Background Art Before commencing digging or boring at a site where underground cables are present it is necessary to mark out their location.

Site maps, drawn up when a cable was originally installed may be available, but are not always accurate. Telecommunication cables are typically installed in conduits, usually of plastics material, with personnel access chambers located at regular intervals along the conduits. However, one cannot assume that a conduit will take a straight line course between adjacent chambers, as the line taken by the conduit will depend on the soil structure and on any obstructions in the path between the access chambers.

Where the telecommunication cables are of the optic fibre type, accidental severing of one of these cables can be very costly as it will disrupt communication along the cable. This results in costs being

incurred by the companies using the cable which can result in compensation claims. For this reason cables are heavily insured.

Devices are available for tracking underground cables, prior to digging, by employing metal detectors. The line of the cable is marked out on the surface of the ground. However, these devices are subject to inaccuracies, particularly in the measurement of the depth of the cables underground. It will be understood that a number of conduits may be layed between adjacent chambers and they will not all travel the same path or be at the same depth below ground.

Other systems employ conductive lines associated with the conduits through which a current may be passed which in turn is detected at the surface. Again such systems are subject to inaccuracies in measurement and can only be used where the conductive lines were layed with the conduits from the outset.

Patent Publication DE A 19614707 describes a system which employs an auxiliary conductor in close proximity to the cables or pipes to be protected. The conductor is activated by low power RF energy at e. g. 10 kHz which is radiated through the intervening ground to a machine equipped with a receiver and suitable warning instruments.

Thus, when the machine comes close enough to the cable the receiver will pick up the signal from the auxiliary conductor and provide a warning to the machine operator.

However, again this system requires the laying of the auxiliary conductor along with the cables from the outset and is therefore of limited usefulness.

Thus, what is required is a device for detecting the proximity of underground cables during digging or boring which overcomes the disadvantages of the devices and methods described above.

Disclosure of Invention Thus, the invention provides a device for detecting the proximity of an underground cable conduit during digging or boring, comprising a microphone suitable for locating in close proximity to the conduit, the microphone being in communication with a detector unit, and means within the detector unit for converting a sound picked up by the microphone due to digging or boring adjacent the conduit into a warning signal detectable by a person engaged in the digging or boring.

The device in accordance with the invention is typically used where digging is taking place with a mechanical digging machine or digger in an area where underground cables have been layed in conduits. A microphone is placed in close proximity to the conduit ahead of the digging and movement of the digger bucket close to the conduit will be detected by the microphone resulting in a warning signal being generated.

The person operating the digging machine would have a rough idea of the position of the cables and would be on the alert for any warning signal generated by the device. As soon as a warning signal is

detected digging can be stopped, before any damage is caused to the cable, and the actual position of the cable can be noted. Thus, use of the device reduces the risk of damaging a cable and also stress on the person operating the digging machine resulting from the fear of causing some damage.

The device in accordance with the invention may also be used where a directional boring system is in operation. Again any movement of the boring head close to the conduit will be detected by the microphone resulting in a warning signal being generated.

Preferably, the microphone is adapted for location in the conduit at a conduit access chamber.

Underground cables are typically layed in conduits provided with personal access chambers at regular intervals along the line of the conduit. The microphone can thus be placed in the conduit at an access chamber ahead of the digging or boring.

Alternatively, preferably, the microphone is adapted for location in close proximity to an external surface of an exposed section of the conduit.

Where a personal access chamber is not located near the area where the digging or boring is to take place, a section of the conduit can be exposed by careful removal of the overlying earth. The microphone can then be placed against the outer surface of the conduit.

Suitably, the microphone is mounted on means for maintaining the microphone in close proximity to the conduit.

It is important to locate the microphone close to the conduit surface and to maintain it in position during the digging or boring.

Therefore, mounting the microphone on a suitable means for maintaining it is position increases the efficiency of the device.

Preferably, the means for maintaining the microphone in close proximity to the conduit is spring-loaded.

