GB2167618A

GB2167618A - Electric protective circuit

Info

Publication number: GB2167618A
Application number: GB08528881A
Authority: GB
Inventors: Terence Edward Corbyn; Ivarsz Kocins
Original assignee: INDEP TV NEWS Ltd
Current assignee: INDEP TV NEWS Ltd
Priority date: 1984-11-22
Filing date: 1985-11-22
Publication date: 1986-05-29
Also published as: GB8429523D0; GB2167618B; GB8528881D0

Abstract

In order to provide accurate monitoring of earth impedance and neutral/earth voltage of a power supply of unknown integrity so that portable electrical equipment, eg. television equipment, may be connected to the power supply, means (30,32) determine the value of the earth impedance of the power supply, means (68,70) determine the value of the voltage between neutral and earth, voltage comparator means (34,36) and monostable means (38,40) are operative to actuate LED's (42,44,46) depending on the value of the earth impedance to show whether the earth is good, may only be used with caution, or is unsafe, and a switch (48) is operative in response to an unsafe earth condition or an excessive voltage between neutral and earth to actuate the switch contacts (4) of an ELCB. The means for determining earth impedance (30,32) include a resistor (32) of known value and a thyristor (30) which is switched on for short periods of time at a low voltage point in each supply cycle in order to sample current to earth without actuating the ELCB. <IMAGE>

Description

SPECIFICATION Electrical protective circuits This invention relates to an electrical protective circuit for protecting personnel from the consequences of faulty connections of equipment to a mains supply. Although the circuit has application to many different types of equipment, for example machine tools, robots, communication equipment, it has particular application to television equipment where a faulty mains connection may result in the exterior of the equipment having a dangerously high voltage level.

It is frequently necessary in the particular case of televised outside broadcasts to connect the camera equipment to an untested power supply which may for example be the power supply of a nearby domestic home. The earthing of the equipment may therefore be inadequate since it will depend on whether there is a proper earth in the power supply. An earth leakage current breaker (ELCB) is normally employed as a safety precaution, but this normally trips only when it detects earth leakage current by an imbalance in the currents flowing in the live and neutral cables. Thus if an earth on a piece of equipment becomes disconnected, or if the mains earth has a high impedance, then equipment could become live under fault conditions without actuating the ELCB.If a mains earth is not present, then the metal casing of equipment can also assume a high potential due to filter capacitors being connected between live and earth. Another fault which may occur with domestic power supplies is that the live and neutral lines may be reversed, or more generally there is too great a voltage between the neutral conductor and earth, it being understood that a voltage value of 40 RMS or 40 V DC regarded as a dangerously high voltage.

It is an object of the invention to provide a circuit which is relatively simple and which can be inexpensively mass produced to carry out automatically an earth testing function. It is also necessary that this testing function should be carried out continuously (preferably during each mains cycle) since earth impedances could vary in the course of time due perhaps to a loose earth connection.

Another object of this invention is that the earth testing circuit should provide executive action in that should the earth impedance be unduly high the power supply should automatically be interrupted so that there is no danger to personnel.

A further object of the invention is that the earth testing circuit should not disturb the earth potential or provide interference spikes on the supply line which may adversely affect other equipment.

These objects are achieved by providing in accordance with the invention in an electrical protective circuit for protecting personnel from the effects of a fault in the mains supply to electrical equipment, the circuit comprising a semiconductor circuit for sampling periodically for a predetermined period in each mains cycle at a relatively low voltage value a current flow between the live line of the mains power supply and the local mains earth without significantly disturbing the earth potential of the power supply and providing an output signal indicative of the current flow, means for determining the value of the output signal in order to determine the earth impedance and circuit breaker means controlled by the comparing means for disconnecting the power supply from the equipment in the event of an unacceptably high earth impedance.

By sampling the current flow at a relatively low voltage value, large amounts of current are not extracted from the mains supply and hence there are no interference spikes introduced into the line such as may interfere with other equipment such as an earth leakage circuit breaker (ELCB) which operates by detecting an imbalance between live and neutral currents. Preferably, the voltage values between which current sampling takes place is CV and 0V at the falling part of the sinusoidal povi,Ei- supply in each cycle.

