Hazard Related Topics
Guidance Note GS6 produced by the Health and Safety Executive "Avoidance of Danger from Overhead Electric Lines" and the HSE Booklet HS(G)47 "Avoiding Danger from Underground Services" gives detailed guidance and sets out procedures for working in the vicinity of overhead and underground power lines of any voltage. There is a legal obligation that any activity shall be carried out in such a manner as not to give rise to danger. Designers have a responsibility in this area by designing structures with an awareness of power cable constraints on the positioning of the structures and indeed, where possible, designing access routes to avoid contact by vehicles and plant.
It must always be assumed that overhead and underground cables are live unless categorically proved to the contrary!! Contract documents should require that all operations are carried out in accordance with this advice.
Where existing overhead telephone cables are retained beyond the completion of the project they should be identified within the Health and Safety File.
All existing overhead High and Low Voltage Cables must be identified within the Health and Safety Plan. They should be illustrated on plan and particularly described where they are affected by the project. In particular site access routes for vehicles and especially heavy plant such as cranes should be planned to avoid contact with such cables.
The advice, conditions, and codes of safety of the electricity authorities should be identified and copied as necessary into the Pre-construction Information.
Where existing overhead electricity cables are retained beyond the completion of the project they should be identified within the Health and Safety File.
Specimen or Standard Clause
- The position of existing electric cables identified on the drawings is based on information supplied by the (named) Electricity Company. The Client does not guarantee accuracy and the Contractor must make his own inquiries to determine the position of electricity cables and services.
- The Contractor shall comply with Guidance Note GS6 produced by the Health and Safety Executive "Avoidance of Danger from Overhead Electric Lines" and the HSE Booklet HS(G)47 "Avoiding Danger from Underground Services".
- The Contractor must have in place and have issued to his employees written procedures for carrying out works within the vicinity of live electric cables.
- The Contractor shall comply with the relevant clauses of the specification detailing how operations shall be carried out in the vicinity of electric cables.
Where the routes of any such services are not definitely known or where services can be damaged by machines carrying out excavation work, the services must be uncovered by hand and supported, if necessary. Where the service is not in the direct line of the excavation, adequate protection (mats) must be provided where necessary.
The use of Plans
Cable plans are not completely accurate scaled drawings but will give a good indication of the line, depth, number and configuration of recorded cables in any particular area, and will thus help to make the subsequent tracing by locators as accurate as possible. Anyone in charge of the work on site, and operators of cable locators, should appreciate that the accuracy of cable plans is limited because:
- cables may be found snaked within the width of the original cable trench, even though marked as straight lines on plans;
- cable routes may be marked on plans relative to reference points whose position has changed since the plans were drawn;
- cable depths shown on plans may be incorrect due to regrading the surface after the cables were laid;
- cables may have been moved without the authority or knowledge of their owners;
- not all service cables, or cables serving street lighting columns etc. are necessarily marked on the plans provided by electricity companies and other owners. The existence of such cables should be assumed even if not shown on the plan.
Adequate instruction and training in how to read and interpret cable plans should be given to anyone who needs to use them. The symbols employed on these plans will vary between electricity companies, therefore advice should be sought from the issuing office.
The use of Cable Locating Devices
Cable locating devices should be used to trace the line of electricity cables before excavation starts: it may be necessary to carry out repeat tests as the excavation proceeds. Although cable locators can be used by themselves to trace cables, where possible they should be used in conjunction with cable plans or similar suitable information. This will help the operative to interpret the locator signal, and hence give the maximum information to those involved with the work before digging starts.
Locating devices should always be used in accordance with the manufacturer's instructions, and employees should receive adequate training in their use.
Live cable detectors are simple to use, but they do not respond to unloaded or direct current cables and they may fail to detect lightly loaded low voltage cables, such as those used for street lighting, or well balanced high voltage cables.
If VLF detection is used other metallic objects may re-radiate the signal and results may vary appreciably according to locality, length of the buried cable or pipe and distance from the termination, and geographical orientation.
