WO1993012436A1

WO1993012436A1 - Fault indicator for power lines

Info

Publication number: WO1993012436A1
Application number: PCT/AU1992/000659
Authority: WO
Inventors: Campbell Houston Keenan
Original assignee: Campbell Houston Keenan
Priority date: 1991-12-09
Filing date: 1992-12-09
Publication date: 1993-06-24

Abstract

A fault indicator for a power distribution line has a sensor unit (1) mounted on the distribution line to be monitored. The sensor unit (1) includes a current sensor (6), a voltage sensor (9), a control unit (8) which compares instantaneous magnitude and phase of sensed voltage and current with steady state values to determine the presence of abnormal current levels and reversal of current flow. The sensor unit also carries local indicator (4) to indicate a fault condition and the direction of the fault on the line. The sensor unit also includes a data transmitter (5) to transmit data to a local data collator (20) which has a cooperating data receiver (21), a controller (22) for collecting and interpreting data from a number of grouped conductors and an indicator (23) for indicating fault conditions and the direction of the fault.

Description

FAULT INDICATOR FOR POWER LINES The present invention relates to a fault indicator for indicating when a fault has occurred in the transmission of electrical power along a power line, such as the power lines used in countrywide electrical distribution networks, for example.

Previously known fault indicators are mounted in situ at strategic points on Electricity Supply Utility distribution networks to quickly pinpoint faults. When a fault occurs, such as a short circuit to earth, for example, all fault indicators trigger from the supply sub-station up to the fault, but not beyond. Consequently, a "trouble man" following the line, or the control operator on his screen, can follow the fault indicator flashing light (a flashing light display is provided locally on each fault indicator for the "trouble man"), identify and isolate the faulty section, then restore power to the remainder of the network.

Prior art fault indicators suffer from a number of problems which have resulted in their general performance being notoriously unreliable and has resulted in their use being restricted to limited applications. These problems include an inability to detect troublesome low earth currents and an inability to identify the correct location of a fault when some particular types of fault occur, such as "brown out" (open circuit), or the situation where a fault occurs halfway between the load and the power supply and the load has enough electromotive force to push the charge back along the power line to the fault. In this case, all fault indicators from the power supply to the load will trigger and it will be impossible to identify the location of the fault.

These problems arise because the traditional fault indicator is designed to trigger when the current on the line reaches a predetermined set value. Therefore any increase in current to this value anywhere on the line will cause the traditional fault indicator to trigger, whether the increase in current is from the power supply to the fault or from the load to the fault, or the result of any other cause.

The fact that standard fault indicators trigger on a predetermined current threshold level also has the added disadvantage that different thresholds must be set for networks using different current fault values. One known prior art device which provides some improvement over standard fault indicators relies upon the fault indicator being isolated from the actual power conductor by being mounted on a conductor support pole, and monitoring the status of conductor/conductors magnetic and electric fields.

While this prior art device has overcome some of the problems of the traditional fault indicator, and has the additional advantage of being isolated from the conductors so that the device can be easily interfaced for distribution automation, some problems still remain, and in particular such prior art devices will still trigger when the current surge due to a fault comes from the load towards the fault as well as coming from the power supply to the fault. The present invention provides a fault indicator for indicating when a fault has occurred on an electrical distribution line, the fault indicator being mountable on an electrical distribution line and comprising means for sensing the direction of current flow on occurrence of a fault and indicating means for indicating occurrence of a fault.

The indicating means is preferably arranged to indicate the direction of current flow on occurrence of the fault. It will be appreciated that by current "direction" in an ac network there is no actual absolute flow direction of current, actual flow periodically reversing. However, convention dictates that current flow "direction" is from the generator to the load. When a back surge current occurs on a fault, from the load towards the fault, the direction of current flow therefore reverses.

In the present invention direction sensing is carried out by monitoring the phase angle of the ac signal. In a preferred embodiment, the fault indicator includes means for detecting the current wave form and means for detecting the voltage wave form, comparison of voltage and current wave forms on occurrence of a fault enabling a determination of current direction.