The advantage of employing a semiconductoi circuit for sampling the current flow is that semiconductor components may be chosen which can be activated precisely at the required part of tine mains cycles and switch the minimal time delay.

Preferably a thyristor or other power semiconductor switching device is employed in series with a current sensing device connected between live and earth. The thyristor is controlled by integrated circuits comprising voltage comparators, monostable circuits and AND gates, all c: which react accurately and quickly.

In this connection, reference is made to British Patent Specification No. 888327. This discloses method of periodically sampling current in order to determine earth impedance. However the currents sampled amount to somewhere between 50A and 500A which causes significant disturbances to the earth potential. In addition the circuit components employed include gas filled valves which cannot be constrolled with sufficient accuracy to enable operation at the low voltage portion of the mains waveform.

The current sensing device preferably comprises a resistor of low value across which a voltage is developed which may be determined by a voltage comparator arrangement. Alternatively the sensing device may comprise a current transformer, light emitting diode, or reed relay array.

The provision of circuit breaker means controlled by the current sensing arrangement ensures that the equipment is disconnected from the mains supply when an unacceptable earth impedance is present. Thus there is provided an executive mode of operation.

As preferred a cautionary mode of operation is provided where an indication is provided to an operator that earth impedance is not satisfactory although not actually unsafe.

In a further aspect, the invention provides an electrical protective circuit for protecting personnel from the effects of a fault in the mains supply to electrical equipment, the protective circuit comprising a circuit breaker having switch contacts for connection to the live line of the power supply; means for determining the value of the earth impedance of the power supply; indicating means coupled to the earth impedance determining means, for indicating in a cautionary mode of operation whether the earth impedance is above a certain level at which the equipment may be used but only with caution; and switch means operative to actuate the circuit breaker and responsive to the means for determining earth impedance, the means for determining earth impedance causing said switch means to operate in an executive mode of operation when the earth impedance rises above a certain value which is indicative of an unsafe condition.

There is thus provided in accordance with the invention a cautionary mode of operation where an indication is provided to an operator that earth impedance is not satisfactory although the equipment is not in immediate danger and secondly an executive mode of operation wherein earth impedance is found to be unsafe and an ELCB or other circuit breaker is actuated to dis-connect the equipment.

The provision of cautionary and executive modes of operation is a particularly useful feature where television equipment is concerned in view of the requirement to connect the equipment to mains supplies of unknown integrity. Thus an estimate can be formed of the value of an earth impedance of the mains supply. If for example the earth impedance is shown to be higher than a perfectly safe value, then if the equipment is to be connected only for a short period of time, the mains supply may be satisfactory. If however the equipment is to be connected to the mains supply for a considerable length of time, then since earth impedances tend to vary with respect to time, a different mains supply may be required in case the impedance becomes dangerously high.If the earth impedance became dangerously high during transmission, the executive mode of operation would be invoked to switch off the equipment which would clearly be embarrassing.

Any suitable indication of earth impedance may be provided for example a visual indication by means of a light emitting diode of a certain colour.

As preferred a plurality of thresholds are provided so that the quality of the earth connection can be indicated in terms of the threshold impedance which is sensed. For example if there is a very low earth impedance (say below 30 ohms) then a light emitting diode of a certain colour may be actuated to indicate a good earth connection. If the earth impedance is at a higher value (say between 30 and 100 ohms) then although this earth impedance may give some grounds for concern it is nevertheless permissible to use television equipment wich such earth impedance and a light emitting diode in a different colour may be actuated. If however the impedance is found to exceed a further threshold, say about 100 ohms, then a light emitting diode of a different colour may be actuated to indicate an unsafe earth impedance.

