IE80684B1 - Residual current circuit - Google Patents

Abstract

A residual current circuit includes a current transformer 11 for detecting an earth leakage current associated with a load 14 connected across the live L and neutral N conductors of an AC mains, and an IC circuit 10 for providing a fault signal when such current exceeds a certain threshold. A capacitor C4 is connected to the live conductor and a transistor switch in the IC 10 is responsive to the fault signal such that in the absence of a fault signal the switch shorts out the capacitor C4 to prevent it from charging up and when there is a fault signal the switch permits the capacitor C4 to charge up. The circuit further including a device such as a buzzer 19 which is activated when the voltage across the capacitor C4 reaches a certain level.

Description

RESIDUAL CURRENT CIRCUIT This invention relates to a residual current circuit for detecting an earth leakage current in a load connected to the mains. The circuit may be used to operate a device which disconnects the load from the mains if an earth leakage current is detected, or it may simply be connected to an audible and/or visual alarm.

According to the invention there is provided a residual current circuit comprising means for detecting an earth leakage current associated with a load connected across the live and neutral conductors of an AC mains, the detecting means including a current transformer having the live and neutral conductors as primary windings and a secondary winding into which is induced a current which is the vector sum of the currents induced by the live and neutral conductors, the induced current being zero in the absence of an earth leakage current, means for comparing the induced current with a reference level which is independent of the mains voltage to provide a continuous fault signal during periods when the induced current exceeds the reference level, a capacitor connected to the live conductor, and a switching means responsive to the fault signal such that in the absence of a fault signal the switching means diverts current away from the capacitor to prevent it from charging up and when there is a fault signal the switching means permits the capacitor to charge up continuously at a rate independent of the magnitude of the earth leakage current, the circuit further including a device which is activated when the voltage across the capacitor reaches a certain level.

The device may be a visible and/or audible alarm, or it may be a solenoid which disconnects the mains by opening contacts inserted in the live and neutral conductors.

Embodiments of 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 a first embodiment of residual current circuit connected to operate a audible and visual alarm, Figure 2 shows components of the internal circuitry of the integrated circuit chip in figure 1, Figure 3 is a circuit diagram of a second embodiment of residual current circuit connected to operate a device which disconnects the load from the mains if an earth leakage current is detected, Figures 4 and 5 respectively show two further embodiments of the invention; and Figure 6 is a modification of the embodiment of Figure 1.

In the figures the same reference numerals have 20 been used to denote the same or equivalent components.

Referring to figures 1 and 2, the residual current circuit shown in figure 1 is based around a customised integrated circuit (IC) 10 whose main internal components, for the purpose of describing this embodiment, are shown in figure 2. However, it will be recognised that the embodiment could just as well be constructed from discrete components. 1 The circuit comprises a current transformer having a toroidal ferromagnetic core 11. Conductors 12 and 13 respectively connected to mains live L and mains neutral N of an A.C. mains pass through the core 11 to a load which is schematically illustrated by the resistor 14.

The conductors 12 and 13 constitute primary windings for the current transformer. A secondary winding 15 is wound on the core 11 and is connected across an amplifier and rectifier circuit 16 within the IC 10 (figure 2) . , In the absence of an earth leakage current the e current flowing in the live conductor L is equal and opposite to the current flowing in the neutral conductor N. In this case the vector sum of the currents induced into the secondary winding 15 by the two conductors L and N will be zero. However, when there is an earth leakage current the currents flowing in the conductors L and N will not be equal, so the vector sum of the currents induced in the secondary winding 15 will be non-zero. The residual current circuit is designed to detect this non-zero current.

Supply voltage is applied to the IC 10 via the live conductor L, a diode DI, resistor R2 (consisting of R2A and R2B), the IC 10 and the neutral conductor L. The IC 10 has an internal voltage regulator (not shown) which clamps Vcc to about 5 volts. C3 provides smoothing of the rectified supply voltage.

