GB2507895A

GB2507895A - Locating Insulation Faults

Info

Publication number: GB2507895A
Application number: GB1401827.9A
Authority: GB
Inventors: Christopher John Hodges
Original assignee: Schneider Electric Ltd
Current assignee: Schneider Electric Ltd
Priority date: 2009-04-24
Filing date: 2010-01-29
Publication date: 2014-05-14
Anticipated expiration: 2030-01-29
Also published as: GB2507894A; GB0907085D0; GB201401826D0; GB2507895B; GB201001420D0; GB2507894B; GB2469706B; GB201401827D0; GB2469706A

Abstract

A monitoring system is described for monitoring Independent Terra earthing arrangements, for example in a railway trackside power supply network. Low-cost monitoring devices (DR1, DR2 etc) are installed at the principal supply point (PSP) and each functional supply point (FSP). They detect and report imbalances between currents in the pair of live conductors at each monitoring point. In response to detection (ELCD) of a first earth fault, which does not cause a power interruption, a deliberate earth connection is induced at the PSP end of a the affected conductor. The circuit section affected by the earth fault can be deduced from which monitoring devices report an imbalance. The monitoring devices also store and report historical current values in response to a power interruption. In the event of a double earth fault causing power interruption, the location of earth faults can be deduced by comparing the stored historical current measurements.

Description

I

SYSTEM AND METHOD FOR LOCATING INSULATION FAULTS

The invention relates to the identification of earthing (ground) faults and/or short circuits in electrical distribution networks, and their location, and in particular to such identification on an IT (Independent Terra) type earthing arrangement.

One area of application for which the invention is designed is the supply of power to trackside equipment (signals, points etc.) in a railway network. This supply is essential to the operation of the rail network, but also is vulnerable to physical damage: both accidental and deliberate; as well as the natural deterioration caused by environmental and operational stresses.

In the UK, the national railway network uses the IT' (Independent Terra) earthing arrangement for supply of power to trackside equipment (signals, points etc.). IT earthing is referred to in the lEE Wiring regulations (BS7671).

For example, in the 16th edition of BS7671, it is discussed in sections: 413-02- 21 to 413-02-26, 542-01-04, and 542-01 -07. Further background is available in Schneider Electric's Cahier Technique publication ECT1 78 available at http://www,schneder-&ectric.com. The content of these documents is incorporated herein by reference.

In the trackside supply example, there are two live alternating current (ac) conductors, Li and L2, with a phase to phase potential of 650V, supplied from a Principal Supply Point (PSP). However, there is no neutral conductor, as the supply is a delta wound transformer. Thus there is no earth reference for the supply.

The reason for using this arrangement is that when an earth fault occurs on one of the live conductors, the supply does not trip. Thus, supply reliability is improved as it is partially fault tolerant (a short circuit will still trip the supply).

There is, however, a legal requirement that a (first) earth fault detection mechanism must be installed, so allowing the presence of the first fault to be identified, and thus a repair process started. The first fault detection mechanism is installed at the PSP, but in its simplest form is only able to identify the presence of a fault, not its location. To locate the fault, a trackside inspection is required, but these are severely constrained for safety reasons.

Thus, a first fault will frequently remain unrepaired for an extended period of time, during which a second earth fault will often occur on the other conductor, thereby leading to the supply tripping, and consequential interruption to rail services.

It is desirable therefore to provide a system and method to address the above issues. Very sophisticated monitoring apparatuses are available which could be applied to the task, but it is also desired to minimise both the cost and the physical space involved in providing this monitoring. This is particularly so because a sizable rail network may include tens of thousands of circuit sections to be monitored.

IT earthing is used in other situations besides trackside supply, including hospitals, and mining. The invention is applicable in all these fields, not only railways. The invention is applicable in three-phase and multiphase as well as two-phase IT supplies.

In a first aspect of the invention there is provided a system for locating insulation faults on a power supply network of a type having an IT type earthing arrangement thereby having no earth reference, the network comprising circuit sections of at least two live conductors, the circuit sections connecting together in series a source supply and a plurality of local supply points, the system comprising a plurality of current monitoring devices for detecting and reporting current imbalances between the conductors at respective monitoring points between sections, said system being operable to identify the circuit section subject to said insulation faults by processing the reported current imbalances.

Said system may be arranged to determine current imbalances between each of said at least two conductors, at the end of a circuit section. Said monitoring points may comprise points at or near each local supply point, and ideally may include points either side of each local supply point. Additionally they may comprise a point at or near the source supply.