Further, preferably, the means for maintaining the microphone in close proximity to the conduit is a generally U-shaped resilient holder, to which the microphone is attached at one end thereof, the distance between the ends of the U-shaped holder being substantially equal to the diameter of the conduit, such that in use the ends of the U-shaped holder may either be squeezed together for placement within the conduit, or the ends may be forced apart for placement on the outside of the conduit.

The U-shaped resilient holder is advantageous as it can be used for maintaining the microphone in close proximity to the conduit either within the conduit or adjacent an external surface thereof.

Suitably, the microphone is rotatably attached to the end of the U- shaped resilient holder.

By rotably mounting the microphone on the U-shaped resilient holder the microphone can be orientated towards either the inner or outer surface of the conduit.

Preferably, the warning signal is visual.

Thus, the warning signal can take the form of a warning light which will be seen by either the operator of the digging machine or by a banksman. A banksman is a person who overseas the digging operation by standing next to the area being dug.

Further, preferably, the warning signal is in the form of a flashing light.

Provision of a flashing light increases the probability of notice being taken of the warning signal as early as possible.

Alternatively, the warning signal is audible.

The warning signal could take the form of a loud siren which would be audible over the noise of the digging machine.

Preferably, the warning signal is both visual and audible.

Having both a visual and audible signal results in a more efficient device. Where either the visual signal is obscured or the audible signal is missed due to other interfering noises in the site, the complimentary signal should work and thus alert the operators.

In one embodiment of the invention, the detection unit is locatable adjacent the part of the conduit where the microphone is located in use.

Such a device would be located within the conduit at an access chamber or against an outer surface of the conduit with a warning signal being provided by either a light visible above ground or a sound generated also above ground. Such a device would be relatively simple in construction.

In a further embodiment of the invention, the detection unit is locatable at a place remote from the part of the conduit where the microphone is located in use.

The advantage of such a device is that the detection unit could be placed in the cab of the digging machine close to the operator. Thus, any warning signal would be immediately detected by the operator, who would then stop digging.

Alternatively, the detection unit could be carried by the banksman who would relay any warning to the operator. In certain situations the banksman could probe the area for digging ahead of the digger and any sound transmitted to the microphone by such probing would be detected by the banksman or operator, and the exact location of the cable could be established. The banksman can also use the probe within a trench being dug where there is a fear that the digger bucket could be close to the conduit.

Preferably, the probe used by the banksman would be calibrated along its length so that when the conduit is located the linesman can note the depth that the conduit is below the surface.

Suitably, the microphone is connected to a radio transmitter unit and the detection unit has means for receiving a signal from the transmitter unit.

A radio transmitter and receiver provide a convenient method for transferring the warning signal to a location remote from the conduit access chamber.

A radio frequency in the citizen band range can be used.

Alternatively, a licenced frequency can be used and the use of such a closed frequency would reduce any radio interference to a minimum.

Advantageously, the device has a plurality of microphones in communication with the detector unit.

Thus, in situations where a number of conduits have been laid between access chambers, each conduit can be monitored using an individual microphone. Movement near any of the conduits will then activate the device to provide a warning signal to the operator.

Brief Description of Drawings The invention will be further illustrated by the following description of an embodiment thereof, given by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of a device according to the invention;

Fig. 2 is a schematic representation of the transmitter unit of the device of Fig. 1; Fig. 3 is a schematic representation of the detection unit of the device of Fig. 1; Fig. 4 is a representation of a microphone mounted on a U-shaped resilient holder according to the invention; and Fig. 5 is a schematic representation of the microphone of Fig. 4 located within a conduit.

Modes for Carrying Out the Invention Referring to Fig. 1, there is illustrated a device according to the invention for detecting the proximity of an underground cable conduit during digging, comprising four microphones 10 located within conduits 11, within a conduit access chamber 12. The microphones 10 are connected to a radio transmitter unit 13 by cables 14. An antenna 15 is connected to the radio transmitter unit 13 by a cable 16. The antenna 15 is provided with a spike 17 for fixing the antenna 15 in the ground 18. The microphones 10 are in communication, through the radio transmitter unit 13, with a detector unit 19 located in a digging machine shown generally at 20. The detector unit 19 is located in the cab 21 of the digging machine 20. The digging machine operator 22, located in cab 21 of the digging machine 20, is provided with a headset 23 which is connected by a cable 24 to the detection unit 19. A receiving antenna 25 is connected to the detector unit 19 by a cable 26 (see Fig. 3).

