Flashcards in 8.4 Safe working in the vicinity of high voltage systems Deck (6)
As mentioned in Element C8.1, high voltage is generally considered to be
greater than 1000 V (1kV) AC or 1500 V DC.
A safe system of work for working near buried services has four basic elements:
(1) Planning the work (2) Plans (3) Cable-locating devices (4) Safe digging practices.
Outline the precautions that can be taken to minimise the risks associated with a cable strike when excavating near underground electrical cables. 20 marks
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.
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:
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.
Precautions depend on the nature of the work - there are three broad categories of work:
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.