Preferably means are provided for detecting the voltage between earth and neutral capable of providing relatively accurate measurement in view of the fact that it may be necessary to connect equipment to less than perfect power supplies. It is therefore necessary to monitor the parameters of the power supply relatively accurately so that while imperfect power supplies may be used, there will nevertheless be no use of a power supply which is unsafe for the equipment and operating personnel, it being understood that a neutrallearth voltage of 40V RMS or 40V DC is regarded as unsafe.Thus means for measuring the neutral/earth voltage may comprise a diode bridge connected between neutral and earth to rectify the voltage thereacross and zener diode means connected to the bridge and which is operative to provide a threshold for providing a clear indication when the voltage between neutral and earth exceeds a predetermined amount which is determined to be an unsafe amount.

When this occurs, switch means is actuated to provide a visual indication that the neutral/earth voltage is at an unsafe level. This may comprise a light emitting diode of a certain colour to indicate that the neutral/earth voltage is at a certain level. As preferred when such light emitting diode is operated switch means is also actuated to cause the ELCB to trip. Thus as preferred cautionary and executive modes of operation are invoked at one and the same time when measuring neutral/earth voltage.However if desired, the diode bridge arrangement could be arranged to actuate a first threshold device for indicating that the neutral/earth voltage is above a certain level and thus provides grounds for concern although it is nevertheless safe for the television equipment to be used therewith and a second threshold device when the neutral/earth voltage reaches a danger level which danger level causes the ELCB to trip.

As preferred test means are provided for manually testing the operation of the neutral/earth voltage monitoring means by applying a predetermined voltage derived from the live phase to the input of the diode bridge arrangement.

In addition means are preferably provided for testing the earth impedance monitoring arrangement by inserting a resistance of predetermined value into the earth line and thus permitting the earth impedance monitor to monitor the value of this resistance inserted in the line.

A preferred embodiment of the invention will now be described with reference to the accompanying drawing which is a circuit diagram of an earth validity tester.

The earth validity tester shown in the drawing is arranged in a physical embodiment as an in-line adaptor unit to be inserted between a power sup ply system of domestic house for example and television equipment for outside broadcasting. The apparatus comprises an ELCB 2 connected in the live and neutral lines and comprising two sets of relay operated contacts 4, a test button 6 and a further set of relay contacts 8 connected between the neutral and live lines. A neon lamp 10 is connected between the live and neutral lines. It will be under stood that the ELCB is controlled in the normal way by a toroidal cone (not shown) having windings connected in the live and neutral lines, so that upon substantial earth leakage current occurring, there is an imbalance between the live and neutral currents causing the ELCB to operate.

A diode 12 is connected via a fuse 14 to the live line, to provide unidirectional flow to a circuit comprising a resistor 16 connected in series with a zener diode 18 to an earth point E which is connected to the earth line via a switch 80. Connected between resistor 16 and diode 18 are inputs to two comparator amplifiers 20 and 22. The other inputs of these amplifiers are connected to reference voltages V, ref and V2 ref. The output amplifier 22 is connected to a monostable 24 which is in turn connected to one input of AND gate 26. The output of amplifier 20 is directly connected to the other input of AND gate 26. The output of AND gate 26 is connected to a thyristor gating circuit 28 which is in turn connected to the gate of a thyristor 30.The anode cathode path of thyristor 30 is connected between diode 12 and sensing resistor 32 of known value (47, 52) the other end of sensing resistor 32 being connected to earth point E. The junction between thyristor 30 and resistor 32 is connected to two inputs of voltage comparators 34, 36, the other inputs of these comparators being connected to voltage reference sources V3 ref and V4 ref. The outputs of voltage comparators 34, 36 are connected to monostable vibrators 38, 40. The outputs of monostable vibrator 38 are respectively connected to a green light emitting diode 42 and a yellow light emitting diode 44.One output of multivibrator 40 is connected to a red light emitting diode 46 while the other output is connected to the base of a switching transistor 48, a relay coil 50 being connected in the collector-emitter path for operating the relay contacts of the ELCB.

In addition there is provided a neutral-earth voltage sensor circuit comprising zener diode 60 connected in series with a resistor 62 and diode 64.

This circuit is connected between fuse 14 (which in turn is connected to the live terminal) and the neutral terminal. A test switch 66 has one contact connected to the neutral line and has another contact connected between diodes 60 and resistor 62.

