Background Serious electrical accidents can happen to anyone, not necessarily just those who are inexperienced, but often to well trained professionals. Every day, many people must work with, or near, potentially dangerous electrical circuitry. Often, circumstances make maintaining adequate safety measures difficult. Potentially dangerous situations relating to electricity are commonly encountered by construction workers, service crews, firemen and countless others who may not realise the danger that is present.

Many people use, often on a daily basis, electrical appliances of many types in many different places. Such electrical appliances include refrigerators, freezers, and washing machines, to name but a few. These electrical appliances are located in many places, including homes, hotels, restaurants and hospitals. These appliances may very well, at one time or another, have an insulation fault due to age, or due to being installed in damp areas, or due to a slight mechanical accident, causing a low undetected current leakage. Such low leakage currents cannot be felt, because the low leakage currents are well below the threshold of human sensation. However, each low leakage current, even in micro amps, will slowly evolve and cause heavy damage and probably a fatal electric shock.

Taken from the symposium on electrical risk in medicine held in Paris 1981, the effects of electrical currents passing through an organism are shown in Table 1 below:- 0.5 mA very little sensation; 10 mA muscular contraction; 30 mA respiratory paralysis; 75 mA irreversible cardiac fibrillation; 1 A cardiac arrest.

Table 1

A leakage of a few milliamps can produce a spark and start a fire, but is not enough to blow a fuse or trigger a typical safety device.

People tend to rely on earthing systems and on earth leakage relays, also known as ground fault interrupters, for protection from electrical faults, yet when a rated leakage current occurs, the relay trips off and power is interrupted at the time when power is most needed.

Earthing was first designed for protection against lightning. However, for maximum safety and protection, an ungrounded current supply is the most highly recommended form of protection by all standards, because when an insulation fault occurs in the system, most of the capacitive currents passing through conductors are very small. Thus, a single pole protection breaker may not trip, so isolated power is recommended for safety and a monitored current continues to be provided.

The early detection of an insulation fault and consequent repair of the fault are much less costly than partial or complete replacement.

An earth leakage relay sensing device, which is electro-mechanical, normally senses current leakage in the range of 30 mA and up, and then breaks the circuit. A current of such magnitude, or even less, may very well cause a muscular contraction and possible respiratory paralysis to a human exposed to such a current. This is a disadvantage of current earth leakage relay sensing devices. Thus, a need exists to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements by providing an improved apparatus for detecting a low leakage current in an electrical appliance.

Summary According to a first aspect of the present disclosure, there is provided an apparatus for detecting leakage current from an electrical appliance. The apparatus includes a chassis, an isolating transformer, an electrical fitting and a detection circuit. The isolating transformer has first and second poles coupled to first and second power terminals. The first and second power terminals have a potential difference isolated from earth by the isolating transformer. The electrical fitting includes first and second terminals connected to respective first and second power terminals. The electrical fitting further includes a third terminal connected to the chassis for coupling a ground terminal of the electrical appliance to the chassis.

The detection circuit is coupled to the chassis and, in parallel with the electrical fitting, to the first and second power terminals. The detection circuit includes a device for warning of a leakage current from the electrical appliance. The leakage current passes through the electrical fitting to the chassis and energises the detection circuit to activate the warning device.

The apparatus maintains power supply to the first and second power terminals, until an isolation resistance on both of the first and second poles of the isolating transformer falls below a predetermined threshold and while an overload condition does not occur.

Other aspects of the invention are also disclosed.

Brief Description of the Drawings One or more embodiments of the present invention will now be described with reference to the drawings, in which: Fig. 1 a is a perspective view of a device for providing a protective power supply interface in accordance with an embodiment of the invention; Fig. lb is a rear plan view of the embodiment shown in Fig. 1 a ; Fig. 2a illustrates a terminal block of the embodiment shown in Figs 1 a and lb ; Fig. 2b is a schematic diagram of circuitry of the embodiment shown in Figs la and lb ; Fig. 3 is a schematic diagram of an embodiment of a protective power supply interface; and Fig. 4 is a schematic diagram of sensing, trigger and power circuits of the schematic diagram of Fig. 3.