Said system may comprise two live conductors with said current monitoring devices having sensing elements (such as current transformers) arranged in pairs at each of said number of points. Alternatively a three-phase embodiment would include three live conductors and a trio of sensing elements at each monitoring point.

In a further embodiment, a monitoring device has a trio of sensing elements, two of which are arranged to measure currents on said live conductors of a first circuit section at one side of a local supply point while the third sensing element is arranged to measure current flowing to a local load, the monitoring device or system being further arranged to deduce current flowing in an adjacent circuit section on the other side of said local supply point. A commercially available three-phase monitoring device can be reprogrammed to form a low cost monitoring device for use in the present system.

Said system may comprise means for intentionally introducing a connection from each of said live conductors to earth. This allows current to flow into the real' earth fault, and aids location of a single earth fault. Said means for intentionally introducing a connection from each of said live conductors to earth may introduce said connection at or near said source supply.

Such a system enables the location of a first earth fault to particular circuit section, by forming said intentional earth connection and noting which of said monitoring devices reports a current imbalance. The monitoring devices may report by telemetry to a central controller for this purpose.

In specific embodiments, described below, following a supply trip, the system is able to locate all earth faults on a circuit, thus allowing the circuit to be completely restored to its original IT earthed condition. The monitoring devices can be of low cost so as to be installed permanently at each monitoring point.

The system may include a further monitoring device for detecting the presence of a first earth fault on said conductors, said further monitoring device not indicating the location of the fault. The system may be arranged to respond automatically to such detection to initiate a fault location procedure. The fault location procedure may comprise the introduction of an intentional connection to earth, as mentioned above. The further monitoring device may comprise the mandatory first fault detector of the IT system, typically an earth leakage current detector.

Said further monitoring device may, on detection of a single first earth fault, indicate on which live conductor the earth fault is located, whereupon the system will intentionally introduce a further connection to earth on said same live conductor as said single earth fault, and each of said current monitoring devices which are located between said intentional connection to earth and said single earth fault, will to detect and report a current imbalance.

Said system may be further operable, upon detection of the location of said single earth fault, but prior to its rectification, to break said intentional connection to earth and to monitor for further earth faults on the same live conductor, said monitoring comprising noting whether pairs of current monitoring devices detect a current imbalance, and if so, which pairs do so, these pairs of current monitoring devices being located between at least two earth faults.

Said system may be operable to locate multiple first earth faults by noting which of said pairs of current monitoring devices detect a current imbalance, these current monitoring devices being located between at least two earth faults.

Each of said current monitoring devices may be operable to store historical current values. Said historical current values stored may comprise current values measured at a plurality of predetermined past time points. Said current values measured at a plurality of predetermined past time points, may include any or all of instantaneous, average and maximum current values. In a preferred embodiment, said current monitoring devices are responsive to a trigger condition to retain and report the last historical current values stored.

The trigger may in particular be a loss of supply input.

The network will generally include a circuit breaker for interrupting supply in the event of a short circuit between two live conductors. The system may be operable such that, where supply is lost due to there being earth faults on more than one of said live conductors, each of said earth faults can be located to a particular circuit section by processing the recorded data from the monitoring devices either side of the affected section. In particular, an earth fault can be located by noting where there occurred a difference in the current measured between adjacent current monitoring devices on the same live conductor, shortly before the loss of supply.

The aforementioned responses and processing may be automated, where the monitoring devices report to a central processing unit, or performed manually after collating reports from the monitoring devices. For the railway trackside application, the distances and hazards involved make telemetry and central processing particularly advantageous.

In a further aspect of the invention there is provided a method for locating insulation faults on a power supply network of a type having an IT type earthing arrangement thereby having no earth reference, the network comprising circuit sections of at least two live conductors, the circuit sections connecting together in series a source supply and a plurality of local supply points, said method comprising: (a) providing current monitoring devices for monitoring current on each live conductor, at a number of monitoring points between circuit sections; (b) using said current monitoring devices to determine the presence of current imbalances at said source supply and at said number of points along the length of said live conductors, and (c) determining, from said determination of the presence of current imbalances, the circuit section in which an earth fault is located.

Said monitoring points along the length of said live conductors may comprise points at each local supply point, and ideally may include points either side of each local supply point. A first monitoring point is typically provided at or near the source supply.