Switch 66 is connected to one terminal of a diode bridge 68, the opposite terminal of which is connected to earth point E and the two adjacent terminals being connected to a zener diode arrangement 70 constituting a voltage threshold device and a light emitting diode isolator 72. The photosensitive transistor 74 of the isolator is connected to the base of a transistor 76, the collector path of which has a red light emitting diode 78. Earth point E is connected via a resistor 82 of known value in parallel with a normally closed test switch 80 to the earth line.

In operation, the circuit senses the voltage developed between live and earth point E via diode 12 and the voltage comparator 20 provides an output when the mains waveform is 10 volts or less and provides a trigger pulse via gate 26 to the gating circuit 28 to fire the thyristor.

Voltage comparator 22 and monostable 24 form a protective circuit which operates to prevent operation of gating circuit 28 during the rising part of the mains cycle; otherwise a very large current would be injected through resistor 32 to earth, which would raise earth potential. This is effected by ensuring than an "0' signal is provided from monostable 24 during the initial part of the rising part of the mains cycle whereas when in the falling part of the mains cycle the monostable provides an "I' signal and permits the trigger signal from comparator 20 when the mains voltage falls below 10V to be passed through AND gate 26 to trigger circuit 28. This has the effect of switching on thyristor 30 for a duration of 37 MS to provide a current of about 85 milliamps.The thyristor is commutated when the mains voltage becomes negative and the insignificant current taken from the live line means that the ELCB is not triggered and that even in the absence of an earth, the earth potential does not become unsafe.

Triggering of thyristor 30 results in a current pulse through resistor 32 of known value and the voltage developed across resistor 32 is applied via a voltage divider network 33 to the input of voltage comparators 34, 36 as a voltage indicative of earth impedance, since the earth impedance will be in series with sensing resistor 32 and therefore the current and voltage across resistor 32 will be proportional to the value of the earth impedance. For low mains impedances, a large current pulse is provided, the comparators 34, 36 produce an output signal which continuously triggers monostable vibrators 38, 40. With monostable multivibrators 38, 40 being continuously re-triggered, the 0 output of multivibrators 38, which is inverted, permits light emitting diode 42 to be illuminated.This light emitting diode is coloured green and indicates that the mains impedance of the biphase system is less than 30 ohms. In addition the Q output of multivibrator 40 is positive thereby switching on resistor 48 and energizing coil 50 to hold on relay contacts 8. Thus the ELCB will not be triggered in this condition. In the event of component failure within the earth testing circuit, transistor 48 will be switched off which will cause relay contacts 8 to open and for the ELCB to be triggered for safety. As the impedance of the earth increases, the voltage developed across resistor 32 decreases and hence the comparator V3 ref being comparatively high will not produce a positive output at any point in the supply cycle. This will occur when the mains earth impedance is 30 ohms and in this case multivibrator 38 is not retriggered and hence an output is provided at the Q output of multivibrator 38. This output which is inverted enables light emitting diode 44 to illuminate which provides yellow light indicating that the mains impedance is above 30 ohms and is not entirely satisfactory.

As the earth impedance rises even further and reaches a value of about 100 ohms the output from comparator 36 remains low throughout the mains cycle since the voltage across resistor 32 is not greater than V4 ref (V4 ref being relatively low).

Hence multivibrator 40 is not retriggered and an output is provided at logical output Q. This output is inverted so as to enable red light emitting diode 46 to be illuminated, which indicates that the mains impedance is greater than 100 ohms and is therefore at a dangerous level. Simultaneously the Q output multivibrator 40 goes low and since this is not inverted, transistor 48 is switched off thereby triggering the ELCB.

In addition, circuitry is provided for testing the voltage existing between neutral and earth which may arise because of live neutral line reversal or because of an unsafe voltage between neutral and earth arising for example from out of balance phases applied from the local power station. A diode bridge 68 rectifies the neutral/earth voltage and when the voltage rises above a threshold value 70V peak or 50V RMS, diode 72 of the opto isolator is illuminated whereby to conduct current through transistor 74 thereby to switch on transistor 76 which causes red light emitting diode 78 to be illuminated. in addition this causes triggering of multivibrator 40 to trigger the ELCB.

