How to Locate Buried Cables

Locating Undergound Cables and Pipes.............................................

In September 2006, a construction worker suffered burns to his face and hands after picking up a cable termination box on a live three-phase 400v cable thinking it was a piece of rubbish. His employer had been aware there were live underground cables at the site but workers had not been informed and no prior cable location scaning or avoidance was carried out prior to commencing digging. The investigating HSE inspector said the victim was extremely lucky to escape with superficial burns.

The majority of accidents involving underground cable services are caused by contractors and subcontractors either failing to locate cables before excavation with suitable cable avoidance tools (CAT's) or not taking all practicable precautions while the excavation is in progress.

As with other areas of hazard management, it is safest to expect the worst - you should always assume that live underground electric cable services are present until it is proved otherwise. It is essential to devise and implement a safe system of work which ensures that those responsible gather all the available information on buried cable services in the area and locate services as accurately as possible with cable detection (Ezi-CAT) or cable avoidance tools (CAT's).

Private property
Wherever possible locate underground cable plans and other information about all buried services in the area well before work starts. You may be able to alter projects to avoid existing services or divert services away from the excavation area.

Most cables belong to electricity suppliers, but even on the highway, some are privately owned. Street lighting cables, for example, may belong to the lighting authority. Electricity suppliers have a legal duty to maintain records and plans of underground cables and to make the information available to enquirers. Similarly, where underground cables belong to private owners, they should make the details available on request. Note that low, medium and high voltage cables may be shown on separate plans - you will need all of these.

Plans held by utilities and private owners are not normally drawn to scale, do not rely on them to obtain distances. Their accuracy may also be limited by changes to reference points such as kerb lines, or due to regrading of the surface since the plans were drawn. That said, they can give a good indication of the position, layout and number of underground services and will help accurate cable location.

Whoever has overall responsibility for the works should record the information on site plans so that it can be understood by the contractor or organisation carrying out the work. It's essential that the consultants or main contractors who receive the information from the electricity authority actually pass it on to subcontractors. Ideally the subcontractor who needs the safety information should be directly involved at the work planning stage.

Location, location, location
To pinpoint the precise position of services in or near the work area, you will need a cable locating device.

There are various types of cable locators :

Hum Devices: these are receiving instruments which detect the magnetic field radiated by live electricity cables. They work only when current is flowing through the cables so, for example, will not detect street lighting cables during the daytime.

Radio Frequency Detectors: these are receiving instruments that respond to low-frequency radio signals, which may be picked up and re-emitted by cables and long metallic pipes. The results can be confused by other metallic objects which may re-radiate the signal.

Transmitter-Receivers: small portable transmitters or signal generators can be connected to a cable or pipe, so that the signal is induced into it ; the receiver can then detect this signal. These cable locators can provide useful information in difficult situations but demand more skill of the operator than other types.

Metal Detectors: conventional metal detectors can be useful in locating flat metal covers and joint boxes, but may well miss round cables and pipes.

Cable locators should be used strictly in line with manufacturers' instructions and should be checked regularly and properly maintained.

The exact line of cable services should be noted on site plans. Paved surfaces should be marked with waterproof crayon, chalk or paint and grassed or unsurfaced areas should be marked by wooden pegs. Avoid steel pins or spikes, which could damage services close to the surface.

After using cable locators, you should carefully dig trial holes, using hand tools, to confirm the cable position. Dig alongside the service, rather than directly above it, and then expose the service digging horizontally. Normally you should stick to spades and shovels and only resort to using picks or forks - carefully - to free lumps of stone or to break up hard layers of chalk or sandstone. A proprietary air-knife, operated by compressed air, is recommended to expose buried services safely.

Once you've uncovered underground services, you must identify them correctly. You should assume all services are live unless you have written confirmation of disconnection from the utility or owner. Electrical buried services will usually be black or red (11kV).

Cable care
Generally, there will be no sign of a buried electrical cable. Underground cables are normally laid in trenches between 45cm and 1m deep, but they are sometimes found just below the surface, particularly where structures have prevented them being laid at standard depths.

Cables may be laid with a bed or surround of fine soil or sand, or in earthenware or plastic pipes or ducts. High voltage cables normally have a layer of cable tiles, slabs or coloured plastic marker tape laid above them. This underground cable protection may have been disturbed and moved - you should not rely on it giving an accurate indication of a cable position.

Occasionally cables are terminated in the ground with a seal, sometimes with external mechanical protection. These "pot-ended" cables should be treated as live; don't assume the cables are abandoned or disused.

Where practicable, avoid using power tools within 500mm of the indicated line of a cable buried in or below a paved surface. When power tools are used to break the surface away from the indicated line, the position of the cable should be positively located by hand digging (as described above). Where a cable is embedded in a paved surface, you should use a cable locator, where possible, to find its depth. Because of the difficulty in confirming depth accurately, you should avoid using hand-held power tools directly over a cable.

Where it's necessary to break away concrete around an embedded cable, you should either ask the electricity supplier to cut power to the cable, or agree an alternative safe method of excavation with the cable owner before work starts. Where cables are dead, work should proceed under a permit-to-work system to ensure the circuit is not re-energised until the work is done and everyone is clear.

Protect all exposed electricity cables against damage, and support those more than 1m long with slings or props. Place planks over cables crossing a trench.

You should protect any cables lying on the bottom of an excavation with wooden planks, troughing or other means, but take care not to use materials or equipment that could penetrate the cables' outer protective sheath. Never tip anything into open cable trenches that could damage the cable.

There is a statutory duty to give adequate notice to the electricity authority of intention to demolish premises. Work should not start until either the authority has confirmed in writing that supplies have been disconnected, or other appropriate safeguarding action has been taken.

Health and Safety Executive guidance document HS(G)47 'Avoiding Danger from Underground Services' provides excellent detailed advice on staying safe from the hazard of underground services. It is strongly recommended that this is read carefully before commencing work on site.

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How to Locate Buried Cables