Live-line working

In electrical engineering, live-line working is the maintenance of electrical equipment, often operating at high voltage, while the equipment is energised. 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 of high voltage, it is normally necessary for equipment to be isolated from the supply before being worked upon, termed a planned outage.

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