The phases of the respective current and voltage wave forms are preferably compared. If there is no relative change in the phases the current is considered to be flowing in its original direction, i.e. away from the generator, whereas if there is a reversal in phase the current is considered to be flowing in the opposite direction i.e. towards the original generator.

Current sensing is preferably carried out by a coil with an iron core placed adjacent the conductor.

Voltage sensing is preferably carried out by a pair of capacitive plates placed adjacent the conductor, one plate being nearer the conductor than the other, which act to sense the presence or absence of voltage.

In a further preferred embodiment actual voltage values may be detected.

The arrangement of the present invention has the advantage that, where the indicating means provides an indication of direction, each indicating means on each fault indicator points towards the fault, even if a back surge current sufficient to trip the detectors has occurred between the load and the fault. In a preferred embodiment, the fault indicator comprises a "doughnut" housing clampable onto a conductor and incorporating in the housing a display for indicating occurrence of a fault and current direction on occurrence of the fault. In another embodiment the fault indication is provided by a fault indicator system, comprising a sensor mounted in a housing on the conductor, which senses current and voltage information, and a data collator mounted within the vicinity of the conductor mounted sensor. The data collator may, where the network is a over-ground network, be mounted on the conductor support pole or, where the network is an underground network, be mounted adjacent the conductors.

In the case of this embodiment the sensor mounted conductor includes a transmitter for transmitting sensor information to a complementary receiver in the data collator. The data collator can therefore monitor the conductor or, where there are a number of conductors, each conductor, and when a fault occurs a display, preferably mounted on the data collator, indicates which conductor is affected and the direction of current flow on occurrence of the fault.

The system has the advantage of line isolation of the data collator to allow ease of interfacing for distribution automation of the information obtained from the conductor sensors. One collator may deal with a number of fault indicators, i.e. one for each conductor associated with the collator.

The transmission medium, for transmission of the information between the sensor and the collator, is preferably a wireless communication link. In a preferred embodiment this is an ultrasonic transmitter and receiver arrangement, although it is possible to use other arrangements, such as radio frequency transmissions in the electromagnetic spectrum. The range required for the transmission will, in virtually all cases, be very low (a few metres) .

Unlike conventional fault indicators, the fault indicator of the present invention is preferably arranged not to trigger on an absolute value of current but on a ratio value of current, e.g. if the current on the line increases by 100% from its normal current value the fault indicator will then trigger. Alternatively it may trigger when di/dt exceeds a predetermined value. One type of fault indicator can therefore be used for all types of electricity distribution networks, whatever the network current value.

To avoid triggering on transient current surges, such as load changes, for example, which do not constitute faults, the fault indicator of the present invention preferably waits a predetermined time period for the line current to drop to near zero before triggering (e.g. 20 seconds). Current sensing followed by triggering on current drop guards against false triggering. Where transient surges do constitute a fault so that the protection system operates, but then automatically recloses successfully the fault indicator will indicate for a period of time, say 4 hours, and will not be reset by return of current. In this instance the indication will be at a different flash rate than for a permanent fault, thus permitting troublesome transient faults to be located and distinguished from permanent faults.

The fault indicator of the present invention may also sense temperature information, turning the fault indicator into a complete power analyser for a power line. Where actual value voltage is sensed, all of the parameters necessary for a complete power analyser are available. Arrangements can be made for transmission of this data to a base station, giving a controller all the information he needs to carry out a complete power analysis of the network, as well as providing the normal fault indication function.

One other factor which is problem in designing fault indicators is that of current in-rush. When a circuit is first switched on, there is a current surge of several times full load for a second or two to provide energising current for transformers, motor start-up, etc. This would normally trigger all standard fault indicators and if there is a fault on the line at switch on, the indicators would not distinguish between in-rush and fault current, making it impossible to identify the location of the fault due to the triggering of all of the indicators by the in-rush. For this reason, in-rush restraint is used in standard fault indicators to prevent "setting" of indicators until in-rush has subsided.

However, in-rush current does not last long. Typically it would not last more than a cycle or so of the current waveform. In a preferred embodiment of the present invention the fault indicator is set so as not to indicate a fault unless the fault current is maintained over at least three cycles. This automatically distinguishes from in-rush current.