Said method may comprise the step of intentionally introducing a connection from each of said live conductors to earth in order to aid location of a single earth fault. Said connection may be introduced at or near said source supply.

The method may include deducing the location of said single earth fault by noting which of said monitoring devices detect a current imbalance between the live conductors, when said intentional connection to earth is made.

Said method may include, on detection of a single first earth fault, determining on which live conductor the earth fault is located and intentionally introducing a further connection to earth on said same live conductor as said single earth fault, thereby detecting a current imbalance in each of said pairs of current monitoring devices located between said intentional connection to earth and said single ground fault.

Said method may comprise, upon detection of the location of said single earth fault, but prior to its rectification, breaking said intentional connection to earth and monitoring for further earth faults on the same live conductor, said monitoring comprising noting whether pairs of current monitoring devices detect a current imbalance, and if so, which pairs do so, these pairs of current monitoring devices being located between at least two earth faults.

Said method may comprise locating multiple first earth faults by noting which of said pairs of current monitoring devices detect a current imbalance, these current monitoring devices being located between at least two earth faults.

Said method may comprise storing historical current values measured by each current monitoring device. Said historical current values stored may comprise the current values measured at a plurality of predetermined past time points.

Said current values measured at a plurality of predetermined past time points, may include any or all of instantaneous, average and maximum current values. Said method may include, on receiving a trigger, retaining the last historical current values stored, for example as a result of receiving a loss of supply input.

Said method may comprise, where supply is lost due to there being earth faults on more than one of said live conductors, locating each of said earth faults by noting where there occurred a difference in the current measured between adjacent current monitoring devices on the same live conductor, shortly before the loss of supply.

In a further aspect of the invention there is provided a monitoring device for use in the system or method as set forth above, the monitoring device having multiple sensor inputs and a signalling output, and being programmed (i) to monitor and compare currents in at least two conductors via said sensing inputs, (ii) to detect and report via said signalling output the presence of an imbalance in current between designated pairs of said conductors, and (iii) to store and report historical values of said currents.

Said historical current values stored may comprise the current values measured at a plurality of predetermined past time points.

Said current monitoring devices may further be operable, on receiving a trigger, to retain and report the last historical current values stored, for example as a result of receiving a loss of supply input.

The invention yet further provides controller for receiving reports from monitoring devices and for processing said reports to implement a system or method according to the invention as set forth above.

These and other aspects, features and advantages of the invention will be understood by the skilled reader from a consideration of the above summary and of the more detailed description of examples which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: Figure 1 shows a prior art trackside circuit arrangement; Figure 2 shows a system according to an embodiment of the invention; Figure 3 shows the system of Figure 2 operational to locate a single first fault; Figure 4 shows the system of Figure 2 operational to locate multiple first faults; Figure 5 illustrates a prior art trackside circuit arrangement having earth faults on both conductors; Figure 6 illustrates the principle behind how faults such as those depicted in Figure 5 can be detected by the system of Figure 2; Figure 7 shows the system of Figure 2 operational to locate two faults, when said two faults result in power loss; Figure 8 shows the system of Figure 2 operational to locate more than two faults, when said faults result in power loss; and Figure 9 illustrates an alternative form of monitoring point.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 shows an example of the UK national railway network's (Network Rail) present trackside supply circuits. It consist of a series of cable circuit sections comprising two live AC conductors Li, [2 that link together Functional Supply Points FSP1, FSP2. At one end is a Principal Supply Point PSP 1 which provides the source supply to the circuit. Each of the FSPs will directly supply power to the trackside equipment such as signals, and points.

The voltage is 650V AC. The FSP will include step-down transformers (not shown in Figure 1 but seen in Figure 9, for example) to provide a local supply of 11 OV AC to the points motor, signal lamp or other load. The step-down transformers also provide isolation of the local load circuits, and so the present description does not concern protection of these load circuits, but rather the trackside supply as such. Traction power is supplied to a train by other means, for example via live rails, overhead cables etc..

Figure 2 shows a fault monitoring system according to an embodiment of the invention. This shows, in addition to the arrangement of Figure 1, a plurality of Disturbance Recorders, DR1, DR2, DR3, DR4, DR5, one being located at the PSP and the others each being located either side of a FSP. ELCD indicates the mandatory (first) earth fault detector mentioned in the introduction above.

