GB2238921A - Ground fault circuit interrupter - Google Patents

Abstract

A ground fault circuit interrupter 5 has supply conductors looped around the core of a toroidal coil 14 to sense out-of-balance currents. The trip circuit uses a solenoid 13 for opening the supply. The energising circuit for the solenoid comprises a full wave rectifier 17 and SCR 16 with its gate connected directly to the secondary winding 15 of the toroidal coil 14. A bridge circuit 25-29 provides a bias voltage for the gate of the SCR 16 which also has a snubber 60. The interrupter has a test circuit 19-21 with protective diodes 22, 23 and a varistor 24, and may be incorporated in a plug or socket for a hair dryer. <IMAGE>

Description

Ground Fault Circuit Interrupter This invention relates to ground fault circuit interrupters and in particular to "ALCI's".

Such devices are already known especially to isolate the power supply to a distribution system such as in a domestic dwelling or building. Certain safety regulations or guidelines exist and are proposed to require interrupters at local points and in some cases for each individual appliance that is to be used. At present the interrupters are either too bulky and/or too costly so that they are cumbersome or represent, respectively, a significant cost as compared to the cost of the appliance itself. This is particularly true for hair dryers and the like.

Presently, ground fault interrupters use a circuit based on an integrated circuit (I.C.) such as Raytheon's "RV4145". A typical circuit diagram for such an interrupter is shown in Figure 12. From this diagram it can be seen that 2 coils 81 and 82 are required as well as the IC 83. As can be seen, the circuit contains a large number of components which all add to the expense and size of the interrupter.

It should be noted that this circuit does not show a surge suppressor circuit or a test circuit facility.

It is an object of the invention to provide a ground fault circuit interrupter which takes up a small space and can be produced and supplied at a comparatively low cost.

According to the invention, there is provided a ground fault circuit interrupter for an electrical supply including normally closed contact means for the supply conductors, an electrically operating means arranged to open the contact means and be supplied via switch means with power from the conductors, a winding formed on a toroidal type core electrically connected to the switch means, in which the supply conductors are looped around the core to generate a net magnetic flux whenever there is an im-balance in the supply due to a fault or an earth leakage, such that in use the net flux creates a signal in the winding to operate the switch means and cause the contacts to open.

The switch means may comprise a silicon controlled rectifier (SCR) having its gate connected to the winding. The switch means may also include a snubber connected in parallel to the SCR. Preferably, the snubber comprises a resistor and a capacitor connected in series.

The conductors are preferably looped around the core at least two or more times.

An auxiliary electrical supply may be provided to bias the switch towards an operative. condition.

The solenoid may be arranged to be supplied via a full-wave rectifier circuit.

Optionally, a switched test circuit connected between the conductors, may be included to simulate a fault condition when the switch of the test circuit is operated.

The contact means may include movable spring loaded contacts biassed towards an open position with respect to cooperating fixed contacts, and a latch to hold each movable contact in the closed position, the latch being mechanically connected to and arranged to be released by the operating means.

The earth fault circuit interrupter may be housed in a plug for an appliance or in an outlet socket of an electrical supply distribution system. The interrupter may also be housed in a plug or socket of an extension cable so that the interrupter can be used for a number of different appliances. The interrupter can then-be used beyond the life of the appliances which is not the case if the interrupter is housed in a sealed plug of the appliance.

Ground fault circuit interrupters, according to the invention, will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a circuit diagram of one circuit for an interrupter; Figure 2 is a circuit diagram of another circuit for an interrupter; Figure 3 is a circuit diagram of a test circuit for an interrupter; Figure 4 illustrates a variation of the circuit of Figure 1; Figure 5 illustrates a variation of the circuit of Figure 2; Figure 6 is a plan view of a plug incorporating a ground fault circuit interrupter; Figure 7 is a top view of the plug with its upper cover removed; Figure 8 is a sectioned side view of the plug with the interrupter in its non-operative position; Figure 9 is a sectioned end view corresponding to Figure 8; Figure 10 is a sectioned side view of the plug with the interrupter in its operative condition;; Figure 11 is a sectioned end view corresponding to Figure 10; Figure 12 is a respective view of the other side of a plug incorporating a ground fault circuit interrupter with a cover removed; Figure 13 is a plan view from below of the plug of Figure 12; Figure 14 is a sectional elevational view of the plug of Figure 12; and Figure 15 is a typical prior art circuit diagram.

