GB2191591A

GB2191591A - Earth monitoring

Info

Publication number: GB2191591A
Application number: GB08614128A
Authority: GB
Inventors: Clive William Dempster Andrews; Malcolm Hutchinson
Original assignee: Kenure Developments Ltd
Current assignee: AB Electronic Components Ltd
Priority date: 1986-06-10
Filing date: 1986-06-10
Publication date: 1987-12-16
Also published as: EP0278147A1; GB8614128D0

Description

SPECIFICATION Improvements in or relating to earth monitoring The present invention relates to an apparatusfor earth monitoring. Electrical and Electronic equipment is generally provided with an earth connection for safety reasons. For instance, when such equipment is provided with a metal case or chassis, the case or chassis is connected to earth so that no harmful or dangerous voltage can appear on it if a fault develops in the equipment. Forfixed equipmentsuppliedwith mains power, the earth connection may be provided by the mains earth connection. For mobile equipment mounted in a vehicle, a local earth connection isfrequently provided by driving a metal spike into the ground. In recentyears, it has become more common to provide earth leakage protection. A common technique for providing earth leakage protection involves monitoring the voltage orthe currentflowing between an earthing point and an equipment earth terminal. If the level of this voltage or current exceeds a predetermined value, the power supply to the equipment is disconnected so as to avoid injury or damage. However, such protection is only effective if the earth connection presents a sufficiently low impedance to current resulting from an undesirable voltage appearing atthe equipment earth terminal. If the earth connection has too high an impedance, the earth leakage protection can become ineffective and dangerous voltages may appear on the equipment chassis or case. According to the invention, there is provided an apparatusfor monitoring an earth connection, comprising means for injecting a current into a conductor connected between a fi rst terminal fo r connection to an earth and a second terminal for connection to an equipment earth terminal, and measuring meansfor measuring the resulting current orvoltage between the first and second terminals. It is thus possible to provide an apparatus which monitors the condition of an earth connection for equipment whose chassis or case is to be earthed for safety reasons. Even where such equipment is provided with earth leakage protection ofthetypewhich monitors current or voltage in the earth connection, it is necessary to monitorthe earth connection to ensurethatthe earth leakage protection is effective. Preferably the injecting means is arranged to inject an alternating current. The frequency of the alternating current preferably lies in a range whose lower limit is 2 kHz, more preferably 5 kHz, and whose upper limit is 100 kHz, more preferably 50 kHz. Afrequency substantially equal to 20 kHz has been found advantageous as this is generally too high to cause audible interference and low enough notto produce noise or radio frequency interference.This also allows the earth impedance to be monitored at a frequencywhich is sufficiently removed from normal mains frequencies and power supply frequencies so thatthe injected alternating current does not interfer with the operation of earth leakage protection and monitoring is substantially unaffected by mains noise or breakthrough. Preferably the injecting means includes a transformer whose secondary winding is connected in series with the conductor. This provides a convenient means for injecting alternating current into the conductor between the first and second terminals. Preferably the measuring means includes means for detecting current whose frequency is the same as that of the injected current. The current detecting means preferably includes a phase sensitive detective arranged to compare the alternating currentfrom the injecting means with the current orvoltage from the conductor. This ensures that the detecting means is sensitive only to the injected current and allows the apparatus to be used in noisy environments. The apparatus is thus made substantially insensitive to signals which are of a different frequency from the injected current, and provides additional security in that only signals with a fixed phase relationship to the injected current are measured. Preferably the measuring means includes a bandpassfilterconnected between the conductor and the current detecting means and tuned to the frequency of the alternating current.This provides additional security against incorrect measurements in a noisy environment. Preferably the measuring means includes a currenttransformerwhose primary winding is connected in series with the conductor. This provides a convenientway of connecting the measuring means to the conductor without substantially affecting the impedance between the first and second terminals. Preferably the measuring means includes a comparatorfor producing an output signal when the resulting current is less than a predetermined value. Thus, an output signal is produced when the earth impedance is higherthan a predetermined value, for instance such that earth leakage protection ceases to be properly effective. Preferably the measuring means includes indicating means for providing an indication in response to the output signal of the comparator. It is thus possible to provide visual or audible warning when the earth impedance is too high for correctfunctioning of earth leakage