CB indicates the circuit breaker which can isolate the circuit when required. In particular, breaker CB is arranged to trip when and isolate both live conductors in response to a short circuit current detected between them. Earth fault detector ELCD and breaker CB would be provided as part of a conventional IT installation, and do not represent additional cost of the novel solution, though they play roles in the diagnostic functions to be described.

The DR units and detector ELCD are preferably connected by telemetry to a supervising controller SUP PLC, which may be located at the PSP, or some other control panel location. Breaker CB may also have some signalling function. Reports from these various devices are combined to determine fault types and location, which can be displayed and printed for attention by maintenance and engineering staff. This avoids the need for trackside access to collect the reported data. Furthermore, switches SW are provided at the PSP which can be controlled by the supervising controller to introduce additional earthing faults' as pad of a location diagnosis procedure, described below. Other remote control functions may be provided by the same type of signalling.

Each DR comprises some analogue and digital circuitry as well as inputs for connection to one or more current transformers (CT5). The Disturbance Recorders can be adapted from monitoring devices such as the Sigma Fault Passage Indicator (FPI) of Bowden Bros. Ltd., Dorking, Surrey, UK (www.bowden-bros.com). In its normal form, this device monitors the current on a circuit using a Current Transformer (CT). A fault current threshold is configured into the unit. The unit latches an indicator when the monitored current exceeds the threshold.

In the present embodiment, each Disturbance Recorder (DR) is also programmed to record a history of recent current values, and freeze the historical record when triggered by an external loss-of-supply input.

Furthermore, the unit used is able to monitor the current in at least two conductors using two separate CTs, and is able to automatically compare these values and raise an alarm if the currents are different. This Residual Current Monitoring (RCM) mechanism is known, per se, and retained for use in the present application. Units with inputs for three CTs are also available, which can be applied in three-phase systems, or in a reduced-cost implementation of the present two-phase system, as described below with reference to Figure 9.

Locating a single first fault When a first fault only has occurred, there is no current flow to earth, so the DRs cannot provide fault location data. However, the mandatory first fault identification mechanism ELCD is at least able to detect the presence of the fault and identify which conductor is faulty.

Figure 3 shows the novel system operating to locate a single first fault. Once it has identified which conductor the fault is on, the appropriate switch SW is closed to induce an additional earth fault' on that same conductor at the PSP (upstream of the first DR's CT). As a result, some of the supply current will be delivered via the earth path to the original first fault, as illustrated by the bold path in Figure 3. Consequently the RCM mechanism in all of the bypassed DRs will trigger (highlighted in Figure 3) because of current imbalance.

Furthermore, the real-time current readings from the DRs will also indicate which is the circuit section with the first earth fault. Therefore the section of the circuit on which the single fault is located is known and can be displayed and printed for attention by maintenance personnel.

The operation of the switch can be temporary, lasting probably a few seconds if the circuit is very extensive and so has a high inherent capacitance; shorter on a smaller network. The introduction of the deliberate earth fault might appear hazardous at first, but it should be remembered that the IT system is inherently tolerant of multiple first faults (faults on the same conductor), and the knowledge gained can lead to faster rectification of the fault in practice, with higher overall safety and reliability as a result. The impedance of the induced fault is a matter of implementation choice, but the detection will be most robust if the impedance is comparable to that of the first earth fault, and so a low impedance is favoured. (It should be remembered in this regard that the network is subject to many transients.) Locating multiple first faults Figure 4 shows the system operating to locate multiple first faults (a first fault is one that does not result in the system tripping). Following the occurrence of a first fault, it is possible for an additional fault to occur in a different circuit section on the same conductor. This will not lead to the supply tripping. But, the occurrence of this additional fault in a different circuit section on the same conductor will now be notified by a current imbalance alarm from the RCM mechanism on the DR units that are between the faults. The bold path in this illustration indicates that there is an earth (insulation) fault between DR1 and DR2, and another fault between DR3 and DR4. Thus DR2 and DR3 register imbalances, and the sections affected by the faults are known.

Location of faults following supply loss Figure 5 shows the situation where there is an earth fault on each conductor.

This results essentially in a short circuit as highlighted, and the system will trip the breakers in the PSP, resulting in supply loss to the whole network. The two faults on the different conductors may be at the same location, or they may be at very different locations in the network.

Figure 6 illustrates that, according to Kirchhoffs law, while there is an earth fault on a conductor, there will be a difference between what was entering the conductor at one end (say 25 amps) and what was leaving the conductor at the other end (say 20 amps). This difference is caused by some of the current (5 amps) being diverted to earth partway along the conductor.

