GB2279761A - Detecting cable faults - Google Patents

Abstract

A capacitive probe has a first plate 11 substantially surrounding a second plate 12. Preferably the plates are coplanar, of equal area, and have circular symmetry. There may be a detector coil 14 or coils (14, 14' Fig. 4 not shown) to assist in locating a buried cable (4). Preferably these are two coils, spaced equal small distances from the axis of the plates. The use of the concentric coils allows greater sensitivity near the fault location than known capacitive probes. <IMAGE>

Description

DETECTING CABLE FAULTS The present invention relates to a method and apparatus for use in the detection of cable faults. It is particularly concerned with buried cables, especially those intended to carry electrical signals, e.g. telephone cables.

At the time of installation or at some time after burial, an underground cable may suffer damage to its outer sheath. This will result in water ingress and almost certainly, as a result, noise faults appearing on the cable. There are many causes of damage, e.g. subsidence, building and roadworks, rodents and insects or the original placement in the ground. One modern low cost method of placing a cable underground is by "ploughing" it in literally dragging it underground with a plough and tractor. This method can severely damage the outer sheath and cause later problems.

Locating a fault may be difficult. Any tool that aids this process may save a great deal of time and abortive excavation.

Two modern methods of fault location are: resistance fault location, and use of time domain reflectometers.

Both have their particular advantages and in skilled hands may give academically accurate fault locations. Their big drawback is that they may accurately measure the distance along a cable but they do not indicate the actual position on the surface of the ground. The distance is not exactly the length along the cable. Cables do not follow a straight path and surface mapping relies on potentially variable reference points. What is needed is a fault locator which will accurately determine where to dig when only a coarse assessment of the fault location is available.

Known techniques of fault location involve applying a signal to the cable such that the signal should be emitted from the fault and the sign current leakage in the ground above should be detectable. There have been two methods.

In the first an "A" frame has a probe in the form of a spike attached to each limb and these are pushed into the ground around the suspected location of the fault so as to pick up currents directly. Using a high gain amplifier it is possible to make audible the signal current leaking into the ground by detecting the difference in electrical potential (usually AC) between the probes.

In a second method, a pair of metal plates mounted some distance (e.g. 60-100cm) apart are used as capacitive signal pick-ups or probes. In this case it is not necessary to penetrate the ground: sufficient signal may be obtained by just placing the plates on the ground.

Both types of device usually also use a coil suitably oriented to receive an electromagnetic signal from the cable. This serves to locate the general direction of the cable so that fault tracing can accurately track the route of the cable underground. It is generally mounted between the probes, so as to be some distance (e.g. 30-50cm) from either of them.

The above methods have a common operating characteristic. Because the input is differential, the signal output is stronger when the input to one probe is stronger than the other. When the input to each probe is identical there is no difference and therefore no output.

These conditions are referred to as peak and null respectively.

A device is moved along the ground above a cable, with the two probes (spikes or plates) spaced along the line of the cable. A reading when the first spike or plate is exactly over a fault should have a peak value. When the device is moved so that the two spikes or plates are symmetrically disposed relative to the fault, there should be a null. Further movement of the device until the second spike or plate is over the fault should then give a second peak. This theoretical output may be observable if tracing a single discrete fault. But faults are usually complex.

The radiation pattern around a fault may not be even and, commonly, the fault is a series of small tears in the cable sheath. Ideally it would be better if the individual faults could be pin-pointed. Typically with the prior art methods the pick-ups, or probes, are spaced apart by about 600-lOOOmm. This can cause very confusing results when faults are multiple and close together. This effect defeats the very purpose of this type of tracer, since a long trench must then be dug to locate the faults accurately.

Of the two above methods the capacitive plate type is better. It does not rely on conductive ground contact to operate and thus can be used for cables extending beneath concrete and asphalt. This method still suffers from the spacing of the plates which causes wide peak-null-peak readings for spot faults and multiple readings for diffuse faults.

The invention provides, in a first aspect, a cable fault detector comprising a capacitive plate detector with first and second sensing plates mounted to support means, and signal processing means for receiving output signals from the sensing plates; wherein the first plate is shaped so as substantially to surround an area, and the second plate is within said area. Thus in general the first plate is annular and surrounds the second plate. The annular first plate is not necessarily a circular annulus, though this is usually preferred. It might be, for example, elliptical, square or rectangular in general form, but preferably with a circular inner periphery. The second plate will generally be symmetrically shaped and disposed in the area. It will usually be evenly spaced from the inner periphery of the first plate. It is preferably circular. Preferably the first and second sensing plates are of equal area. They are generally coplanar, or in parallel planes.