protection. Preferably the apparatus further comprises earth leakage detection means for detecting an earth leakage current orvoltage between the first and second terminals. It is convenientto combine monitoring of the earth connection with earth leakage protection so asto allow the reliability of the earth leakage protection to be monitored continuously. Preferably the earth leakage detection means is arranged to detect alternating earth leakage current. Preferably the earth leakage detection means includes a filterfor attenuating signals whose frequency is outside a frequency band including mainsfrequency. Such filtering improves the reliability of the earth leakage protection by attenuating or rejecting signals which are unrelated to earth leakage caused by fault conditions within equipment being monitored. Thefiltering also allows the injected currentto be ignored by the earth leakage protection. Preferably the earth leakage detection means includes comparator means for producing a fault signal when the amplitude of the positive or negative peaks of the alternating earth leakage current exceeds a predetermined amplitude. The use of an ACcoupled comparator avoids the delays associated with conventional DC comparators which employ rectification and smoothing. Thus, the response of the earth leakage protection is not impaired and a fault condition can be detected within half a cycle of an alternating current mains supply.The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates connections to an earth leakage trip and monitoring apparatus constituting a preferred embodiment of the present invention; Figure 2 is a block diagram of the apparatus of Figure 1; Figure 3 is a more detailed block diagram of part of the apparatus of Figure 1; Figure 4 is more detailed block diagram of another part of the apparatus of Figure 1; and Figure 5 is a detailed circuit diagram of the apparatus of Figure 1. The apparatus of Figure 1 is intended for installation in a vehicle for providing protection against earth faults in equipment installed in the vehicle. The apparatus 1 receives a 32 volt direct current supply which is derived from 250 volt 50 Hz mains supply. A single phase mains supply is shown, but the apparatus works equally well with three phase supplies. The negative line of the 32 voltsupply is connected to a chassis earth stud 2 of the vehicle or the equipment mounted therein, which stud is also separately connected to the apparatus 1. The apparatus is further connected to a safety monitor earth stud 3 which is connected to an earth spike driven into the ground. Red and green lamps 4and 5, respectively, are provided for indicating the condition of the earth connection. If the earth impedance is too high for safe or correct operation of the earth leakage protection, the red lamp 4 is illuminated. If the earth impedance is sufficiently lowforcorrect operation, the green lamp 5 is illuminated. When the red lamp 4 is illuminated to indicate a potentially unsafe situation, an operator can judge whether to continue using apparatus connected to the supply or to disconnect or switch off the supply. Alternatively, a trip arrangement (not shown) may be provided to cut off the supply when a potentially unsafe condition is detected. The apparatus 1 has inputs for receiving a test voltage supply, fortesting correct operation of the earth leakage protection, via a dual pole ELtest switch 6 biased to its off position. One of the poles is connected to the neutral line of a mains supply input whereas the other pole is connected via an earth leakage reset switch 7 and a contactor 8 to the live line. The apparatus 1 also includes a relay which is operated in the presence of an earth leakage current exceeding a predetermined value having a contact connected between the switch 7 and the contactor 8. In many applications, the contactor 8 and its connection can be ommitted without otherwise affecting operation of the apparatus. As shown in Figure 2, the apparatus comprises a conductor connected between the chassis earth stud 2 and the safety monitor earth stud 3. Connected in series with this conductor are: the secondary of an injection test voltage transfomer 9 which has a mains primary winding; a current limiting series resistor 10; the primary of a 50 Hz current transformer 11; the secondary of a 20 kHz injection transformer 12; and the primary of a 20 kHz current transformer 13. The apparatus includes an input power supply regulator 14which receivesthe unsmoothed direct currentsupplywhosevoltage is nominally 32 volts but which may vary between 28 and 36 volts. The regulator 14 provides a regulated + and -12volt direct current supplyforthe electronics of the apparatus as indicated at 15. The secondary winding of the transformer 11 is connected to the input of an earth leakage monitor circuit 16 which provides an output for actuating a trip relay 17 when an earth leakage current orvoltage is detected having a value greaterthan a predetermined value. The relay 17 has a pair of normally closed contacts 18which are connected to the contact breaker coil and reset switch within the earth leakage reset switch 7. The primary winding of the transformer 12 and the secondary winding of the transformer 13 are connected to a 20 kHz injection and monitor circuit 19 whose output controls a relay 20. The relay 20 has a pair of normally closed contacts 21 forthe red light 4 and a pair of normally open contacts 22 for the green light 5.The injection and monitor circuit 19 is therefore fail safe in that, in