Currently available transmitter-receiver instruments require more skill to operate and are less suitable for non-technical personnel.
A cable locator may be unable to distinguish between cables running close together and may represent them as a single cable. If two cables are sited one above the other it may not be possible to detect the lower cable.
Since the detection of cables with a locator cannot be guaranteed, the absence of a positive indication must not be taken as proof that a cable is not present, or that any cable that is present is not live.
If a cable recorded on an electricity company plan cannot be located, appropriate assistance or advice should be sought. If digging has to start before such assistance or advice has been obtained, extreme care must be taken.
Safe Digging Practice
In residential and urban areas, private land near to substations, waterway footpaths, railway property, etc., the presence of live cables must be assumed, particularly where there are no overhead lines. In most cases, there will be no permanent surface marker posts or other visible indication of the presence of a buried cable. Even if no cable is shown on plans or detected, a close watch should be kept for any signs which could indicate the presence of a cable.
Although underground cables are normally laid in trenches between 450mm and lm deep, they can often be found just below the surface. This should always be borne in mind, particularly if the ground has been disturbed or if there are cellars or structures such as bridges in the area which may have prevented cables being laid at standard depths. Even the shallowest excavations may be a source of danger.
Cables may have been laid directly in the ground with a bed or surround of fine soil or sand, or in cement-bound sand, or in earthenware or plastic pipes or ducts, or they may have a layer of tiles, slabs or coloured plastic marker tape laid above them. Such protection may have been disturbed and moved and should not be taken as an accurate indication of a cable position.
Although high voltage (HV) cables normally have tiles or marker tapes laid over them, low voltage (LV) cables may be laid without separate protective cover. This lack of warning underlines the importance of safe digging practice.
Some cables have a protective layer of steel wire or tape, while others have a light serving of hessian or PVC overlaid on aluminium or lead sheaths. Plastic sheathed cables are usually coloured black or red and usually have the legend "electric cable" embossed through their length.* However, some plastic sheathed electricity cables look very much like plastic water pipes and can also be easily mistaken for telephone cables, while in some coal mining areas the National Coal Board cables are coloured yellow and may be mistaken for gas pipes. In cases of doubt the electricity company or probable owner should be consulted.
* NJUG Publication No 4 "The identification of small buried mains and services."
Hand tools used incorrectly are a common source of accidents but their cautious use can normally provide a satisfactory way of exposing buried cables once the approximate positions have been determined using plans and cable locators. Spades and shovels should be used in preference to other tools; they should not be thrown or spiked into the ground, but eased in with gentle foot pressure. Picks, pins or forks may be used with care to free lumps of stone, etc. and break up hard layers of chalk or sandstone. Picks should not be used in soft clay or other soft soils near to cables.
Using hand-held power tools to break up paved surfaces often leads to accidents. Where practicable such power tools should not be used within half a metre of the indicated line of a cable buried in or below a paved surface. The cable should be positively located by excavating trenches half a metre from the indicated line and digging carefully by hand under the paved surface: where possible a cable locator should be used as a depth guide down the side of the excavations. If the cable cannot be found by this method it should be assumed to be embedded within the paved surface. The half metre safety margin may be reduced in certain circumstances, for example where congestion of buried cables renders it impracticable, but only if the line of the cable has been positively identified by plans and confirmed by a locator. Because of the difficulty in confirming depth, hand-held power tools should never be used over the line of the cable except where the cable has already been exposed by digging under the surface to be broken out and physical precautions taken to prevent the tool striking the cable.
Where machine excavators are used in the possible vicinity of underground cables, the work should be arranged so that everyone is kept well clear of the excavator bucket while it is digging. If a cable is struck, the driver should stay in the cab. If the driver has to leave the cab he should jump clear and not climb down, otherwise electrocution may result. No one should go down into the excavation or touch the excavation until the cable owner has attended to make the damaged cable safe.