Referring to the drawings, Figure 1 shows an electrical plug for a supply cable to an electrical domestic appliance, such as a hair dryer, including a normally closed two pole switch 8 having movable contacts 9 and 10 connected to a neutral conductor 11 and a line conductor 12 respectively. A solenoid actuator 13 is arranged to open the switch 8, as explained below, and is supplied in use with power from the conductors 11 and 12. A toroidal-type coil 14 has a winding 15, with a large number of turns (shown only diagramically in Figure 1). The conductors 11 and 12 are looped through the coil 14, usually two to six times, and extend to the hair dryer (not shown). A silicon controlled rectifier (SCR) 16 is connected between the coil of the solenoid actuator 13 and the neutral conductor 11 via a full-wave rectifier circuit consisting of four diodes 17.The winding 15 is connected to the gate of the SCR 16 and to a capacitor 18 which absorbs low intensity high frequency excursions generated as a result of noise in the power supplied along the conductors 11 and 12.

In use, a net flux will be produced in the magnetic core of coil 14 if there is an im-balance in the current in the conductors 11 and 12. Such an im-balance is an indication of a ground fault or earth leakage occurring in the supply cable or at the appliance. The net flux in the core generates an electrical signal by inducing a voltage across the winding 15. This voltage is applied to the gate of the SCR 16 and when it exceeds the turn on voltage of the SCR, i.e. the gate trigger voltage VGT which is about 0.8 volts for the preferred embodiment, electrical power will be supplied via the SCR 16 to energise the solenoid 13 so that the contacts 9 and 10 are opened. As a result, the power supply for the hair dryer is cut off.

In this embodiment, the magnetic core of coil 14 has an outside diameter of about 20 mm so that the coil can be housed in a plug body of generally normal size. As the conductors 11 and 12 are looped around the core more than once, and preferably about six times, the net flux produced when a fault occurs is suitably increased to make the interrupter more sensitive to the presence of low fault and leakage currents.

A test circuit is provided by a switch 19 in series with resistors 20 and 21. Opposed diodes 22 and 23 are also provided to ensure that the test leakage current is limited to the minimum trip leakage current. The current through the trip circuit is limited by resistors 20 and 21. However if the supply voltage is surging at the time of the test the true test current may actually be higher than the required minimum amount, also as the interrupter may be used on different source voltages without modification the test current must be established using an independent reference voltage. This reference voltage is provided by the two diodes 22 and 23. The forward bias of a diode is normally about 0.6 volts and this voltage is used as the source voltage for resistor 21 to limit the leakage current through the coil 14 to 5mA or as required for test purposes.

Figure 3 illustrates more clearly the test circuit.

Resistor 20 is 10ohms and provides a current path i1 before the coil 14. For typical voltages in the range 100 volts to 250 volts il will be l0mA to 25mA. Current il is divided into currents i2 and i3 such that il=i2+i3. The voltage across the diode 22 and 23 is a maximum of 0.6 volts the forward voltage drop across either diode. i2 should be 5mA or as required for the test leakage current as i2 injects out-of-balance current to the load side of the primary winding of the coil 14. Resistor 21 of the preferred embodiment is 140Ohms which gives i2 as 4.3mA. This current is fixed by resistor 21 and when the supply voltage varies currents il and i3 vary.

i2 will only vary if il is below the set i2 value, in this instance 4.3mA. This will only happen if the supply voltage is below 43 volts.

A metal oxide varistor (MOV) 24 protects the electrical components against high voltage surges which may otherwise damage them and which occur due to interference in the power supply or possibly at switch on of the hair dryer.