the event of power failure or a power supply fault, the red light 4 is illuminated to indicate an unsafe condition. The earth leakage monitor circuit 16 is shown in more detail in Figure3.The secondary winding of the 50 Hz current transformer 11 is connected to the input of an amplifier and lowpass filter 23. The filter comprises a three pole butterworth lowpass filter with a turnoverfrequency of 200 Hz. This allows earth leakage fault signals, which have a frequency of 50 Hz, to pass without substantial time delay but attenuates or removes higherfrequencysignals. The output of the filter and amplifier 23 is connected to a circuit 24which includes a pair ofAC-coupled comparators for comparing the positive and negative half-cycles ofthe signal with a reference voltage V ref of 1.2 volts. The outputs ofthe comparators are connected together and form the output ofthe circuit 24 which is connected to the trigger input of a monostable multivibrator 25. The monostable multivibrator has a time constant of 100 milliseconds and supplies its output signal to a relay driver circuit 26 whose output 27 controls the trip relay 17. This time constant is sufficiently long to allowthe main contactor8 to open and clearthefault before reset occurs. The injection and monitor circuit 19 is shown in more detail in Figure 4 and comprises a 20 kHzwein bridge oscillator 28 connected to a transformer driver circuit 29. The output of the driver circuit 29 is connected to the input of an oscillator auotmatic gain control circuit 30 which also receives a 1.2 volt reference voltage and controls the amplitude ofthe output signals from the oscillator 28 so that this amplitude is maintained constant. The output of the drive circuit 29 is connected to the primary winding of the transformer 12 which has a primary-tosecondary winding ratio of 1:1. The secondary winding of the currenttransformer 13 is connected to the input of an amplifier and bandpass filter circuit 31. The filter comprises a first-order bandpass filter tuned to 20 kHz and having a Qfactor of 5 so as to improve the noise and interference rejection performance of the monitor circuit. The output of the circuit 31 is connected to the first input of a phase sensitive detector 32 whose second input is connected to the output of the driver circuit 29 so as to receive the signal therefrom as reference signal. The output of the phase sensitive detector 32 is proportional to the amplitude of any current passing between the chassis earth stud 2 and the safety monitor earth stud 3 and having a frequency of 20 kHz and a phase which is in phase or in anti-phase with the output signal of the driver circuit 29.The phase sensitive detector 32 is arranged to have a base-band width of 3 Hz. Thus, the filter in the circuit 31 and the phase sensitive detector 32 provide a very high degree of immunity to signals otherthan the injected alternating current signal of 20 kHz supplied by the oscillator 28, the driver circuit 29, and the transformer 12 to the conductor between the studs 2 and 3. The output signal from the phase sensitive detector 32 is supplied to a comparator and relay driver33 which compares the signal from the phase sensitive detector 32 with a reference signal derived from the 1.2 volt reference. If the current passing through the primary winding of the transformer 13 is such asto indicate an earth impedance greaterthan a predetermined acceptable value, the driver 33 prevents the relay20from operating sothatthe red light 4 is illuminated to indicate an unsafe condition. Alternatively, if the alternating current is such as to indicate an earth impedance less than the predetermined acceptable value, the comparator and relay driver 33 actuates the relay 20 so asto extinguish the red light 4 and illuminate the green light 5 to indicate a safe condition for operation of the earth leakage monitor circuit 16. Figure 5 is a detailed circuit diagram of the apparatus 1. The input regulator 14 includes a voltage dependent resistor or varistor 34 for suppressing voltage spikes and a common mode Rf suppression filter comprising chokes 35 and 36 and capacitors 37 to 40. A diode bridge comprising diodes 41 to 44 prevents damagefrom incorrect power supply polarity connection. Asmoothing capacitor 45 is connected across the output ofthe diode bridge and is connected via a current limiting resistor46to a voltage regulation circuit comprising an integrated circuitvoltage regulatortype LM 117K 47, a tra nsistor 48, a zener diode 49, diodes 50 to 52, resistors 53 to 59, capacitors 60 to 65, and a further zener diode 66. The input regulator 14 is essentially conventional and will not be described further. Voltage dependent resistors orvaristors 67 to 70 are connected to the terminals of the apparatus for suppressing voltage peaks ortransients. The low pass filter 23 comprises an integrated circuit operational amplifier 71 provided with resistors 72 to 75 and capacitors 76 and 77 to form a three pole butterworth lowpass filter with aturnoverfrequency of 200 Hz. The input of the filter is protected against excess input voltages by anti-parallel diodes 78 and 79.An input impedance-defining resistor 80 is connected across the secondary winding of the transformer 11. The output of the amplifier 71 is connected via a capacitor 81 and a resistor 82 to the AC-coupled comparators 24. An operational amplifier 83 provided with resistors 84 and 85 and capacitors 86 to 88 is arranged as a high gain inverting amplifier. The output of the amplifier 83 is coupled via a