According to a second aspect the present invention provides a fault indicator for indicating when a fault has occurred on an electrical distribution line, the fault indicator being mountable on an electrical distribution line and comprising means for monitoring the magnitude of the current flow to determine whether or not a fault current has occurred, the occurrence of a fault current being established when a predetermined magnitude of current flow has occurred for a predetermined time.

When referring to current "magnitude" in the above paragraph it will be appreciated that this can include the determination of fault current by a ratio magnitude or rate of increase evaluation as discussed above. A predetermined time preferably includes the predetermined number of cycles of the fault current flow, e.g. three cycles. This enables the fault indicator to distinguish between in-rush current and a true fault current. A second option may be the sensing of di/dt within a much shorter time to co-ordinate with fast acting fuses.

This aspect of the invention can include all of the preferred features of the first aspect of the invention discussed above, as well as the feature of determining current direction and indicating same.

According to a third aspect the present invention provides a fault indicator for indicating when a fault has occurred on an electrical distribution line, comprising means for mounting the fault indicator on an electrical distribution line conductor, indicating means for indicating the occurrence of a fault, current sensing means for sensing the current on the distribution line and trigger means responsive to the magnitude of the current increasing to a predetermined ratio or di/dt value over the normal current value to cause the indicating means to indicate that a fault has occurred.

The fault indicator of this aspect of the invention may also include all of the features discussed above in relation to the above other aspects of the invention, including current direction sensing, voltage sensing, a fault indication system comprising conductor mounted sensors and remote data collators, etc. as discussed above. According to a fourth aspect the present invention provides a fault indicator system for indicating when a fault has occurred on an electrical distribution line, the system comprising a sensor arranged to be mounted on an electrical distribution line conductor and including means for sensing current on the conductor, and a data collator mounted in the vicinity of the sensor but not being mounted on the conductor, the sensor further including transmitter means for transmitting current sensor information to the data collator, and the data collator including receiver means for receiving the current sensor information, the data collator also being provided with indicator means for indicating the occurrence of a fault on the conductor when the current sensor information indicates that a fault has occurred. The sensor may also include means for detecting voltage and temperature as discussed above in relation to the first aspect of the invention, and may determine the occurrence of a fault by ratio current detection followed by current drop also, as well as all of the other features discussed above in relation to previous aspects of the present invention.

Features and advantages of the present invention will become apparent from the following description of an embodiment thereof, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 is a schematic block circuit diagram of circuitry in a sensor unit arranged to be mounted on an electrical transmission conductor and

Figure 2 is a schematic circuit block diagram of a data collator to be used in conjunction with the sensor of Figure 1 and to be mounted in the vicinity thereof.

Figures 1 and 2 illustrate an embodiment of a fault indicator in accordance with the present invention which is configured as two separate units, although one unit is operable on its own as a local fault indicating unit if required, as will become clear later on in the description.

The first unit, illustrated in Figure 1, is a sensor unit (generally designated by reference unit 1) which, in operation, is mounted within a "doughnut" shaped housing (these types of housings are known for previous fault indicators) which is clampable directly on to a conductor of a power distribution line. The sensor 1 is clamped to the power line such that its various sensors 2 and current transformer 3 are within operating range of the conductor. The fault indicator 1 is arranged to monitor the current and voltage flowing along the power conductor, to determine when changes in these parameters mean that there is a fault somewhere along the conductor, and to provide an indication via indication means 4 of the occurrence of a fault and of the direction of current flow on occurrence of the fault.

The sensor 1 is also provided with a transmitter circuit 5 which is capable of transmitting information relating to the sensed parameters and also whether or not a fault has occurred to the data collator unit illustrated in Figure 2. It should be noted that the indicator means 4 of the sensor unit 1 is optional. Where the sensor unit 1 is being used in conjunction with a data collator of Figure 2, as in this described embodiment, where the data collator has its own indicator means for indicating a fault and current direction, the local indication means 4 is not necessary. The unit 1, however, may be used on its own, without the collator, in which case the local indicator 4 is able to give all of the fault information necessary. The collator may be integrated into a distribution automation system.