During digging work a careful watch should be kept for evidence of cables and repeat checks with a cable locator may be necessary in order to determine the position of any cable more precisely as signals become clearer. It is important to remember that a cable is positively detected only when it has been safely exposed, and even then digging must still proceed with care since there may be other cables, particularly HV cables, adjacent or lower down.
Where it is necessary to break away or disturb concrete in which cables are embedded, the cables should be made dead or an alternative safe method of excavation agreed with the electricity board or other owner of the cable before work starts.
When there is any doubt about the identity of an exposed service it should be treated as a 'live' cable. Occasionally, cables are terminated in the ground by means of a seal, sometimes with external mechanical protection. These 'pot ended' cables should be treated as 'live' and should not be assumed to be abandoned or disused.
Where a cable exposed for more than l metre crosses a trench, it should be supported with slings or props, and suitable planks should be placed over it. Exposed cables should not be used as hand or foot holds by anyone climbing in or out of the trench. Any cables lying in the bottom of an excavation should be protected by nail free wooden planks, troughing or other suitable means but care must be taken not to use materials or equipment which could penetrate the outer protective sheath of cables. In addition, precautions should be taken to prevent access to exposed cables by children or other unauthorised people. If a cable has to be moved, the electricity board or other owner should be consulted.
Hard or sharp materials such as pieces of rock, large stones, hard-core or surplus concrete should not be tipped into open cable trenches. Advice on back filling cable trenches can be obtained from electricity boards.
Particular care is necessary when gas leak search techniques such as barholing are employed: there could be buried cables. Reference should be made to Transco guidance material on this subject.
The principal hazard associated with overhead cables is of electrocution of plant operators. This is often in the form of the excavators and cranes but tipping lorries have discharged the load and driving off site with the body still raised is a continuing source of injury.
Having identified the presence of overhead cables, the following procedure should be adopted:
- Overhead electrical services, which are a source of risk, should be requested to be switched off. This may be achieved on a temporary basis by the controlling electricity company providing an isolation certificate for the duration of at least part of the works.
- If it is not possible to have the service isolated, an approach should be made to the electricity authority to have the service diverted to a safe part of the site or indeed of the site altogether.
If (a) or (b) above are not possible then steps should be taken to ensure that no item of plant can be sited in a position where either contact with its service or arcing can occur this will normally be achieved by the provision if a fences or other ground level barriers paced at a distance of 1 jib or boom length plus 6 metres from the service. In circumstances where there is insufficient space to permit this clearance distance, then the maximum possible distance may be permitted provided that an absolute minimum of 6 metres horizontal clearance between the service and the barrier is maintained. If the distance between the service and the barrier is less than the jib length and 6 metres then in addition to the ground level barrier, high level bunting should additionally be provided in line with the barrier.
If it is necessary to gain access under the electric service then a gap should be made in the fence and good posts should be installed on either side of the service and the ground level barrier provided linking the goalposts to delineate the access route. The height of the goalposts will be specified in each case by the controlling electricity authority. Goalposts should preferably be made of timber or other non conducting material. These shall be clearly marked normally by painting in red and white stripes. A notice, warning of the goalpost should be placed approximately 30 metres in advance of them to give crane drivers warning that they have to lower their jib.
Where the access route under the cable is particularly wide such that timber crossbars cannot be used then tensioned ropes may be substituted. These should be no closer than 12 metres from the nearest cable, bunting or similar should be attached to the rope. Steel ropes must be effectively earthed.
If it is necessary to work under live overhead electric cables then only plant which is not capable of reaching the arcing distance may be used. Alternatively, machines may be fitted with restraints so that their movement is adequately restricted.
Steps must be taken to ensure that no person is endangered by contact with the overhead cable during the erection of goalposts. This may be achieved by ensuring that the goalposts are sufficiently far back from the cables.
Warning notices and crossbars will be illuminated during hours of darkness whilst work is in progress.
No offices, mess rooms, stores, toilets, stockyards, etc. must be sited under any overhead electric cables.