capacitor 89 to the input of a first comparator comprising a comparator integrated circuittype LM 193 90 and associated resistors 91 to 94 and capacitors 95 and 96. The output of the amplifier 83 is also connected to a unity gain inverting stage comprising an operational amplifier97 and associated resistors 98to 100 and capacitors 101 and 102. The output of the amplifier 97 is connected via a coupling capacitor 103 to the input of another comparator circuit comprising an integrated circuit comparator 104 (second half of the LM 193) and associated resistors 105to 107. The two comparators are connected to receive a reference voltage of 1.2 volts provided by a band gap reference diode 108 (integrated circuit type LM 185). The diode 108 is provided with a dropping resistor 109 and a noise-suppressing capacitor 110. The outputs of the comparators 90 and 104 are connected together and via a diode 111 to thetrigger input of a monostable multivibrator providing a pulse width of 100 milliseconds. The monostable multivibrator 25 comprising an integrated circuit (type "555") 112 and associated resistors 113to 115 and capacitors 116to 119. The output of the monostable multivibrator is connected via a potential divider comprising resistors 120 and 121 to the relay driver 26 comprising a transistor 122 connected as a common-emitter switching transistor and provided with back-EMF suppression by a diode 123 and a zener diode 124. The 20 kHzwein bridge oscillator 28 comprises an operational amplifier 125 provided with a wein network comprising resistors 126 and 127 and capacitors 126 and 127, and capacitors 128 and 129. The operational amplifier is provided with a decoupling capacitor 130. The output of the oscillator is connected to the transformer drive circuit 29 which comprises an operational amplifier driving a push-pull transistor output stage comprising transistors 132 and 133,diodes 134to 137, resistors 138to 140,and an output capacitor 141. Negative feedback is provided by resistors 142 and 143 and a capacitor 144. The output of the drive circuit 29 is also connected tothe input of the oscillatorAGC circuit30. The circuit30 comprises an operational amplifier 145, a potential divider comprising resistors 146 and 147, a 1.2 volt voltage reference comprising a band gap diode 148 together with a dropping resistor 149 and a filter capacitor 150, and an output circuit comprising a resistor 151, a diode 152, resistors 153 to 155 and a smoothing capacitor 156. The output of the circuit 30 is connected to a gain control circuit of the oscillator 28. The gain control circuit comprises a field effecttransistor 157 which, together with resistors 158 and 159,formsthe bottom limb of a potential divider including a resistor 160 connected across the output of the amplifier 125. The tapping point of the potential divider is connected to the inverting input of the amplifier 125 so thatthe amplitude of the output signal from the oscillator 28 is stabilized. The amplifier and filter 31 is provided with antiparallel diodes 161 and 162 for input protection and an impedance-setting resistor 163 connected across the secondary winding of the transformer 13. The circuit 31 includes an input amplifierformed by an operational amplifier 164, resistors 165 to 168, and capacitors 169 to 171. The resistor 167 is adjustable so as to allowthe gain of the amplifier to be adjusted. The output of the amplifier is connected to the input of the bandpass filter comprising an operational amplifier 172, resistors 173 to 176, and capacitors 177 and 178. The output of the filter is connected via a capacitor 179 and a resistor 180to the input of an operational amplifier 181 arranged as a differential high gain amplifier including resistors 182 to 184 and capacitors 185 to 187.The output of the amplifier 181 is connected to one input of the phase sensitive detector 32. The phase sensitive detector comprises a pulse squaring stage including a comparator 188, antiparallel diodes 189 and 190, resistors 191 and 192, and capacitors 193 and 194. The pulse squaring stage receives the output of the drive circuit 29. The output of the comparator 188 is connected via a diode 195to provide a first clock signal 1, and via a network comprising resistors 196 and 197, a capacitor 198 and a diode 199, to an inverting stage comprising a common-emitter connected transistor 200 with a collector load resistor 201. The inverting stage provides a second clock signal 2. The clock signals 1 and 2 are supplied to the control inputs of integrated circuitanalogue gates 202 and 203 formed by an integrated circuittype DG 200A. The output signal of the operational amplifier 181 is connected via the first gate 202 and a resistor 204 across a capacitor 205.The second gate 203 is connected in series with a resistor 206, and the series connection is connected across the capacitor 205. The input of a comparator circuit, comprising a comparator 207 and resistors 208 to 210, is connected to the comparator 205. The output of the comparator 207 is connected via a potential divider comprising resistors 211 and 212to a relay driving stage comprising darlingtonconnected transistors 213 and 214, a resistor 215, and back/EMF protection diodes 216 and 217. The non-inverting input of the comparator 207 receives the reference voltage from the band gap diode 148. The operational amplifier 97 is an integrated circuit operational amplifiertype TL 071 M whereas the other operational amplifiers are provided by twin operational amplifier integrated circuits type TL 072M.