In more detail, the sensor 1 comprises a current sensor 6, which comprises an iron coil core situated adjacent the power conductor. The iron coil core senses the current wave form on the conductor, the signal is filtered in a 50/60 Hz band pass filter (not shown) and then amplified in a current amplifier 7.

The signal from the current amplifier 7 is input to an analog to digital converter input of a control unit 8, to convert the ac current signal into a digital signal. The control unit 8 comprises a central processing unit, analog to digital converters (perhaps on board the CPU) , a memory and various outputs. A person skilled in the art will be able to design a control unit to perform the various functions required by the control unit of the present invention, from a standard knowledge of electronics.

The gain of the current amplifier 7 is adjusted by a pulse width modulated output from the control unit 8. The pulse width modulated output from the control unit 8 is used as a feedback signal to constantly adjust the gain of current amplifier 7 and maintain it such that the output of the current amplifier 7 remains unsaturated even when the absolute current magnitude value increases. The absolute magnitude value of the current can be calculated in the control unit 8 from the digitally converted output of the current amplifier 7 and the value of the pulse width modulating signal at any particular time. In an alternative embodiment (not shown) the gain of the amplifier 7 may be adjusted by switching a number of resistors digitally to provide various known gains. Again, the current magnitude is calculated from the signal level at the A/D converter and gain at the time. This gain setting will be continually updated to keep signal levels appropriate for fault detection. This alternative embodiment does not suffer from the possible inaccuracy problems which can occur with the previous, pulse width modulated arrangement, due to temperature variations. A voltage detector 9 is also provided and comprises a pair of capacitive plates placed adjacent the conductor, one plate being further away from the conductor than the other.

The magnitude of the voltage is not measured by the detector 9, but the capacitive sensor 9 does produce a signal, filtered by a 50/60 Hz band pass filter (r t shown) amplified by a voltage amplifier 10.

In addition, a temperature sensor is provided, reference numeral 11, which provides information on the temperature of the conductor, which is useful where a complete power analysis of the conductor is required. The signal from the temperature sensor 11 is amplified in a temperature amplifier 12 and input to analog to digital converter in the control unit 8, for subsequent onward transmission.

The indicating means 4 comprises four LEDs arranged in a diamond shape, which display provides a simple indication of current flow direction when three of the LEDs are illuminated. The LEDs are preferably "super bright" LEDs, and flashing indication is used (e.g. a flash every two seconds) to conserve power and increase visibility.

The transmitter circuit 5 includes an ultrasonic or ultralow power radio transmitter which takes the serial output from the control unit c and transmits it to the data collator illustrated in Figure 2.

The circuit is provided with power from a power supply 13 which takes its power from a current transformer 3 hooked to the conductor, and being provided with standard over-current and voltage protection 14. Standard technology is used in blocks 3, 13 and 14.

Where local indication of a fault current is required by indicator 4, a back-up battery (e.g. a lithium battery) may be provided to continue powering the display 4 when there is no longer any power being provided by the conductor (e.g. when the supply circuit breaker has opened) .

In operation, as far as current sensing is concerned, the control unit 8 determines when the current has increased by a predetermined ratio amount or di/dt, comparied to its former, normal value. It may do this by storing in memory the normal average value of current magnitude and subsequently comparing this normal average value with the magnitude of current presently being sensed. If the current changes to the trigger values this provides the first indication to the control unit that a fault may be occurring. The second indication, confirmatory of the fact that a fault has occurred, is when the current detected by the current detector drops to near zero, or more precisely to less than 1% say of normal value, i.e. no current pulse has been detected. When both these events have occurred, the control unit indicates that a fault has occurred by driving the display 4 and/or transmitting the fault information to the collator of Figure 2 so that the collator can indicate a fault on its display.

Current direction is determined in the control unit 8 by comparing the signal received from the voltage detector amplifier 10 and the signal received from the current amplifier 7. If the phase angle between the two wave forms remains the same, the direction of the current is the same as when no fault was occurring, i.e. from supply to load. However, if the phase angle between the two signals reverses, the current direction has reversed. In the preferred embodiment, the control unit 8 is arranged to compare the times that the voltage and current waves cross 0 in a positive direction. The power factor can then be calculated from the following formula:- PF = cos φ = cos (ΔT/T x 360) where ΔT is the time difference between the zero crossing points of the voltage and current waveforms and T is the time for one cycle, φ is the phase difference and by watching for sudden change in this phase difference it can be determined whether or not current flow direction has reversed. In this embodiment, ΔT only is transmitted to the collating unit, since T is known, and the power factor can be calculated at the collator.

