim 1; and a housing for encapsulating the control circuit.

7. A protection device according to claim 6 wherein the detector includes a detector relay for providing the fault signal, and the switching device includes a switching relay that is responsive to the fault signal for moving between a first state and a second state that correspond with the first mode and the second mode.

8. A protection device according to claim 7 wherein the detector relay includes a detector relay coil and the switching relay includes a switching relay coil both of which, in the absence of a fault, are not energized.

9. A control circuit for a generator device that provides an AC voltage between at least one phase terminal and a neutral terminal, the control circuit including: an input terminal for electrically connecting with the phase terminal; an output terminal for electrically connecting with a load terminal of an electrical load wherein, in use, the electrical load is also electrically connected with the neutral terminal; a detector for providing a fault signal in response to a fault condition; and a switching device that is responsive to the fault signal for progressing between a first mode and a second mode wherein: in the first mode the input and output terminals are respectively electrically connected for allowing the load terminal to receive the AC voltage from the phase terminal via the switching device; and in the second mode the input and output terminals are electrically disconnected for preventing the phase terminals from supplying the AC voltage to the load terminal via the switching device.

10. A control circuit according to claim 9 wherein the generator device includes one of: an inverter; and a generator.

11. A control circuit according to claim 9 or claim 10 wherein the detector provides the fault signal while the fault condition persists, and the switching device remains in the second mode while the fault signal persists.

12. A control circuit according to any one of the preceding claims 9 to 11 wherein the switching device, in progressing from the first mode to the second mode, progresses the detector from an enabled state to a disabled state.

13. A control circuit according to any one of the preceding claims 9 to 12 wherein the generator device is mounted to a support frame that is electrically isolated from the terminals, and the detector is electrically connected to the frame.

14. A control circuit according to claim 13 wherein the frame defines, at least in part, a housing for the generator device.

15. A control circuit including: an input terminal for electrically connecting with a power source that, in use, provides a source voltage; an output terminal for electrically connecting with a load; a detector having: an input that, in use, is connected to the power source; and an output, wherein the detector is responsive to a reference signal being within a predetermined range for electrically connecting the input and the output to apply the source voltage to the output; and a switching device that is electrically connected to the output of the detector and which is responsive to the source voltage being applied to the output for progressing between a first mode and a second mode wherein: in the first mode the input terminal and the output terminal are electrically connected for allowing the load to receive power from the source via the switching device; and in the second mode the input terminal and the output terminal are electrically disconnected for preventing the source from supplying power to the load via the switching device.

16. A control circuit according to claim 15 wherein the detector includes a sensor relay having a sensor coil and a first sensor contact and a second sensor contact, wherein the sensor coil, when energized by the reference signal, connects the first sensor contact to the second sensor contact.

17. A control circuit according to claim 16 wherein the first and second sensor contacts respectively define the input and the output.

18. A control circuit according to any one of the preceding claims 15 to 17 wherein the switching device includes a switching relay having: a switching coil that is electrically connected to the output; and a first switching contact and a second switching contact, wherein the switching coil, when energized by the source voltage at the output, progresses the switching device between the first mode and the second mode.

19. A control circuit for an electrical appliance having a load element for receiving electrical power from a power source having a floating earth, the appliance having at least one conductive element which is not, in use, electrically connected with the floating earth, the circuit including: at least two input terminals for electrically connecting with the power source; at least two output terminals for electrically connecting with the load element; a detector that is responsive to the voltage of the element for providing a reference signal; and a switching device being interposed between the input and output terminals and being responsive to the reference signal being within a predetermined range for progressing from a first mode to a second mode wherein: in the first mode the input and output terminals are respectively electrically connected for allowing the load element to receive power from the source via the switching device; and in the second mode the input and output terminals are electrically disconnected for preventing the source from supplying power to the load via the switching device.

20. A control circuit according to claim 19 wherein the appliance is mounted to a conveyance having a body that is electrically connected with the floating earth.

21. A control circuit according to claim 19 or claim 20 wherein the power source is separate from the mains supply and is preferably a generator device.

22. A control circuit according to claim 19 or claim 20 wherein the power source is mounted to the conveyance.