The earth neutral voltage test switch 66 applies a volt potential to the input of the bridge to check whether there is an adequate earth neutral voltage existing.

An earth impedance test switch 80 connects a resistor 82 in series with the earth input to the tester so that the earth impedance testing circuitry may be tested.

It may thus be seen there is disclosed an electrical protective circuit for protecting personnel from possible harmful effects of faulty main supplies possessing electrical equipment. Although the invention has primary application to outside television broadcasting equipment, it has application to in-house television broadcasting equipment and broadcasting equipment in general. It also has application to machine tools, robotic systems and in general in any electrical equipment where a faulty mains supply may endanger an operator.

The present invention has applicability to all types of AC power supplies.

Claims

1. An electrical protective circuit for protecting personnel from the effects of a fault in the mains supply to electrical equipment, the circuit comprising a semi-conductor circuit for sampling periodically for a predetermined period in each mains cycle at a relatively low voltage value a current flow between the live line of the mains supply and the local mains earth without significantly disturbing the earth potential in the power supply and providing an output signal indicative of the current flow, means for determining the value of the output signal in order to determine the value of the earth impedance and circuit breaker means controlled by the comparing means for disconnecting the power supply from the equipment in the event of an unacceptably high earth impedance.

2. A circuit as claimed in claim 1 wherein the semi-conductor circuit includes a thyristor connected in series with a current sensing device for connecting between the live line and earth of the power supply, and means for switching on the thyristor for a predetermined interval during each cycle of the mains supply voltage.

3. A circuit as claimed in claim 2 wherein the current sensing device comprises a resistance of known value.

4. A circuit as claimed in claim 2 or 3 wherein the means for switching on the thyristor comprises comparator means for comparing the mains voltage with a reference voltage for producing a signal for turning on the thyristor.

5. A circuit as claimed in claim 3 wherein the means for determining the value of the voltage developed across the resistance comprises a voltage comparator for comparing the resistance voltage with a reference voltage, the comparator means providing an output for operating switching means for operating said circuit breaker means.

6. A circuit as claimed in any preceding claim, including means for determining the voltage value between the neutral and earth lines of the power supply, indicating means coupled to the earth impedance determining means, for indicating in a cautionary mode of operation whether the earth impedance is above a certain level at which the equipment may be used but only with caution, and switch means operative to actuate the circuit breaker and responsive both to the means for determining earth impedance and to the means for determining the neutral/earth voltage, the means for determining earth impedance or neutral/earth causing said switch means to operate in an executive mode of operation when the earth impedance rises above a certain value which is indicative of an unsafe condition.

7. A circuit as claimed in claim 6 as dependent on claim 5 wherein the means for determining the value of the voltage developed across the resistance comprises a first voltage comparator for comparing the resistance voltage with a first relatively high reference voltage, and a second voltage comparator for comparing the resistance voltage with a relatively high second reference voltage the comparator means providing outputs for illuminating light sources for providing an indication of the resistance voltage and hence the earth impedance.

8. A circuit as claimed in claim 7 wherein the first voltage comparator means is connected to a first monostable multivibrator having a first output connected to a light source for indicating that the resistance voltage is above the first reference voltage and the earth is therefore good quality, a second output connected to a light source for indicating that the resistance voltage is below the first reference voltage and that the earth should only be used with caution, and wherein the second comparator means is connected to a second monostable multivibrator which provides outputs when the second reference voltage is not exceeded indicating an unsafe earth impedance for illuminating a light source and for actuating switch means for operating the circuit breaker.

9. A circuit as claimed in claim 6 wherein the means for determining the voltage value between neutral and earth lines comprises a diode bridge connected between neutral and earth and coupled to a threshold voltage means for providing an output when the voltage between neutral and earth rises above a value determined by the threshold means.

10. A circuit as claimed in claim 9 as dependent on claim 8 wherein the output of the threshold voltage means is connected to said second monostable multivibrator for actuating said switch means for operating the circuit breaker.