If the current direction has reversed, the indicating means 4 is appropriately adjusted to show this.

The direction indication is very important as it now allows the location of faults to be easily detected even where the faults are due to one of the three following reasons:- (1) Due to an open circuit where the fault current is back fed via the load side transformer.

(2) When back feeding by large motors or capacitance occurs on the load side of the fault.

(3) Occurring on a closed loop with fault current being fed from either side of the fault.

These three situations cannot be adequately coped with by standard fault indicators.

As discussed above, the sensor unit 1 can operate as a stand alone unit by virtue of being provided with a local indicator 4, and need not be operated in conjunction with the data collator illustrated in Figure 2. Where a cheaper system is required, therefore, sensor units 1 on their own will be sufficient.

However, where the advantages of line isolation for a user interface for distribution automation are required (e.g. a SCADA system) then a fault indication system comprising the sensor of Figure 1 operating in conjunction with the collator of Figure 2 is desirable.

SCADA is an acronym for "Supervisory Control and Data Acquisition", i.e., in this case, a remote control system for automating the distribution system, by receiving information at a control room and enabling all switching and control to be carried out from the control room. Referring to Figure 2, the collator unit 20 is provided in a housing which may be similar to the housing provided for prior art pole mounted fault indicators, the housing being mountable in the vicinity of the conductor mounted sensor. In an over-ground electricity network, for example, the collator 20 would be mounted on the conductor support pole. In an underground network, the collator 20 could be mounted in any accessible easily visible area in the vicinity of the conductor mounted sensor.

The collator 20 comprises a receiver, which may be ultrasonic or radio depending upon the transmission media, for receiving information from the sensor 1, a control unit 22 for processing the received information, an LED display 23 for indicating faults and current direction of fault and a user interface 24, which may be an RS232 interface, for example, for interfacing with a distribution automation SCADA network or interfacing with the controller base to provide power line information and/or fault information directly to the network controller. The power supply 25 for the unit 20 includes a battery which is chargeable by a solar panel 26 (obviously the solar panel may only be useful in over-ground units and other sources of power will usually be required for underground units) . Note that, as regards underground units, it is quite possible to have the sensors remaining underground while the collator units are situated over-ground. The solar panel may still be used for these.

A collator unit 20 would usually be used with a plurality of conductor mounted sensor units 1, i.e. one for each conductor phase in the vicinity of the collator unit 20. This gives the advantage over prior art devices of being able to gather information on each conductor separately while keeping the advantage of line isolation for the collator. The information sent from each conductor to the receiver 21 will include the current, phase angle (or just ΔT) , which conductor (i.e. address of the particular sensor sending the information) , status (whether there is a fault or not) and temperature of the conductor. For a complete power analysis, the sensor 1 would also be arranged to determine voltage magnitude and would transmit this to the receiver.

Where there are a number of sensors 1 for each collator 20, the sensed informed will be sent from each sensor 1 using random delays to avoid clashes of information.

The control unit 22, if a fault occurs, determines from the address of the faulty conductor which conductor it is and indicates on its display 23 which conductor and the direction of current and occurrence of the fault.

The present invention allows more information to be shown about a fault on a line, i.e. whether the fault is phase to phase, phase to earth, brown out, short or open circuit, etc. A fault indicator in accordance with the present embodiment is preferably arranged to be automatically reset after the occurrence of a fault by the return of a voltage for a continuous predetermined time period, for example 20 seconds. A timeout for resetting the device is also possible. For example, the device may be set to automatically reset within a predetermined suitable time period, such as four hours.

