8.4 Safe working in the vicinity of high voltage systems

Question 1

As mentioned in Element C8.1, high voltage is generally considered to be

Answer 1

greater than 1000 V (1kV) AC or 1500 V DC.

Question 2

A safe system of work for working near buried services has four basic elements:

Answer 2

(1) Planning the work (2) Plans (3) Cable-locating devices (4) Safe digging practices.

Question 3

Outline the precautions that can be taken to minimise the risks associated with a cable strike when excavating near underground electrical cables. 20 marks

Answer 3

If excavation work is to be carried out near underground electrical cables, it is essential that the work is properly planned before it starts. This involves:  seeking advice from the relevant body such as the appropriate electricity supply company or the highway/roads authority  assessing electricity cable plans on the understanding that symbols may vary between utilities and that high and low voltage cables can be shown on separate plans  making the supply dead if this can be done or investigating the possibility of diversion  avoiding the use of mechanical excavation and utilising a permit to work system. Before the actual excavation work starts, it will be necessary to locate the presence of cables by the use of cable avoidance tools (CATs), such as Hum or radio frequency detectors, and their identification and routes clearly marked. If a cable, which is recorded on a plan, cannot be located, assistance and advice should be sought from the appropriate authority, with extreme care being taken if excavation work begins before the assistance and advice has been obtained. The excavation work should be carried out only by personnel who have been trained and are competent in safe digging practice. They would need to be aware that although no cables are indicated on plans or located by a detector, they may still be present and though cable depths are often prescribed, they should not assume that this is always the case. Additionally, though cables often have protective covers, the assumption should not be made that this is universal practice. Workers should also be aware that though a cable has been exposed, there may be further cables at a lower level, and if cables are found which are terminated with pot ended or bottle ended seals, they may still be live. When cable joints are discovered, they should be supported, should not be moved, and should not be roughly treated.Further precautions that should be taken during the excavation work include the need to look out for permanent marker posts or other visible indication of buried cables; to establish positive location by careful hand digging, exposing the cables by digging from the side and using hand held power tools only at a distance of 500 mm or more from the indicated lines of cables. If mechanical excavators have to be used in the vicinity of cables, then damage should be avoided as far as is reasonably practicable. Personnel should be kept well away from the excavator, and drivers instructed to stay in the cab if a cable is struck. If they have to leave the cab in such circumstances, they should jump clear. Once cables are exposed, any damage noted should be reported to the cable owners and no work undertaken in the vicinity. Support should be provided if more than one metre of cable is exposed, and protection, such as the use of planks or sandbags, should be provided to prevent damage from on-going work. Finally, cables should not be moved aside unless the work is supervised by the cable owners.

Question 4

The following hierarchy of precautions, from the HSE guidance Avoiding danger from overhead power lines (GS6), should be applied to work at or near overhead power lines:

Answer 4

 Avoid working under or near overhead lines.  Divert overhead lines clear of the work area.  Make lines dead while the work is in progress.  Work around the live overhead lines using appropriate precautions.

Question 5

Precautions depend on the nature of the work - there are three broad categories of work:

Answer 5

1 No scheduled work or passage of plant under the lines Use barriers to prevent close approach. 2 Plant will pass under the lines Defined passageways should be made. 3 Work will be carried out beneath the lines Further precautions are required in addition to the erection of barriers with passageways.

Question 6

Construction work is to take place in a rural area where electrical power for the site is to be gained from an existing 11kV overhead supply that cuts across the site on wooden poles. Outline control measures that should be taken to reduce risks associated with the: (a) overhead supply 8 marks (b) provision and use of electricity on site. 12 marks

Answer 6

(a) Control measures associated with work near an overhead electricity supply are covered in the HSE guidance GS6 – Avoiding danger from overhead power lines, and include starting by considering the possibility of re-routeing the lines and pointing out that, before any actions were recommended by way of protective measures, the utility supplier would need to be consulted. Other control measures include:  identification of safety distances  the use of barriers, marking tape and bunting  the use of goal posts and tunnels where access routes pass under the power lines  height restrictions on plant  the introduction of safe systems for working near to power lines  a restriction on the storage of materials within six metres of the overhead lines  the prohibition of using hand-held long metal equipment such as ladders  the erection of warning signs  the provision of training and information to the workforce on the precautionary measures that should be taken. (b) Control measures include:  planning and assessment for the development of the electricity supply by a competent person  the suitable connection and positioning of step-down transformers  the routeing, marking and protection of cables  the use of protective measures such as reduced low voltage systems, residual current devices and double insulated equipment  arrangements for the regular inspection and testing of the fixed supply including the integrity of the earthing of the system  exercising control on the equipment brought on to the site including portable items  the use of competent persons for the installation of and work on the electrical supply.