The state of the circuit when there is no earth leakage current is as follows. A comparator 17 inside the IC 10 has its positive input connected to the output of the amplifier and rectifier circuit 16 and its negative input connected to an internal voltage reference, Vrefl, which is derived from Vcc. The output of the comparator 17 is high in the absence of an earth leakage current, and this high output switches on and holds on an internal transistor TRI. The Osc pin of the * IC 10 is held low by the on transistor TRI and the current flowing through an external resistor R3 is diverted to the 0 volt (ground) rail. A capacitor C4 is shorted out whilst TRI is turned on, and therefore C4 is fully discharged. Ά second internal comparator 18 has its positive input connected to the Osc pin of the IC 10 and its negative input connected to a second internal voltage reference, Vref2. The output of the comparator 18 is low in the absence of an earth leakage current because its positive input is tied to ground by the on transistor TRI. Thus a second internal transistor TR2 is turned off. The Buzzer pin of the IC 10 is held low whilst TRI is turned on by connection to ground through an external buzzer 19.

When an earth leakage current flows in the load circuit, there will be a resultant output signal from the winding 15 which is fed to the IC 10. This output signal is processed by the amplifier and rectifier circuit 16 to produce a DC potential at the positive input of the compatator 17 inversely proportional to the RMS value of the output signal. A capacitor C2 provides smoothing of the DC voltage.

When there is no earth leakage current, the DC potential at the positive input of the comparator 17 is higher than Vrefl so that the output of the comparator 17 is high, keeping the transistor TRI turned on as mentioned above. However, when there is an earth leakage current the DC potential will fall in proportion to the magnitude of the output signal from the winding 15. Therefore, when the DC potential at the output of the circuit 16 falls below Vrefl, corresponding to a :certain level of earth leakage current, the comparator output goes low, so that the transistor TRI is turned off.

The current through R3 now flows through C4, enabling C4 to charge up via R3. When the voltage across C4 reaches about 15 volts, the voltage at the positive input of the comparator 18 exceeds Vref2 so that the output of the comparator 18 goes high and the transistor TR2 is turned on. C4 now starts to discharge through the buzzer 19 since a path to ground is now completed via the Buzzer pin and the on transistor TR2.

When the voltage across C4 falls to about 5 volts, TR2 turns off again and C4 starts to charge up again.

The charging and discharging cycle of C4 is repeated all the while the DC potential at the output of the circuit 16 remains below Vrefl, that is, while the earth leakage current exceeds a certain threshold level. When the DC potential at the positive input of the comparator 17 once again exceeds the reference Vrefl, meaning that the output signal from the winding 15 has fallen below the threshold level, the output of the comparator 17 will go high once again and TRI will turn on. This will discharge the capacitor C4 and hold it in the discharged state, thereby restoring the circuit to its quiescent state.

Each time C4 discharges through the buzzer 19, the buzzer emits a loud audible sound, thereby providing an audible indication of the fault condition. The repetition rate of the audio alarm is determined by the time constant of C4/R3. A visual fault indication is also provided by a light emitting diode (LED) which is connected in parallel with the buzzer 19 via the resistor R4.

It is advantageous to fit a residual current circuit with a test facility to enable the user to verify its correct operation. Most such circuits provide a test circuit comprising a wire passing through the toroidal core connected to a test switch which is in series with a resistor, the resistor and test switch ? being connected across the mains supply. This requires a high wattage resistor which is capable of dissipating t the power generated by the test current for the duration that the test switch is held closed. This in turn makes the test resistor bulky and expensive. When the test switch is open, it will have the full mains voltage across its contacts. To meet national and international clearance requirements, the contact gap will need to be of the order of 3nun. In addition, the test switch will require contacts which can withstand the mains voltage and test current. All of these requirements will result in a test switch that will tend to be bulky and costly.