Figure 7 shows the conductors that are subject to this fault current when supply loss is caused by just two faults (one on each conductor).

The historical current values recorded by the DRS at both ends of each circuit section, just before the loss of supply, are compared by the supervising controller. Thus an earth fault can be located to a specific conductor on a specific circuit section by comparing the pre-trip current readings for the conductors in each circuit section. Specifically, the fault in Li can be identified by the pre-trip current differences between DR 3 and DR 4's CIs, and the fault in L2 can be identified by the pre-trip current differences between DR 1 and DR 2's CTs.

The DRs have inputs used as trigger inputs to inform them of the power loss.

A simple implementation of this would be a relay (not shown), connected across the supply at the monitored location. In the event of loss of supply, the relay would de-energise, and a contact connected to the trigger input would open or close, according to the design. The range of historical values stored in response to this trigger should be recent enough to record the fault currents immediately before the trip event, but also long enough to be sure that they pre-date the trip event itself. In the preferred embodiment, a range of samples from different time points prior to the trip are stored as a precaution, as illustrated in the Appendix. Software or human inspection can be designed to use a particular sample, if the best sample is known, or to identify the most reliable data by comparison of all the recorded values after the event.

Figure 8 illustrates the system when there are more than two faults present when supply loss occurs, the difference in current flows at the DRs just prior to the supply loss will reveal the location of all of the faults.

The differences in fault currents allow the location of all faults to be determined as illustrated in Table 1 below. In Figure 8, sections of the circuit between the nearest two fault locations are highlighted in solid bold lines, while the section between there and the third fault location is highlighted in dotted bold line. The comparison of reported current values can be applied to locate any number of faults, in principle.

DR 1 Fault in DR 2 Fault in DR 3 Fault in DR 4 fault this fault this fault this fault curient secflon? cuirent section? current section? curient ________ level __________ level __________ level _________ level Li CT Full Yes Partial No Partial Yes None L2 CT Full Yes None No None No None

Table 1

It should be noted that cables sometimes get short-circuit faults, where one conductor becomes connected to the other. A short-circuit fault will be seen' by the system disclosed herein as an earth fault on both conductors at the same point on the feeder. Normally, a shod circuit fault will also connect to earth, but not always. If a short circuit fault does not also connect to earth, then the system, as described, will be unable to detect any other pre-existing earth faults on the circuit.

Variations & Modifications In the examples above, the current transformers connected to a single DR are applied to the pair of conductors at a single monitoring point on a circuit section, to facilitate local detection of imbalances in current between the two conductors at that point. In other implementations, it may be desirable to apply the current transformers to two points on a single conductor (e.g. conductor Li at the input and output sides of the FSP).

Figure 9 illustrates a variation of the monitoring arrangement at one FSP, which can further reduce the size and cost of the monitoring system. Also seen at the foot of this drawing is the step-down transformer which provides the 11 OV AC supply to the local loads. Instead of using two DRs each having two CTs at the terminations of the upstream and downstream circuit sections, a DR having three CT inputs is used to achieve substantially the same information. The commercially available Bowden Bros Sigma FPI has three CT inputs, intended naturally to monitor three phases at a single point. Two of the CTs are applied to the conductors Li and L2 at (for example) the input side of the FSP, while the third CT is applied to one of the conductors feeding the local load transformer. Provided that we assume that the transformer will draw a balanced load at all times, the currents on Li and L2 on the right hand side can be determined by subtracting (or adding) the transformer current from the left hand Li and L2 currents. These calculations are made digitally in real time within the DR, and the results displayed locally and/or reported by telemetry to the supervising controller.

All the above examples are for illustration only and other embodiments and examples can be envisaged without departing from the spirit and scope of the invention. In particular the foregoing has been exclusively described using the example of a railway circuit, but it is clear that the invention is equally applicable to any IT earthed circuit, regardless of its utility. Therefore no limitation to any particular utility or industry should be inferred.

Furthermore, the description above is inherently simplified and the skilled reader will understand that a practical implementation will include the usual sophisticated signal processing to avoid false triggering by noise, to distinguish fault currents from capacitance and inductance effects, and so forth.