In operation, when the bucket 27 of the digging machine 20 is operated in close proximity to one of the conduits 11, a sound wave is transmitted therethrough and is picked up by the corresponding microphone 10. A signal from the microphone 10 is converted in the radio transmission unit 13 into a radio signal which is transmitted, via the antenna 15, and is picked up by the receiving antenna 25. The signal is then converted in the detector unit 19 into an audible signal detectable on the headset 23 and a visual display (not shown) located on the detector unit 19.

Referring to Fig. 2, the radio transmitter unit 13 of the device according to the invention is illustrated in more detail.

The radio transmitter unit 13 comprises a housing 30 having connectors 31 mounted on a surface 32 thereof, and to which the microphones 10 are connected via cables 14. An on/off switch 33 is mounted on the surface 32 together with an associated indicator light 34 and a fuse 35.

A connector 36 on the surface 32 receives the cable 16 of the antenna 15. A battery 37 is connected via a cable 38 to the housing 30 at a connector 39.

Each microphone 10 is partially surrounded by sound-proofing material 40 to cut down on any interference due to background noise.

The sound-proofing material 40 also acts as a means for maintaining each microphone 10 in close proximity to the conduit (not shown).

The radio frequency used by the radio transmitter unit is 28 mHz.

When a number of devices in accordance with the invention are being used on the same site separate frequencies can be used to avoid confusion.

Referring to Fig. 3, the detector unit 19 of the device according to the invention is illustrated in more detail.

The detector unit 19 comprises a housing 41 having an on/off switch 42 mounted on a surface 43 thereof, together with an associated indicator light 44 and a fuse 45.

The antenna 25 is connected via the cable 26 to the housing 41 by a connector 46 on the surface 43. A battery 47 is connected to the housing 41 at a point 48 on the surface 43 via a cable 49. The headset 23 is connected via the cable 24 to the surface 43 at a connector 50. The cable 41 includes a volume control 51.

A visual display unit 52 is connected via a cable 53 to the housing 41 at a point 54 on the surface 43. The visual display unit 51 includes an array of indicator lights 55 which progressively light up, in use, depending on the level of the warning signal. A suction cup 56 is attached to the visual display unit 51 and is used for mounting the visual display unit 51 on an internal surface of the cab 21 within view of the operator 22.

Referring to Fig. 4 there is indicated generally at 80 a U-shaped resilient holder made from a steel bar 81 and having a microphone unit 82 rotatably mounted at one end 83 thereof. The microphone unit 82 can

swivel so that microphone head 84 can be orientated away from the other end 85 of the resilient holder 80, as illustrated, or alternatively, it can be orientated so that the microphone head 84 is facing the other end 85.

In use the microphone head 84 is orientated away from the other end 85 when the microphone unit 82 is to be placed in a conduit.

Alternatively, the microphone head 84 is orientated towards the other end 85 when the microphone unit 82 is to be placed against an outer surface of a conduit.

The resilient nature of the holder 80 means that the microphone unit 82 will be maintained firmly in place whether it is within or without the conduit.

A microphone cable 86 is attached to the holder 80 using suitable clips 87.

Referring to Fig. 5 the microphone unit 82 of Fig. 4 is shown in position within a conduit 88. The microphone head 84 is maintained in position against an inner surface 89 of the conduit 88 due to the resilient nature of the holder 80.

In order to place the microphone unit 82 into conduit 88, the ends 83 and 85 of the holder 80 are first squeezed together. The microphone unit 82 is then pushed into the conduit 88 and when in position the holder 80 is released causing the ends 83 and 85 to move apart and the microphone unit 82 to be held firmly in position.

If it is desired to attach the microphone unit 82 to an outside surface of a conduit then the ends 83 and 85 of the holder 80 are first forced apart and then clipped to the outside of the conduit.