Detailed Description Where reference is made in any one or more of the accompanying drawings to steps and/or features, which have the same reference numerals, those steps and/or features have for the purposes of this description the same function (s) or operation (s), unless the contrary intention appears.

Electric appliances, at one time or another, can be exposed to serious electric malfunction caused by a short circuit resulting from a current leakage evolving over a long time, mainly from an excess of moisture, a loose wire or weak insulation, particularly in the absence of scheduled maintenance. Early detection and repair of

current leakage is much less costly than stoppage and then major repair or complete replacement.

A protective power supply interface device is disclosed that can sense a leakage current as low as a few micro-amps. The device warns of such a leakage current while maintaining the load, as indicated on an ammeter, and will continue to provide safe power supply when a single pole earth fault occurs, allowing the consumer reasonable time for checking and repair. In a preferred embodiment, the warning is provided through the use of audible and visual warning devices. Preferably, a warning light is utilised in conjunction with a buzzer and an indicating panel mount meter on the device that indicates the magnitude of the leakage current.

The device preferably includes a built-in isolation transformer that is shielded and of a desired capacity for small or medium application. The most popular voltages are 240/120 vac, 50/60 c/s on primary and secondary coils for single phase. The size of the device is determined by the size of the isolation transformer used. Isolation transformer capacities up to 10 kVA are recommended. If the isolation resistance, on both poles, falls below a minimum value and/or an over-load occurs, the circuit breaker will trip. Hence, observation is recommended.

In operation, the interface device connects between a power supply and an appliance to ensure continuous safe operation (no stoppage), even under hazardous conditions. Non-stop power supply is most needed at such times. If a warning device, preferably in the form of a red light and/or an audio buzzer, is activated, a current leakage exists in the appliance. A user can monitor meters on the device and schedule an appropriate time for maintenance of the electronic appliance. The device provides continuous safe operation, protects against electric shocks and prevents fire-starting sparks.

The device preferably includes a metal case consisting of two part mild steel cases for portable use. A top cover provides protection for front and rear panels. A front panel comprises a main circuit breaker, a green power on indicating light, a leakage monitoring meter, a single pole single throw (SPST) audio selector button, a momentary test push button, a red leakage indicating light, a load ammeter, and spaces for labels. A rear panel comprises output sockets, a protection fuse and an electrical cord. This portable unit is designed for use in homes, schools, hospitals, restaurants, hotels, chemical plants and wherever electric appliances are present.

Fig. la shows a perspective view of an embodiment 100 of the protective power supply interface device. The protective power supply interface device 100 includes a metal case 110 consisting of two part mild steel cases suitable for portable or bench top uses. A top cover 115 provides protection for respective front and rear panels 112,114.

Ventilation louvres 125 in side panels 135,145 aid cooling. The device also has a built- in, chassis-mounted, miniature piezoelectric buzzer with a wide operating voltage of 1-16 vdc. Preferably, both parts of the metal case 110 are fixed together by four screws, two on each side, or as necessary. A rubber foot 120 is positioned on each corner of the base of the device to enable soft placement of the device 100.

The front panel 112 includes a double pole circuit breaker 130 in a thermo- magnetic, panel mount. The circuit breaker 130 is rated to match the load of the transformer. The front panel 112 also includes a green power on indicating light 140 of panel mount design and rated as 240/120 vac. A panel mounted indicating meter 150 has a range of 0-500 I1A. The front panel 112 includes first and second panel mounted switches 160,170. The first switch 160 is a Single Pole Single Throw (SPST) push button switch and the second switch 170 is a momentary push button switch.

The front panel 112 further includes a panel mounted red indicating light 180, rated at 240/120 vac. A panel mounted ammeter 155 indicates the magnitude of an applied load. First and second label spaces 165 and 175 are for labels to indicate a brand name or trade name and technical specifications.