Appendix It is proposed, in a particular embodiment, that each DR unit provides the following data: Digitals -8 bits Bit no 0-state 1-state 0 not triggered triggered 1 circuit not live circuit live 2 RCM alarm clear RCM alarm set 3 spare 4 spare spare 6 spare 7 spare Analogues -32 (12-bit values stored in 16 bits) 0 Cm Real-time current value 1 Cli Real-time value lOs before the trip signal 2 Cli Real-time value 20s before the trip signal 3 Cli spare 4 Cli maximum value detected in the 2Oms period prior to the trip signal 5 Cli maximum value detected in the 4Oms period prior to the trip signal 6 Cli maximum value detected in the 6Oms period prior to the trip signal 7 Cli maximum value detected in the BOms period prior to the trip signal 8 Cli maximum value detected in the 1 OOms period prior to the trip signal 9 Cli maximum value detected in the l2Oms period prior to the trip signal 10 CT1 maximum value detected in the l4Oms period prior to the trip signal 11 Cli spare 12 Cli spare 13 C12 Real-time current value 14 C12 Real-time value lOs before the trip signal 15 C12 Real-time value 20s before the trip signal 16 Cl2 spare 17 CT2 maximum value detected in the 2Oms period prior to the trip signal 18 CT2 maximum value detected in the 4Oms period prior to the trip signal 19 C12 maximum value detected in the 6Oms period prior to the trip signal 20 CT2 maximum value detected in the SOms period prior to the trip signal 21 C12 maximum value detected in the lOOms period prior to the trip signal 22 C12 maximum value detected in the i2Oms period prior to the trip signal 23 CT2 maximum value detected in the l4Oms period prior to the trip signal 24 CT2 spare 25 CT2 spare 26 spare 27 spare 28 spare 29 Residual current value when RCM alarm became set 30 time in minutes since last RCM alarm became set 31 time in minutes since last trip signal Controls -4 P and 4 0 CtlnoP 0 0 reset trigger set trigger 1 reset RCM alarm set RCM alarm 2 spare spare 3 spare spare End of Appendix This application is a divisional application of UK patent application number GB1001420.7 (the "parent application") also published under number GB2469706A.

The original claims of the parent application are repeated below as clauses in the present specification and form part of the content of this divisional application as filed.

Clauses 1. A system for locating insulation faults on a power supply network of a type having an Independent Terra (IT) type earthing arrangement thereby having no earth reference, the network comprising circuit sections of at least two live conductors, the circuit sections connecting together in series a source supply and a plurality of local supply points, the system comprising a plurality of current monitoring devices for detecting and reporting current imbalances between the conductors at respective monitoring points between sections, said system being operable to identify the circuit section subject to said insulation faults by processing the reported current imbalances.

2. The system of clause 1, wherein said system is arranged to determine current imbalances between each of said at least two conductors, at the end of a circuit section.

3. The system of clause 1 or 2, wherein said monitoring points comprise points at or near each local supply point.

4. The system of any preceding clause, wherein said monitoring points include points either side of each local supply point.

5. The system of any preceding clause, wherein said monitoring points include a point at or near said source supply.

6. The system of any preceding clause, wherein said system comprises two live conductors with said current monitoring devices having sensing elements arranged in pairs at each of said number of points.

7. The system of any of clauses 1 to 5, wherein said system comprises three live conductors with said current monitoring devices having sensing elements arranged in threes at each of said number of points.

8. The system of any of clauses 1 to 5, wherein said monitoring device has a trio of sensing elements, two of which are arranged to measure currents on said live conductors of a first circuit section at one side of a local supply point while the third sensing element is arranged to measure current flowing to a local load, the monitoring device or system being further arranged to deduce current flowing in an adjacent circuit section on the other side of said local supply point.

9. The system of any preceding clause, wherein said system comprises means for intentionally introducing a connection from each of said live conductors to earth.

10. The system of clause 9, wherein said means for intentionally introducing a connection from each of said live conductors to earth introduces said connection at or near said source supply.

11. The system of any preceding clause, wherein said monitoring devices reports by telemetry to a central controller.

12. The system of any preceding clause, wherein said system further includes a further monitoring device for detecting the presence of a first earth fault on said conductors, said further monitoring device not indicating the location of the fault.

13. The system of clause 12, wherein said system is arranged to respond automatically to such detection to initiate a fault location procedure.

14. The system of clause 13, wherein said fault location procedure comprises the introduction of an intentional connection to earth.

15. The system of any of clauses 12 to 14, wherein said further monitoring device comprises said mandatory first fault detector of said IT system.