Claims

1. An apparatus for monitoring an earth connection, comprising means for injecting a current into a conductor connected between a first terminal for connection to an earth terminal and a second terminal for connection to an equipment earth terminal, and measuring means for measuring the resulting current orvoltage between the first and second terminals.

2. An apparatus as claimed in claim 1, in which the injecting means is arranged to inject an alternating current.

3. An apparatus as claimed in claim 2, in which the injecting means is arranged to inject an alternating current having a frequency substantially equal to 20 kHz.

4. An apparatus as claimed in claim 2 or3, in which the injecting means includes atransformer whose secondary winding is connected in series with the conductor.

5. An apparatus as claimed in any one of claims to 4, in which the measuring means includes means for detecting current whose frequency is the same as that of the injected current.

6. An apparatus as claimed in claim 5, in which the current detecting means includes a phase sensitive detector arranged to compare the alternating current from the injecting means with the current orvoltage from the conductor.

7. An apparatus as claimed in claim 5 or 6, in which the measuring means includes a bandpassfilter connected between the conductor and the current detecting means and tuned to the frequency of the alternating current.

8. An apparatus as claimed in any one of claims 2 to 7, in which the measuring means includes a currenttransformerwhose primary winding is connected in series with the conductor.

9. An apparatus as claimed in any one of the preceding claims, in which the measuring means includes a comparatorfor producing an output signal when the resulting current is less than a predetermined value.

10. An apparatus as claimed in claim 9, in which the measuring means includes indicating means for providing an indication in response to the output signal of the comparator.

11. An apparatus for monitoring the earth impedance of an installation substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

11. An apparatus as claimed in anyone of the preceding claims,furthercomprising earth leakage detection means for detecting an earth leakage current or voltage between the first and secondterminals.

12. An apparatus as claimed in claim 11, in which the earth leakage detection means is arranged to detect alternating earth leakage current. 13. An apparatus as claimed in claim 12, in which the earth leakage detection means includes a filter for attenuating signals whose frequency is outside a frequency band including mains frequency.

14. An apparatus as claimed in claim 12 or 13, in which the earth leakage detection means includes comparator means for producing a fault signal when the amplitude of the positive or negative peaks of the alternating earth leakage current exceeds a predetermined amplitude.

15. An apparatus for monitoring an earth connection, substantially as hereinbefore described with re- ference to and as illustrated in the accompanying drawings. Amendments to the claims have been filed, and have the following effect:- Claims 1-15 above have been deleted. Newclaims have been filed asfollows:- 1. An apparatus for monitoring the earth impedance of an installation which is connected to earth by a conductor and which is arranged to receive alternating supply current of a firstfrequency, comprising earth leakage detection means arranged to detect alternating earth leakage current ofthefirst frequencythrough the conductor, means for injecting alternating current of a second frequency different from the firstfrequency into the conductor, and measuring meansfor measuring the resulting current of the second frequency flowing through the conductorto earth.

2. An apparatus as claimed in claim 1, in which the injecting means is arranged to inject an alternating current having a frequency substantially equal to 20 kHz.

3. An apparatus as claimed in claim 2 or 3, in which the injecting means includes a transformer whose secondary winding is connected in series with the conductor. 4. An apparatus as claimed in any one of the preceding claims, in which the measuring means includes a phase sensitive detector arranged to perform phase detection between the alternating current from the injecting means and the resulting current.

5. An apparatus as claimed in claim 4, in which the measuring means includes a bandpass filter connected between the conductor and the phase sensitive detector and tuned to the second frequency.

6. An apparatus as claimed in any one of the preceding claims, in which the measuring means includes a current transformer whose primary winding is connected in series with the conductor.

7. An apparatus as claimed in any one of the preceding claims, in which the measuring means includes a comparatorfor producing an output signal when the resulting current is less than a predeterminedvalue. 8. An apparatus as claimed in claim 7, in which the measuring means includes indicating meansfor providing an indication in response to the output signal of the comparator.

9. An apparatus as claimed in any one of the preceding claims, in which the earth leakage detection means includes afilterfor attenuating signals whose frequency is outside a frequency band including the firstfrequency.

10. An apparatus as claimed in any one of the preceding claims, in which the earth leakage detection means includes comparator means for producing a fault signal when the amplitude of the positive or negative peaks of the alternative earth leakage current exceeds a predetermined amplitude.