Live-line working

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. 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. Electricity is hazardous: an electric shock from a current as low as 35 milliamps is sufficient to cause fibrillation of the heart in vulnerable individuals. Even a healthy individual is at risk of falling from a high structure due to loss of muscle control. Higher currents can cause respiratory failure and result in extensive and life-threatening burns. The first recorded human fatality occurred in 1879 when a stage carpenter in Lyon, France touched a 250 volt wire. The lack of any visible sign that a conductor is energised, even at high voltages, makes electricity a particular hazard. At high voltages, it is unnecessary to come into direct contact with charged equipment to be shocked. An electric field surrounds all charged devices. Bringing a conducting object such as a human body into that field can intensify the field enough for electrical breakdown of the air and an arc to jump from the equipment to earth via that person. In the U.S., the Occupational Safety and Health Administration establishes clearance guidelines. Solid materials such as rubber, while excellent insulators at low voltages, are also subject to electrical failure if subjected to a high enough field. Electricity utilities wish to avoid loss of supply, for which they receive customer complaints or are financially penalised. At the same time they are obligated to maintain and replace their electrical equipment on a regular basis. Due to the hazard to maintenance workers of high voltage, it is normally necessary for portions of the electric power grid to be disconnected from the power supply before being worked upon; this is termed a planned outage. In a radially-supplied system, a planned outage results in a loss of supply for all customers supplied by the equipment, unless equipment is connected in parallel, back up supplies are available or the grid is reswitched to transfer the electrical load elsewhere. An interconnected grid results in no loss of power, but security of supply is compromised, and out-of-merit generation may need to be ordered to maintain system security, which can be expensive. 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 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.