16. The system of clause 14 or 15, wherein said further monitoring device, on detection of a single first earth fault, indicate on which live conductor the earth fault is located, whereupon the system intentionally introduces a further connection to earth on said same live conductor as said single earth fault, and each of said current monitoring devices which are located between said intentional connection to earth and said single earth fault, will to detect and report a current imbalance.

17. The system of clause 16, wherein said system is further operable, upon detection of the location of said single earth fault, but prior to its rectification, to break said intentional connection to earth and to monitor for further earth faults on the same live conductor, said monitoring comprising noting whether pairs of current monitoring devices detect a current imbalance, and if so, which pairs do so, these pairs of current monitoring devices being located between at least two earth faults.

18. The system of clause 17, wherein said system is operable to locate multiple first earth faults by noting which of said pairs of current monitoring devices detect a current imbalance, these current monitoring devices being located between at least two earth faults.

19. The system of any preceding clause, wherein each of said current monitoring devices is operable to store historical current values.

20. The system of clause 19, wherein said historical current values stored comprise current values measured at a plurality of predetermined past time points.

21. The system of clause 20, wherein said current values measured at a plurality of predetermined past time points, includes any or all of instantaneous, average and maximum current values.

22. The system of any of clauses 19 to 21, wherein said current monitoring devices are responsive to a trigger condition to retain and report the last historical current values stored.

23. The system of clause 22, wherein said trigger condition is the loss of supply input.

24. The system of any preceding clause, wherein said network includes a circuit breaker for interrupting supply in the event of a short circuit between two live conductors.

25. The system of any of clauses 17 to 24, wherein said system is operable such that, where supply is lost due to there being earth faults on more than one of said live conductors, each of said earth faults can be located to a particular circuit section by processing the recorded data from the monitoring devices either side of the affected section.

26. The system of clause 25, wherein said earth fault is located by noting where there occurred a difference in the current measured between adjacent current monitoring devices on the same live conductor, shortly before the loss of supply.

27. A method for locating insulation faults on a power supply network of a type having an Independent Terra (IT) type earthing arrangement thereby having no earth reference, the network comprising circuit sections of at least two live conductors, the circuit sections connecting together in series a source supply and a plurality of local supply points, said method comprising: (a) providing current monitoring devices for monitoring current on each live conductor, at a number of monitoring points between circuit sections; (b) using said current monitoring devices to determine the presence of current imbalances at said source supply and at said number of points along the length of said live conductors, and (c) determining, from said determination of the presence of current imbalances, the circuit section in which an earth fault is located.

28. The method of clause 27, wherein said monitoring points along the length of said live conductors comprise points at each local supply point.

29. The method of clause 27 or 28, wherein said monitoring points along the length of said live conductors comprise points either side of each local supply point.

30. The method of any of clauses 27 to 29, wherein a first monitoring point is provided at or near the source supply.

31. The method of any of clauses 27 to 30, wherein said method comprises the step of intentionally introducing a connection from each of said live conductors to earth in order to aid location of a single earth fault.

32. The method of clause 31, wherein said connection is introduced at or near said source supply.

33. The method of clause 31 or 32, wherein said method includes deducing the location of said single earth fault by noting which of said monitoring devices detect a current imbalance between the live conductors, when said intentional connection to earth is made.

34. The method of any of clauses 31 to 33, wherein said method includes, on detection of a single first earth fault, determining on which live conductor the earth fault is located and intentionally introducing a further connection to earth on said same live conductor as said single earth fault, thereby detecting a current imbalance in each of said pairs of current monitoring devices located between said intentional connection to earth and said single ground fault.

35. The method of any of clauses 31 to 34, wherein said method comprises, upon detection of the location of said single earth fault, but prior to its rectification, breaking said intentional connection to earth and monitoring for further earth faults on the same live conductor, said monitoring comprising noting whether pairs of current monitoring devices detect a current imbalance, and if so, which pairs do so, these pairs of current monitoring devices being located between at least two earth faults.

36. The method of clause 35, wherein said method comprises locating multiple first earth faults by noting which of said pairs of current monitoring devices detect a current imbalance, these current monitoring devices being located between at least two earth faults.

37. The method of any of clauses 27 to 36, wherein said method comprises storing historical current values measured by each current monitoring device.

38. The method of clause 37, wherein said historical current values stored comprises the current values measured at a plurality of predetermined past time points.