There may also be one or more detector coils, e.g. a single coil which is centrally mounted, coaxial with the sensing plates, or a pair of coils preferably mounted equidistant from the axis of the sensing plates. The detector coil or coils will generally have axes that extend vertically. The axes of a pair of coils may be in the same vertical plane but tilted slightly from the vertical so as to diverge symmetrically from the plate axis in an upwardly direction. Thus where the sensor is over a cable, the two coils may each extend approximately radially of the cable.

In a second aspect the invention provides a method of detecting a cable fault by means of a cable fault detector according to the first aspect.

By configuring the sensing plates as a first substantially annular plate extending around the second plate, this reduces the effective distance between the plates to a minimum. The system still produces a peaknull-peak output but these are now very close together and it can be arranged for this to be perceived as a single peak. With this system accuracy improves considerably over the spaced plate or spike method. A further advantage of this system is that lateral drift from the fault is also detectable if the pick-up plate is symmetrical (circular pick-up pattern). This eliminates the problems associated with angular rotation of linear type devices affecting pick-up strength.

Using an embodiment of the invention can offer some or all of the following advantages: Increased tracing accuracy.

Multiple faults are revealed by individual peaks and nulls.

Diffuse faults with no clear centre will be distinct from closely spaced multiple faults.

Freedom from directional orientation problems.

Electrostatic and Electromagnetic pick-ups may be concentric for greater accuracy.

Reduction in far-field noise pick-up.

A more practical physical construction is possible.

There are many variations possible with this design so only a typical description is given below.

Typically but not exclusively the pick-up head will consist of a support plate with two concentric foil plates - one a circle and the other an annular ring of equal surface area. Typically they will be insulated and may be encapsulated with the support plate. This may be mounted on the end of an extension shaft so that the mounting arrangement is similar to a mine detector, with the sensor head held in front of the user. Alternatively the assembly may be adapted to be carried at a user's side. Usually the detector will include one or more electromagnetic pick-up coils mounted concentric to the capacitive plates or equidistant from their centre, for giving an accurate indication of the cable position. When there are two coils, there may be switching means such that their outputs may be combined either additively or in mutual opposition.

If the signal outputs are arranged so that they are electrically opposing and the coils are placed symmetrically about a cable, the induced electrical currents will be in opposite directions and being of opposite polarity these will add to produce a peak signal immediately above the location of the cable. This is a significant improvement over prior art techniques which rely on a null or zero to detect the location of the cable or a gentle peak signal to locate the cable position. In this arrangement the coils also provide a very strong rejection of power cable signals that can otherwise be overpowering.

The coils may alternatively be switched to the opposite connection arrangement such that they are wired to be electrically aiding. In this arrangement the coil pickup signal produced is stronger for wide area searching producing a soft peak as the cable is approached and a sharp null directly over the cable. This wide area search capability may be used to aid the location of a lost cable where the maximum sensitivity is required and accuracy is not essential.

The detected signals may be represented audibly and/or visually or by some other suitable method such that the user may easily tell the difference between a cable location signal and a cable fault signal.

In the preferred arrangement the two signals are made audible in distinct, separate and easily distinguishable form. Separate visual indicators may also be used.

In the preferred form the tracer may also be used as a cable depth-finder using one of the known electromagnetic location methods.

Some embodiments of the invention will now be described in greater detail with reference to the accompanying drawings in which: Fig. 1 is a schematic circuit diagram of a first embodiment of the invention having a single coil; Fig. 2 is a perspective view of the first embodiment; Figs. 3 and 4 are views similar to Figs. 1 and 2 but showing a second embodiment having two coils; and Fig. 5 is a schematic view showing use of the second embodiment to monitor a buried cable.

Figure 1 shows the electrical arrangement of a typical design. An electrostatic differential pick-up 10 has a first sensor plate which is an annular (concentric) ring 11 of equal area to a second sensor plate which is a central disc 12. A coil 14 is mounted concentrically above the electrostatic pick-up 10. The coil and plates are connected typically to an input stage 13 consisting of high gain amplifiers and tuning filters (usually separate). The output(s) are then fed to further amplifier stages 15 for the purpose of boosting the signals to a level suitable for an audio output and/or for driving visual or other indicating devices indicated schematically at 16. There may be gain adjustment controls to facilitate easy and accurate location. These may be Automatic Gain Control (AGC) devices.