The circuit of Figure 2 is generally the same as the circuit of Figure 1. In Figure 2, an electrical bridge is provided between the output of the full-wave rectifier circuit. The bridge consists of 3 resistors 25, 26 and 27, a capacitor 28, and a zener diode 29. The bridge is arranged in use to supply a bias voltage of about 0.5 volts to the gate of the silicon controlled rectifier 16. The zener diode 29 is provided to ensure that the output voltage is maintained at a substantially constant voltage in use, regardless of the supply voltage including during surges. This has the effect of biassing the rectifier 16 towards its switch on condition. Thus, the net flux that is needed to be generated in the core 14 is much less than before to provide operation of the interrupter.This is because a smaller voltage, about 0.3 volts, is now required across the winding 15 in order to turn on the silicon controlled rectifier 16. The capacitor 28 is provided to suppress noise in the circuit. As will be appreciated, the effect of noise in such a circuit would increase the risk of false trips due to an electrically noisy supply or environment. Thus accidental actuation due to noise is reduced by the inclusion of a noise suppressor such as capacitor 28.

It will be appreciated that with many two pin plugs used to-day, the polarity of their connection to a supply socket is not necessarily predictable; the plug need not be inserted in the socket in only one relative orientation. The described circuits in Figures 1, 2 and 3 are not polarity dependent and the interrupter will work satisfactorily whichever way around the plug is connected to the outlet. Further, the circuits are not dependent on the actual supply voltage and are designed to work without alteration at both 120 volts and 240 volts, for example.

It has been noted, however, during testing, that if a "snubber" circuit is placed in series with the SCR 16 then the occurance of false trips during surging is reduced. False tripping during surging was noticed on circuits designed for operation on 250V due to the increased operating voltage of the MOV 24 and the sensitivity of the SCR circuit.

A snubber 60 in the form of a resistor 61 and a capacitor 62 connected in series and placed in parallel to the SCR 16 as shown in Figures 4 and 5 alleviated the problem.

In Figure 6, the plug has on its upper surface a manually operable reset button 30 and the test switch 19. A cable grommet 31 is provided in usual to attach a supply cable 32 for the hair dryer. The supply cable conductors (not shown) are attached inside the plug to the conductors 11 and 12.

Referring to Figures 7 to 11, various major components of the ground fault circuit interrupter will now be described. The contacts 9 and 10 are mounted on spring arms 50 which bear down on each side of a carriage 40 to bias it downwards. A latch 42 fits to a plunger 43 of the solenoid 13. The plunger is biassed towards the right, as seen in Figures 7 and 9. A central post 44 passing through the carriage 40 is provided at its lower end with a crank or displaced portion 45.

The central post 44 is biassed upwardly by springs 41 urging against the reset button 30 which is fitted to the top of the central post 44. The springs 41 bear against a bar or plate 51 located in a fixed position above the carriage 40.

In normal use, as seen in Figures 10 and 11, the contacts 9 and 10 are in their raised position and closed against respective stationary contacts 46 and 47. Although the carriage 40 is urged downwards from its upper position by the spring arms 50, the carriage is prevented from moving downwards because the crank 45 presses upwards against the latch 42 and holds the carriage 40 in its upper position as the relative strength of the springs 41 is greater than the strength of the spring arms 50. Power is therefore supplied to the hair dryer.

If a fault occurs, the solenoid 13 is operated and the latch 42 is moved to the left allowing the crank 45 to be released and the carriage 40 to move downwards. As a result, the contacts 9 and 10 move to the position shown in Figure 8 and power to the hair dryer is interrupted.

When the solenoid 13 is de-energised, the latch 42 will return to the right to the position shown in Figure 10. If the reset button is then pressed, the crank 45 springs past and under the latch 42 to the position shown in Figure 9.

It will be noted that the electrical parts of the interrupter comprise the coil 14 and the small number of electrical components shown in Figures 1 or 2 mounted on a circuit board 49. The coil, significantly the largest of the electrical components, is never-the-less small enough to fit inside the plug body. The plug body is not much, if at all, bigger than a normal size standard plug presently in use for electrical appliances. It has been explained that by using two or more turns of the conductors 11 and 12 on the coil 14 how even a relatively small core can provide a signal of sufficient magnitude in the winding 15. Also, in Figure 2, by biassing the gate of the silicon controlled rectifier 16, the coil may be made smaller or if the same size, is more sensitive in operation.

The circuit board and components mounted thereon take up very little space. Thus the ground fault circuit interrupters described can be housed in plug bodies which are not unsightly or bulky.