The test circuit configuration provided in figure 1 overcomes most of these problems. As mentioned, the resistor R2 comprises two resistors, R2A and R2B. R2A is the main dropper resistor of the mains voltage to the IC 10, and has a resistance of several Rohm. R2B is of much lower value, typically of several ohms. A further winding 20 is provided on the core 11, connected across the resistor R2B, and a normally-open test switch 21 is connected in series with the winding 20.

When the test switch 21 is open, the supply current Icc for the IC 10 flows through both R2A and R2B. When the test switch is closed, Icc is diverted through the winding 20 to the IC 10. This induces a current into the secondary winding 15 which, provided it exceeds the 'magnitude necessary for the DC output of circuit 16 to J fall below Vrefl, causes the alarm buzzer 19 to be activated as previously explained. The number of turns o in the winding 20 is selected to ensure that the ampere turns is sufficient to activate the alarm.

This test circuit arrangement offers the advantage of being able to use a low cost test switch which requires a very low voltage and current rating. In addition, because the normal voltage drop across R2B will be very low, typically a few volts, the contact gap on the test switch can be very small. R2A services the dual functions of providing the IC supply current and the test current, obviating the need for a separate test resistor.

In addition to the buzzer 19 and the LED it is possible to provide a device which provides a mechanical indication of a fault, and in particular a mechanical indication which remains set after the fault condition ceases in order to provide a continuing indication that a fault has occurred at some point in the past.

This could be achieved by placing the coil of a solenoid or relay or other electro-mechanical device in parallel with the buzzer 19. This device would be designed to be set by the first discharge of current from the capacitor C4, and thereafter be unresponsive to further circuit activity so as to remain set during and between subsequent discharges of C4 and also after the fault has ceased and the transistor TRI is once again turned on.

Thus, the device would provide a form of memory or record that a fault condition had previously occurred.

The device could be manually reset to the non-fault 'indicating state.

Alternatively, in addition to the buzzer and LED one could provide a device responsive to the voltage on the capacitor C4 to register a fault condition continuously while the voltage on C4 was greater than a few volts. In the case of a fault condition, even though the capacitor C4 is charging and discharging, its voltage remains at or above the approximately 5 volt level at which the transistor TR2 turns off, so that such a device would register a fault condition cont inuously.

Figure 3 shows an embodiment of the invention in which the mains is disconnected when the transistor TRI is turned off by an earth leakage current exceeding the threshold level, rather than just sounding a buzzer and/or lighting a lamp. In the circuit of figure 3 a solenoid coil 22 replaces the buzzer 19 and the LED.

When the capacitor C4 discharges in response to the transistor TR2 turning on, the solenoid 22 will be energised to cause it to disconnect the mains supply by opening a pair of contacts 23 interposed in the mains live and neutral conductors L and N.

The solenoid may be of the permanent magnet type, whereby the contacts 23 are normally held closed by a permanent magnet operating against a spring, the permanent magnet force being overcome by the electromagnetic force generated in the solenoid coil by the discharging current from C4. Alternatively, the solenoid may be of the type whereby the tripping force is provided by the solenoid operating a plunger or lever in response to the electromagnetic force generated by the discharging current from C4. Both types of device iare well known in the art and need no further description here. Of course, in this embodiment there will be no repetitive charging/discharging of the capacitor C4, because the mains is disconnected the first time the capacitor C4 discharges.

In the embodiment of figure 4 the solenoid coil 22 is connected in series with a switching device 24 such as an SCR which is controlled by a zener diode 25 connected between the capacitor C4 and the gate of the SCR 24. In this case when the voltage on the capacitor C4 reaches the breakover voltage of the zener diode 25 the SCR 24 is switched on so that current can flow through the solenoid coil 22 to open the contacts 23. Instead of an SCR the switching device 24 could be a transistor, and instead of the zener diode 25 a resistor could be used to connect the capacitor C4 to the switching device 24.

In figure 5 the solenoid coil 22 is connected in parallel with the switching device 24 rather than in series with it. In this case the solenoid coil 22 is connected directly across the mains and is therefore normally energised, and de-energisation of the solenoid is required to cause the contacts 23 to open. This is achieved by the SCR 24 or other switching device shorting out the solenoid coil 22 when it is turned on by the zener diode or resistor 25.

