In electrical engineering, live-line working, also known as hotline maintenance, is the maintenance of electrical equipment, often operating at high voltage, while the equipment is energised. Although this is more hazardous for personnel than working on electrical equipment with the power off, live-line maintenance techniques are used in the electric power distribution industry to avoid the disruption and high economic costs of having to turn off power to customers to perform essential periodic maintenance on transmission lines and other equipment. The first techniques for live-line working were developed in the early years of the 20th century, and both equipment and work methods were later refined to deal with increasingly higher voltages. In the 1960s, methods were developed in the laboratory to enable field workers to come into direct contact with high voltage lines. Such methods can be applied to enable safe work at the highest transmission voltages.
Background In general, it is impossible to determine visually whether electrical equipment is energized; in any event, it is often necessary to maintain or repair circuits while they are in operation. In addition, at high voltages, it is unnecessary to come into direct contact with charged equipment to be shocked because an arc can jump from the equipment to a tool or part of the body. Materials such as rubber, while excellent insulators, are also subject to electrical failure at high voltages.
Methods
In general, there are three methods of live-line working which help workers avoid the considerable hazards of live line working. In various ways, they all serve to prevent current flowing from the live equipment through the worker.
Hot stick or Live Line Tool Hot sticks are used in live line work by having the worker remain at a specified distance from the live parts and carry out the work by means of an insulating stick. Tools can be attached to the stick, allowing work to be performed with the worker safely away from the live conductors. Insulating Gloves or Rubber Gloves A live line worker is electrically protected by insulating gloves and other insulating equipment, and carries out the work in direct mechanical contact with live parts. Barehand or Potential The barehanded approach has a live line worker performing the work in direct electric contact with live parts. Before contact, the worker's body is raised to the same electric potential as the live parts, and then held there by electric connection, while maintaining suitable isolation from the surroundings which are at different potentials, like the ground, other people or trees. Because the worker and the work are at the same potential, no current flows through the worker. Unearthed or De-energised Some organizations additionally consider working on unearthed de-energised equipment to be another form of live-line working. This is because the line might become inadvertently charged (e.g. through a back-charged transformer, possibly as a result of an improperly connected, inadequately isolated emergency generator at a customer facility), or inductively coupled from an adjacent in-service line. To prevent this, the line is first grounded via a clamp known as a bond or drain earth. Once this is in place, further work is not considered to be live-line working.
Hot stick
Hot-stick working appeared in the second decade of the 20th century, when insulating poles made from baked wood were used for tasks such as replacing fuses, replacing post insulators, and transferring lines onto temporary supports. The sticks enabled the linemen to carry out the work without infringing on the minimum clearance distances from live equipment. As experience with the techniques developed, then the operating voltages at which the work was performed increased. With the advent of fibreglass poles in the late 1950s, which neither split nor soaked up rainwater, utilities were prepared to carry out hot-stick working to their highest operating voltages, perhaps 765 kV. Tools, such as hooks or socket wrenches can be mounted at the end of the pole. More sophisticated poles can accept pneumatically or hydraulically driven power tools which allow, for example, bolts to be unscrewed remotely. A rotary wire brush allows a terminal to be scoured clean before a connection is made. However, a worker's dexterity is naturally reduced when operating tools at the end of a pole that is several metres long.
Insulating glove or rubber glove working Usually applied for work above 1 kV AC 1.5 kV DC The primary classes are:
Class 00 – phase to phase working voltage 500 V Class 0 – phase to phase working voltage 1.0 kV Class 1 – phase to phase working voltage 7.5 kV Class 2 – phase to phase working voltage 17 kV Class 3 – phase to phase working voltage 26.5 kV Class 4 – phase to phase working voltage 36 kV Gloves protect the worker from exposure to the live part being worked upon sometimes referred to as the 1st point of contact; the point where current would enter the body should an inadvertent contact be made. Covers of insulating material such as blankets and linehose are employed in rubber glove working to protect the worker from exposure to a part at a different potential sometimes referred to as the 2nd point of contact; the point where current would leave the body should an inadvertent contact be made.
Bare hand Bare-hand, or potential working involves placing the worker in direct electrical contact with an energized overhead line. The worker might work alongside the lines, from a platform that is suspended from them, or may sit or stand directly on the line itself. In all cases, the worker's body is maintained at the same voltage as the line. It is imperative that the worker maintain appropriate and adequate limits of approach to any part at a different potential. Such techniques were first used in 1960. There are a number of ways in which the worker can access the live parts:
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