For example, Figures 12 to 14 illustrate a further embodiment of the ground fault circuit interrupter in which the interrupter is incorporated into the housing of a plug for a power cord. In this arrangement, the ground fault circuit interrrupter can be supplied to manufacture for fitting to an existing appliance or even to the power cord of any existing appliance by replacing the plug fitted to the power cord. This may be termed "retrofit".

In the retrofit ground fault circuit interrupter shown in Figures 12 to 14, the interrupter is fitted inside the housing of an electrical plug, such as a two pin plug as shown. The power cord for the appliance or the extension cord, etc may be connected to the plug as shown. The power cord 32 may be connected to terminal 60 by the usual methods. The terminals shown are solder terminals but other types such as screw terminals may be used.

The cord path is serpentine within the plug to provide a good grip on the cord by the housing. The cord path is defined by locating lugs 61 as shown in Figure 13. A cover 62 encloses the connection of the power cord to the terminals and co-operates with the locating lugs, by acting as a cable clamp, to help maintain the cord in the path correctly.

In the physical arrangement of the components of the ground fault circuit interrupter, the toroidal coil is orientated horizontally to provide room for the connection for the power cord. The input to the ground fault circuit of the ground fault interrupter is connected directly to the power pins 63 of the plug and the output of the circuit is connected to the terminals for connection of the power cord. This can be more easily seen in Figure 14.

Further, the electrical components required and described are generally off-the-shelf items so that the cost of providing ground fault circuit interrupters need no longer be a significant cost as compared to say, small electrical appliances, such as the hair dryers.

Claims (19)

Claims

1. A ground fault circuit interrupter for an electrical supply, including, normally closed contact means for the supply conductors, an electrically operating means arranged to open the contact means and be supplied via switch means with power from the conductors, a winding formed on a toroidal type core electrically connected to the switch means, in which the supply conductors are looped around the core to generate a net magnetic flux whenever there is an im-balance in the supply due to a fault or an earth leakage, such that in use the net flux creates a signal in the winding to operate the switch means and cause the contacts to open.

2. A ground fault circuit interrupter according to claim 1, in which the switch means comprises a silicon controlled rectifier having its gate connected to the winding.

3. A ground fault circuit interrupter according to claim 2 in which the switch means includes a snubber connected in parallel with the silicon controlled rectifier.

4. A ground fault circuit interrupter according to claim 3 wherein the snubber comprises a resistor and a capacitor connected in series.

5. A ground fault circuit interrupter according to any one of the preceding claims, in which the conductors are looped around the core at least two or more times.

6. A ground fault circuit interrupter according to any one of the preceding claims, in which an auxiliary electrical supply is provided to bias the switch towards an operative condition.

7. A ground fault circuit interrupter according to any one of the preceding claims, in which the solenoid is arranged to be supplied via a full-wave rectifier circuit.

8. A ground fault circuit interrupter according to any one of the preceding claims, including a switched test circuit connected between the conductors to simulate a fault condition when the switch of the test circuit is operated.

9. A ground fault interrupter according to any one of the preceding claims, in which the contact means includes movable spring loaded contacts biassed towards an open position with respect to cooperating fixed contacts and a latch to hold each movable contact in the closed position, the latch being mechanically connected to and arranged to be released by the operating means.

10. An electrical plug having housed therein a ground fault circuit interrupter according to any one of claims 1 to 9.

11. An electrical plug according to claim 10, wherein there is provided a cord clamp and terminals for connection of a power cord.

12. An electrical plug as defined in claim 11, wherein the terminals are solder connectors.

13. An electrical plug according to claim 11 or claim 12, wherein the cord clamp includes a removable cover corporating with locating lugs to define a serpentine cord path.

14. An electrcial plug as defined in any one of claims 10 to 13, wherein the plug is a two pin plug.

15. A two pin outlet socket having housed therein a ground fault circuit interrupter according to any one of claims 1 to 9.

16. An extension cable with a two pin plug at one end thereof having housed therein a ground fault circuit interrupter according to any one of claims 1 to 9.

17. An extension cable having a plug at one end and a socket at the other end wherein the socket includes a ground fault circuit interrupter according to any one of claims 1 to 9.

18. A ground fault circuit interrupter substantially as herein described with reference to any one or more of Figures 1 to 1t of the accompanying drawings.

19. A ground fault circuit interrupter incorporating a test circuit substantially as shown in Figure 3.