Figure 6 shows a modification of the embodiment of Figure 1 wherein the capacitor C4 is connected to both neutral N and earth E, each via a respective diode Dn and De, so that the capacitor C4 can be connected to earth in the case of a loss of neutral fault. The earth path also includes an impedence Ze to encourage current to flow towards neutral under normal operation of the device.

Any of the preceding embodiments can be build in miniaturised form into an electrical plug, socket, adaptor, or other suitable housing for use on an electrical product, appliance or installation Advantages of the residual current circuit described above are: - It provides a very simple low cost circuit with minimal component count which may be used as an alarm circuit alone or alternatively to disconnect the mains.

- In the case of its use as an alarm circuit the alarm is provided by an oscillator whose repetition rate is independent of the fault signal magnitude or frequency.

- It uses a common capacitor to determine the alarm oscillator repetition rate and provide the alarm activating current.

- Its supply current, and therefore power consumption, does not increase when the circuit switches to the alarm state.

- The quiescent supply current is used as a test current thereby overcoming power dissipation problems in the test circuit, and obviating the need for a large contact separation in the test switch which is required when the test circuit is operated at mains potential, thereby facilitating the use of a micro switch for the test circuit.

- In particular, it has a test circuit which operates by diverting the IC supply current through the toroidal core for the purpose of creating an imbalance current for test purposes, and uses a common resistor for the dual functions of mains dropper resistor for the IC and test resistor.

Claims (10)

CLAIMS:

1. A residual current circuit comprising means for detecting an earth leakage current associated with a load connected across the live and neutral conductors of an AC mains, the detecting means including a current transformer having the live and neutral conductors as primary windings and a secondary winding into which is induced a current which is the vector sum of the currents induced by the live and neutral conductors, the induced current being zero in the absence of an earth leakage current, means for comparing the induced current with a reference level which is independent of the mains voltage to provide a continuous fault signal during periods when the induced current exceeds the reference level, a capacitor connected to the live conductor, and a switching means responsive to the fault signal such that in the absence of a fault signal the switching means diverts current away from the capacitor to prevent it from charging up and when there is a fault signal the switching means permits the capacitor to charge up continuously at a rate independent of the magnitude of the earth leakage current, the circuit further including a device which is activated when the voltage across the capacitor reaches a certain level.

2. A residual current circuit as claimed in claim 1, wherein the switching means is connected in parallel with the capacitor and is controlled by the fault signal such that in the absence of a fault signal the switching means is held on and shorts out the capacitor and in the presence of a fault signal the switching means is turned off thereby permitting the capacitor to charge.

3. A residual current circuit as claimed in claim 2, further including a second switching means which is turned on when the potential on the capacitor reaches a certain level.

4. A residual current circuit as claimed in claim 3, wherein when it is turned on the second switching means completes a discharge path for the capacitor through the said device to activate the device.

5. A residual current circuit as claimed in claim 3, wherein when it is - 12tumed on the second switching means completes a current path through the said device to activate the device. t

6. A residual current circuit as claimed in claim 3, wherein when it is 5 turned on the second switching means diverts current from the said device to activate the device.

7. A residual current circuit as claimed in any preceding claim, wherein the device is a visible and/or audible alarm.

8. A residual current circuit as claimed in any preceding claim, wherein the device is a solenoid coil which disconnects the mains by opening contacts inserted in the live and neutral conductors. 15

9. A residual current circuit as claimed in any proceeding claim, wherein a supply voltage for the circuit is derived from the AC mains across a diode and first and second resistors arranged as a voltage divider with the resistance value of the first resistor being very much greater than that of the second resistor, and wherein a test switch and further primary winding are 20 connected in series across the second resistor.

10. A residual current circuit substantially as described herein with reference to figures 1 and 2, or to figure 3, 4 or 5, of the accompanying drawings.