A transient lock facility may also be implemented which is arranged to provide indication that transient faults are occurring, i.e. a fault which only lasts for a short time period before current comes back on line. A normal fault indicator would be automatically reset, but the transient version can be implemented merely by dispensing with the current reset feature. A transient fault would be distinguishable from a permanent fault by a different (faster or slower) flash rate.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS : -

1. A fault indicator for indicating when a fault has occurred on an electrical distribution line, the fault indicator being mountable on an electrical distrib. on line and comprising means for sensing a fault condi on means for sensing direction of current flow on occurrence of a fault condition and indicating means for indicating occurrence of a fault.

2. The fault indicator of claim 1 said indicator further including direction indicating means to indicate the direction of current flow on occurrence of the fault.

3. The fault indicator of claims 1 or 2, said indicator including means for detecting the current wave form, means for detecting the voltage wave form, and means for comparing voltage and current wave forms on occurrence of a fault to determine current direction.

4. The fault indicator of claim 3 wherein current direction is determined by comparing the phase of the voltage and current signals.

5. The fault indicator of claim 3 or 4 wherein current sensing is carried out by a coil with an iron core placed adjacent the conductor.

6. The fault indicator of claims 3, 4 or 5 wherein voltage sensing is carried out by a pair of capacitive plates placed adjacent the conductor, one plate being nearer the conductor than the other, which act to determine the phase angle of the voltage wave.

7. The fault indicator as claimed in any one of claims 1 - 6 wherein a toroidal or U-shaped housing is provided, said housing being arranged to be clamped around a conductor.

8. The fault indicator of claim 7 wherein said housing incorporates a display for indicating occurrence of a fault and current direction on occurrence of the fault.

9. The fault indicator as claimed in any one of claims 1 - 8 comprising a sensor mounted in a housing on the conductor, and arranged to sense current and voltage information, and a data collator mounted within the vicinity of the conductor mounted sensor, the data collator being in communication with the sensor and incorporating said indicating means.

10. The fault indicator of claim 9 wherein the conductor mounted sensor includes a transmitter for wireless transmission of sensor information to a complementary receiver in the data collator.

11. The fault indicator of claim 10 wherein the transmission medium, for transmission of the information between the sensor and the collator, is an ultrasonic acoustic signal or other interference immune low power signal.

12. The f ult indicator of claim 10 wherein the transmission medium, for transmission of the information between the sensor and the collator, is a radio frequency electromagnetic signal.

13. The fault indicator as claimed in any one of the preceding claims wherein a fault condition is detected when a current value is detected which is greater than a steady state current in the distribution line prior to the fault by a predetermined percentage or rate of rise (di/dt) is greater than a predetermined value.

14. The fault indicator of claim 13 wherein a fault condition is not registered until the line current drops to near zero.

15. The fault indicator according to any one of the preceding claims wherein a fault is not indicated unless a fault current exists for at least three cycles.

16. The fault indicator according to any one of the preceding claims further including communication means for communication of data sensed by said sensors to a central control station.

17. A fault indicator for indicating when a fault has occurred on an electrical distribution line, the fault indicator being mountable on an electrical distribution line and comprising means for monitoring the magnitude of the current flow to determine whether or not a fault current has occurred, the occurrence of a fault current being established when a predetermined magnitude of current flow has occurred for a predetermined time.

18. The fault indicator of claim 17, said fault indicator further including means for sensing direction of current flow on occurrence of a fault condition and indicating means for indicating occurrence of a fault.

19. The fault indicator of claim 18, said indicator further including direction indicating means to indicate the direction of current flow on occurrence of the fault.

20. The fault indicator of claims 18 or 19, said indicator including means for detecting the current wave form, means for detecting the voltage wave form, and means for comparing voltage and current wave forms on occurrence of a fault to determine current direction.

21. The fault indicator of claim 20 wherein current direction is determined by comparing the phase of the voltage and current signals.

22. The fault indicator of claim 20 or 21 wherein current sensing is carried out by a coil with an iron core placed adjacent the conductor.

23. The fault indicator of claims 20, 21 or 22 wherein voltage sensing is carried out by a pair of capacitive plates placed adjacent the conductor, one plate being nearer the conductor than the other, which act to determine the voltage phase angle.

24. The fault indicator as claimed in any one of claims 17 - 23 wherein a toroidal or U-shaped housing is provided, said housing being arranged to be clamped around a conductor.