11. A circuit as claimed in claim 3 including manual testing means for inserting a second known resistance between the thyristor and the earth line for determining whether said electrical protective circuit is operative.

12. A circuit as claimed in claim 9 including manual testing means for applying a voltage derived from the live line of the power supply to the diode bridge for ascertaining whether said means for determining the voltage value between the neutral and earth lines is operative.

13. An electrical protective circuit for protecting personnel from the effects of a fault in the mains supply to electrical equipment, the circuit including: a circuit breaker having switch contacts for connection to the live line of the power supply; means for determining the value of the earth impedance of the power supply; means for determining the voltage value between the neutral and earth lines of the power supply; indicating means coupled to the earth impedance determining means, for indicating in a cautionary mode of operation whether the earth impedance is above a certain level at which the equipment may be used but only with caution; and switch means operative to actuate the circuit breaker and responsive both to the means for determining earth impedance and to the means for determining the neutral/earth voltage, the means for determining earth impedance causing said switch means to operate in an executive mode of operation when the earth impedance rises above a certain value which is indicative of an unsafe condition, and the means for determining neutral/earth voltage causing said switch means to operate when the neutral/earth voltage rises above a predetermined value indicative of an unsafe condition.

14. A circuit as claimed in claim 13 wherein the circuit breaker is constituted by an earth leakage circuit breaker which is operative to detect earth current leakage by imbalance of currents between neutral and earth lines of the power supply.

15. A circuit as claimed in claim 13 or claim 14 wherein the earth impedance determining means comprises a thyristor having its main current path connected in series with a resistance of known value for connection between live and neutral lines of the power supply, means for switching on the thyristor for a predetermined interval during each cycle of the mains supply voltage, and means for determining the value of the voltage developed across the resistance.

16. A circuit as claimed in claim 15 wherein the means for switching on the thyristor is arranged to switch on the thyristor for a relatively short time at a relatively low value of mains voltage.

17. A circuit as claimed in claim 16 wherein the means for switching on the thyristor comprises comparator means for comparing the mains voltage with a relatively low reference voltage for producing a signal for turning on the thyristor and a higher reference voltage for producing a signal for switching off the thyristor.

18. A circuit as claimed in any of claims 15 to 17 wherein the means for determining the value of the voltage developed across the resistance comprises a first voltage comparator for comparing the resistance voltage with a first relatively high reference voltage and a second voltage comparator for comparing the resistance voltage with a relatively high second reference voltage, the comparator means providing outputs for illuminating light sources for providing an indication of the resistance voltage and hence the earth impedance.

19. A circuit as claimed in claim 18 wherein the first voltage comparator means is connected to a first monostable multivibrator having a first output connected to a light source for indicating that the resistance voltage is above the first reference voltage and the earth is therefore good quality, a second output connected to a light source for indicating that the resistance voltage is below the first reference voltage and that the earth should only be used with caution, and wherein the second comparator means is connected to a second monostable multivibrator which provides outputs when the second reference voltage is not exceeded indi- cating an unsafe earth impedance for illuminating a light source and for actuating switch means for operating the circuit breaker.

20. A circuit as claimed in any preceding claim wherein the means for determining the voltage value between neutral and earth lines comprises a diode bridge connected between neutral and earth and coupled to a threshold voltage means for providing an output when the voltage between neutral and earth rises above a value determined by the threshold means.

21. A circuit as claimed in claim 20 as dependent on claim 19 wherein the output of the threshold voltage means is connected to said second monostable multivibrator for actuating said switch means for operating the circuit breaker.

22. A circuit as claimed in claim 18 including manual testing means for inserting a second known resistance between the thyristor and the earth line for determining whether said electrical protective circuit is operative.

23. A circuit as claimed in claim 20 including manual testing means for applying a voltage derived from the live line of the power supply to the diode bridge for ascertaining whether said means for determining the voltage value between the neutral and earth lines is operative.

24. An electrical protective circuit substantially as described with reference to the accompanying drawing.