Figure 2 shows a typical assembly arrangement.

Electrostatic pick-up assembly 19 is a pair of insulating plastic discs 20 between which the foil pick-ups 11,12 are sandwiched and encapsulated. The electromagnetic pick-up coil 14 is mounted coaxially. A mounting shaft/handle 21 extends at an angle to the plane of the assembly 19 so that the lower planar face thereof can easily be swept over the surface of the ground. A box 22 houses the electronic circuit and output devices.

The second embodiment is in most respects very similar to the first. The electrostatic pick-up assembly 19 is the same. However now there are two sensing aids 14,14' which are connected via a switching assembly 24 which enables the coils to be connected so as to be mutually electrically aiding or opposing. Thereafter the input stage 13, further amplification stage 15 and display means 16 are essentially as for the first embodiment.

Fig. 4 shows a possible practical arrangement of the second embodiment. (Either embodiment could have either type of support arrangement.) The pick-up assembly 19 may be as for the first embodiment. It is mounted at the bottom of a carrier 31 which is shown here as a vertically elongate housing having a carrying handle 32, a control panel 33 and a visual display panel 34 adjacent its top.

(Alternatively or additionally there may be audio output means, e.g. employing headphones. The housing is shaped and dimensioned so that it can be carried at a user's side, the user being to the right or left in Fig. 4. The figure shows the coils 14,14' mounted with their axes tilted so that, if extended, they would meet some way beneath the detector. The tilt (which may be absent) is exaggerated in Fig. 4 for clarity. It is desirable that the axes meet at a depth typical of the depth at which cables 40 are to be expected. The coils 14,14' are horizontally spaced by about 100-400mm, typically 200mm.

Figure 5 illustrates the normal connection arrangement and detecting method. A signal source 43 in the form of an oscillator is connected to faulty conductors in a cable 44 and to ground. The buried cable 44 may be traced along its underground route by using the coil arrangement 14,14', and monitoring their outputs as explained above. The presence of the fault will be indicated by a characteristic output from the electrostatic pick-up assembly, generally in the form of a single peak as shown at 47, as the pick up assembly 19 is passed over it. Because a fault's signal radiates spherically the peak will occur when the circular concentric assembly 19 is directly above the fault and centred on it. Thus the location is accurate in all horizontal directions and will be free from the normal errors associated with linearly arranged plates or groundprobe spikes.

The arrangement may be used in this or other forms for cable depth location using prior art triangulation detection methods.

Claims (17)

Claims

1. A cable fault detector comprising a capacitive plate detector with first and second sensing plates mounted to support means, and signal processing means for receiving output signals from the sensing plates; wherein the first plate is shaped so as substantially to surround an area, and the second plate is within said area.

2. A detector according to claim 1 wherein the first plate is annular and surrounds the second plate.

3. A detector according to claim 1 or 2 wherein the first plate substantially surrounds the second plate at a constant spacing.

4. A detector according to claim 2 or 3 wherein the first plate has a circular inner periphery.

5. A detector according to any preceding claim wherein the first plate is a circular annulus which surrounds the second plate.

6. A detector according to any preceding claim wherein the second plate is circular.

7. A detector according to any preceding claim wherein the first and second sensing plates are of equal area.

8. A detector according to any preceding claim including at least one elongate detector coil whose axis extends vertically; and signal processing means for receiving signals from the coil or coils.

9. A detector according to claim 8 having two vertically extruding detector coils disposed symmetrically over the capacitive plate detector and horizontally spaced apart.

10. A detector according to claim 9 including switching means coupled to the two coils for selectively combining their outputs additively or in opposition.

11. A detector substantially as herein described with reference to and as illustrated in the accompanying drawings.

12. A sensor for a detector according to claim 8, 9, or 10, having a pair of capacitive sensing plates and at least one detector coil.

13. A sensor for a detector substantially as herein described with reference to and as illustrated in the accompanying drawings.

14. A method of detecting a cable fault by means of a detector according to any of claims 1 to 11 wherein the detector is moved over the cable and the output of the capacitive plate detector is mounted.

15. A method according to claim 14 wherein the detector is according to claim 8, 9 or 10 and the detector is located in relation to a buried cable by monitoring the output of the coil or coils.

16. A method according to claim 15 wherein there are two coils and the cable is first located roughly by monitoring their added output and then located more accurately by monitoring their output combined in opposition.

17. A method of detecting a cable fault substantially as herein described with reference to and as illustrated in the accompanying drawings.