Fig. lb shows a rear view of the device 100. The rear panel 114 includes a third label space 184 for instructions. First and second panel mounted 3-pin socket outlets 188, 190 are rated to correspond with an applied load. In use, one of the first and second socket outlets 188,190 connects to an electrical appliance that is to be protected. The socket used depends on the electrical rating of the electrical appliance.

A strain-relief bush 196 holds an electric cord with a plug 194 next to a panel mount load matching fuse 192. The fuse 192 is connected in series with the cord 194 and is connected to the primary of the Isolation Transformer.

In use, the protective power supply interface device is connected between a local power source and an electric appliance that is to be protected, and the connecting circuit breaker is switched on. An earth line must be connected between the device and the other equipment.

In a preferred embodiment, the device provides self-monitoring functionality. In the event that weak insulation exists in the isolation transformer and a leakage of a small

current occurs within the device, warning signals are activated. The self-monitoring is performed by connecting the device to the power source and turning the circuit breaker on to determine whether any warning is activated. This self-monitoring functionality avoids any earth fault complication between the device and the electrical equipment that is being protected.

In a preferred embodiment, a solid-state, printed circuit board (PCB) within the device senses, detects and monitors isolation earth faults. Fig. 2a shows an embodiment of a PCB enclosed in a plastic mould 210 covered with an acid-proof, non-conductive resin for protection against moisture, dust, ambient temperature and unnecessary tampering. The PCB is connected to a ten-way terminal block 220. The plastic mould 210 preferably includes a space 230 for a label to illustrate proper connections between the PCB and components on the front panel 112.

Other load-complying components, such as electrical cords, plugs, fuses, outlets, main circuit breakers and corresponding connecting conductors, comply with international specifications.

Fig. 2b illustrates general wiring connections 200 for an embodiment of the device.

A power supply 240 is connected to the circuit breaker 130. Preferably, the power supply 240 represents isolated power from a secondary coil of an isolation transformer. The circuit breaker 130 is connected to top and bottom power rails 245,255. The top power rail 245 connects the circuit breaker 130 in series with the ammeter 155. The ammeter 155 is in series with the electrical appliance that is to be protected, represented by the load 250. The earth connection of the load 250 is connected to a chassis 260 of the device.

The load 250 is also connected to the bottom power rail 255 to complete the circuit to the circuit breaker 130. The size of insulated copper conductors utilised in the circuit are dependent upon the total load.

The ten-way terminal block 220 connected to the PCB is connected in parallel with the load 250. The terminal block 220 contains ten terminals numbered 1 to 10. Terminal 1 is connected to the bottom power rail 255 and to the push button switch 170. The push button switch is connected to the chassis 260. The push button 170 provides self-test functionality. Terminal 2 is connected to the top power rail 245, between the ammeter 155 and the load 250.

Terminals 3 and 4 are connected to the green power indicating light 140. Terminals 5 and 6 are connected to a buzzer 270 via the SPST push button 160 in Fig. 1. Terminals

7 and 8 are connected to the red light indicator 180 to indicate earth leakage if such a leakage occurs.

Terminals 9 and 10 are connected to the micro-meter 150 to indicate the existence and magnitude of an earth leakage current. Terminal 10 is also connected to the chassis 260 in a manner similar to terminal 1, but without the push button. Preferably, the size of connecting electric insulated wires from the terminal block 220 to components is 1.5 mm Sq.

Fig. 3 shows the circuit diagram of Fig. 2b with the addition of an isolation transformer 320 rated at 240/120 VAC 50/60 c/s. Not all of the connections to the terminal block 220 are shown, in an attempt to simplify the drawing and not obscure the invention. However, it should be understood that the connections to the terminal block 220 are the same as shown in Fig. 2b. A mains power supply is connected to first and second terminal 305,310 of a primary coil of the isolation transformer 320. First and second terminals 325,330 of a secondary coil of the transformer 320 connect to the circuit breaker 130 to provide to the device power that has a floating current with no line output connected to earth. In the embodiment shown, the primary and secondary coils are connected in series for 240 volts. For 120 volts, the primary and secondary coils must be connected in parallel.