25. The fault indicator of claim 24 wherein said housing incorporates a display for indicating occurrence of a fault and current direction on occurrence of the fault.

26. The fault indicator as claimed in any one of claims 17 - 23 comprising a sensor mounted in a housing on the conductor, and arranged to sense current and voltage information, and a data collator mounted within the vicinity of the conductor mounted sensor, the data collator being in communication with the sensor and incorporating said indicating means.

27. The fault indicator of claim 26 wherein the conductor mounted sensor includes a transmitter for wireless transmission of sensor information to a complementary receiver in the data collator.

28. The fault indicator of claim 27 wherein the transmission medium, for transmission of the information between the sensor and the collator, is an ultrasonic acoustic signal or other interference immune low power signal.

29. The fault indicator of claim 27 wherein the transmission medium, for transmission of the information between the sensor and the collator, is a radio frequency electromagnetic signal.

30. The fault indicator as claimed in any one of claims 17 - 29 wherein a fault condition is detected when a current value is detected which is greater than a steady state current in the distribution line prior to the fault by a predetermined percentage or the rate of rise (di/dt) is greater than a predetermined value.

31. The fault indicator of claim 30 wherein a fault condition is not registered until the line current drops to near zero.

32. The fault indicator according to any one of claims 17 - 31 wherein a fault is not indicated unless a fault current exists for at least three cycles.

33. The fault indicator according to any one of claims 17 - 32 further including communication means for communication of data sensed by said sensors to a central control station.

34. A fault indicator system for indicating when a f ult has occurred on an electrical distribution line, the system comprising a sensor arranged to be mounted on an electrical distribution line conductor and including means for sensing current on the conductor, and a data collator mounted in the vicinity of the sensor but not being mounted on the conductor, the sensor further including transmitter means for transmitting current sensor information to the data collator, and the data collator including receiver means for receiving the current sensor information, the data collator also being provided with indicator means for indicating the occurrence of a fault on the conductor when the current sensor information indicates that a fault has occurred.

35. The fault indicator of claim 34, said sensor including means for sensing direction of current flow on occurrence of a fault condition.

36. The fault indicator of claim 34 or 35 said collator further including direction indicating means to indicate the direction of current flow on occurrence of the fault.

37. The fault indicator of claims 34, 35 or 36, said sensor including means for detecting the current wave form, means for detecting the voltage wave form, and means for comparing voltage and current wave forms on occurrence of a fault to determine current direction.

38. The fault indicator of claim 37 wherein current direction is determined by comparing the phase of the voltage and current signals.

39. The fault indicator of claim 37 or 38 wherein current sensing is carried out by a coil with an iron core placed adjacent the conductor.

40. The fault indicator of claims 37, 38 or 39 wherein voltage sensing is carried out by a pair of capacitive plates placed adjacent the conductor, one plate being nearer the conductor than the other, which act to determine the voltage phase angle.

41. The fault indicator as claimed in any one of claims 34 - 40 wherein a toroidal housing is provided for said sensor , said housing being arranged to be clamped around a conductor.

42. The fault indicator of claim 41 wherein said housing incorporates a display for indicating occurrence of a fault and current direction on occurrence of the fault.

43. The fault indicator as claimed in any one of claims 34 - 42 wherein the conductor mounted sensor includes a transmitter for wireless transmission of sensor information to a complementary receiver in the data collator.

44. The fault indicator of claim 43 wherein the transmission medium, for transmission of the information between the sensor and the collator, is an ultrasonic acoustic signal.

45. The fault indicator of claim 43 wherein the transmission medium, for transmission of the information between the sensor and the collator, is a radio frequency electromagnetic signal.

46. The fault indicator as claimed in any one of claims 34 - 45 wherein a fault condition is detected when a current value is detected which is greater than a steady state current in the distribution line prior to the fault by a predetermined percentage or the rate of rise (di/dt) is greater than a predetermined value.

47. The fault indicator of claim 46 wherein a fault condition is not registered until the line current drops to near zero.

48. The fault indicator according to any one of the claims 34 - 47 wherein a fault is not indicated unless a fault current exists for at least three cycles.

49. The fault indicator according to any one of the preceding claims further including communication means for communication of data sensed by said sensors to a central control station.