39. The method of clause 38, wherein said current values measured at a plurality of predetermined past time points, includes any or all of instantaneous, average and maximum current values.

40. The method of any of clauses 37 to 39, wherein said method includes, on receiving a trigger, retaining the last historical current values stored, for example as a result of receiving a loss of supply input.

41. The method of clause 40, wherein said method comprises, where supply is lost due to there being earth faults on more than one of said live conductors, locating each of said earth faults by noting where there occurred a difference in the current measured between adjacent current monitoring devices on the same live conductor, shortly before the loss of supply.

42. A monitoring device for use in the system of clauses 1 to 26 or method of clauses 27 to 41 as set forth above, the monitoring device having multiple sensor inputs and a signalling output, and being programmed (i) to monitor and compare currents in at least two conductors via said sensing inputs, (ii) to detect and report via said signalling output the presence of an imbalance in current between designated pairs of said conductors, and (iii) to store and report historical values of said currents.

43. The monitoring device of clause 42, wherein said historical current values stored comprise the current values measured at a plurality of predetermined past time points.

44. The monitoring device of clause 43, wherein said current monitoring devices are further operable, on receiving a trigger, to retain and report the last historical current values stored, for example as a result of receiving a loss of supply input.

45. A controller for receiving reports from monitoring devices and for processing said reports to implement a system of clauses 1 to 26 or method of clauses 27 to 41 as set forth above.

46. A system for locating insulation faults on a power supply network as herein before described with reference to figures 2 to 4 and 6 to 9.

47. A monitoring device as herein before described with reference to figures 2 to 4 and 6 to 9.

Claims

Claims 1. A monitoring device for use in a power supply network, the monitoring device having multiple sensor inputs and a signalling output, and being programmed (i) to continuously detect current values in at least two conductors via said sensing inputs, (ii) to store historical current values, and (iii) in response to a trigger event, to retain and report via said signalling output the last historical current values stored in an interval immediately preceding the trigger event.
2. The monitoring device of claim 1, wherein said historical current values stored comprise the current values measured at a plurality of predetermined past time points.
3. The monitoring device of claim 2, wherein said current values measured at a plurality of predetermined past time points, includes any or all of instantaneous, average and maximum current values.
4. The monitoring device of any of claims 1 to 3, wherein said monitoring device has a trio of sensing elements, two of which are arranged to measure currents on said live conductors of a first circuit section at one side of a local supply point while the third sensing element is arranged to measure current flowing to a local load, the monitoring device being further arranged to deduce current flowing in an adjacent circuit section on the other side of said local supply point.
5. The monitoring device of any preceding claim, wherein said monitoring device is operable to report by telemetry to a central controller.
6. The monitoring device of any preceding claim, wherein said trigger event is the loss of supply input.
7. A controller for receiving reports from said monitoring device of any preceding claim and for processing said reports to identify faults in the power supply network.
8. A monitoring device as herein before described with reference to figures 2 to
9.Amendment to the claims have been filed as follows Claims 1. A monitoring device for use in a power supply network, the monitoring device having multiple sensor inputs and a signalling output, and being programmed (i) to continuously detect current values in at least two conductors via said sensing inputs, (ii) to store historical current values, and (iii) in response to a loss of supply input, to retain and report via said signalling output the last historical current values stored in an interval immediately preceding the loss of supply input.2. The monitoring device of claim 1, wherein said historical current values stored comprise the current values measured at a plurality of predetermined past time points.3. The monitoring device of claim 2, wherein said current values measured at a plurality of predetermined past time points, includes any or all of instantaneous, average and maximum current values.4. The monitoring device of any preceding claim, wherein the historical current values retained and reported by said monitoring devices include a maximum value of current detected in each of a plurality of different time periods preceding the loss of power.5. The monitoring device of any of claims 1 to 4, wherein said monitoring device has a trio of sensing elements, two of which can be arranged to measure currents on said live conductors of a first circuit section at one side of a local supply point while the third sensing element can be arranged to measure current flowing to a local load, the monitoring device can be further arranged to deduce current flowing in an adjacent circuit section on the other side of said local supply point.6. The monitoring device of any preceding claim, wherein said monitoring device is operable to report by telemetry to a central controller.7. A controller adapted to receive reports from said monitoring device of any preceding claim and to identify faults in the power supply network by processing the reported historical current values.8. A monitoring device as herein before described with reference to figures2to4and6to9. aD