Circuitry of the PCB includes a sensing circuit 340, a trigger circuit 350 and a power circuit 360, each of which is represented schematically. The sensing circuit 340 energises the trigger circuit 350 to allow the power circuit 360 to activate the warning devices when a leakage occurs. An earth wire from the electrical appliance must be connected to the device to enable monitoring to occur. The PCB limits the leakage current up to 200 I1A, after which the consumer is advised to schedule time for maintaining the appliance.

In a preferred embodiment, the PCB detects a leakage current right from zero up, however, at 10 uA the PCB triggers all warning devices of the apparatus and continues to monitor the evolving leakage current while maintaining continuous power supply. When isolation falls below a minimum value, the automatic circuit breaker 130 tends to trip.

Fig. 4 is a schematic circuit diagram of the sensing, trigger and power circuits of the circuit diagram of Fig. 3. The encircled numbers shown in the drawing correspond to the terminal numbers of the terminal block 220. Terminals 1 and 4 connect to a first resistor Rl that is connected in series with a first diode D1 oriented in the reverse direction.

Diode D1 then connects to a gate of a silicon controlled rectifier (SCR).

Terminals 1 and 4 also connect, in parallel with the resistor Rl and diode D1, to a second resistor R2 that is in series with a second diode D2 oriented in the forward direction. The diode D2 connects in series to a reverse oriented third diode D3, a third resistor R3, a fourth resistor R4 and a reverse oriented fourth diode D4. The fourth diode D4 then connects to the gate of the SCR. Terminals 2 and 3 connect to the same point in the circuit between the third and fourth resistors R3, R4. The circuit defined by resistor RI and diode D1 being in parallel with resistor R2, diode D2, diode D3, resistor R3, resistor R4 and diode D4 acts as a rectifier for the alternating input power presented across terminals 1 and 2.

Terminals 2 and 3 also connect to a sixth resistor R6 that is in series with diode D5 oriented in the forward direction that connects to terminal 9. Terminals 5 and 8 connect to a point in the circuit between the second and third diodes D2, D3. Terminal 5 also connects to a first side of a first capacitor C1. The second side of capacitor Cl connects to terminal 6 and to a reverse oriented sixth diode D6 that is in series with an eighth resistor R8. The eight resistor R8 connects directly to the cathode of the SCR. Terminal 7 connects to a seventh resistor R7 that also connects to the cathode of the SCR.

Terminal 7 also connects, in parallel with resistor R7, to a first side of a second capacitor C2. A second side of the capacitor connects to terminals 5 and 8.

Terminal 10 connects to a ninth resistor R9 that is in series with a forward oriented seventh diode D7 that connects to the anode of the SCR. The anode and the cathode of the SCR are connected via a fifth resistor R5.

Terminals 1 and 2 provide input power to the PCB. Terminals 3 and 4 are connected by a green indicating light that indicates when power is on. Terminals 5,6 are connected by a piezoelectric buzzer that acts as a leakage current indicator. Similarly, terminals 7 and 8 are connected by a red light that also acts as a leakage current indicator.

Terminals 9 and 10 are connected by a meter that measures currents in the micro-Amp range to indicate the presence of a leakage current.

Operation: Plug the device to a power supply and turn the circuit breaker 130 on; the green indicating light comes on to indicate that power is connected. If the red indicating light also comes on, then an internal isolation fault in the device has occurred. The occurrence of an internal isolation fault is seldom.

Turn the single pole single throw switch 160 on and press test push button 170. The red indicating light 180 comes on and the buzzer 270 sounds, indicating that the PCB is functioning correctly. The device is ready to be used to protect an electrical appliance.

Turn the circuit breaker 130 off, plug the equipment to be protected into one of the sockets 188, 190. Turn the circuit breaker 130 on and the system is in operation.

Industrial Applicability It is apparent from the above that the arrangements described are applicable to the electrical industries.

The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive.