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. 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.
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Background
Electrical hazards
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.
Avoiding loss of supply
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.
In a radially-supplied system, a planned outage results in a loss of supply, 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.
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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 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 himself safely away from the live conductors.
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.
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.
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 1kV ac 1.5kV dc The primary classes are: Class 1 - phase to phase working voltage 7.5kV Class 2 - phase to phase working voltage 17kV Class 3 - phase to phase working voltage 26.5kV Class 4 - phase to phase working voltage 36kV
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.
The first procedures for barehand working were developed in 1960 by Harold L. Rorden, a high-voltage engineer for American Electric Power. Techniques were further refined following field and laboratory tests.
There are a number of ways in which the worker can access the live parts:
- The worker can access from a specialist type of mobile elevating work platform (MEWP) termed an insulating aerial device (IAD) which has a boom of insulating material and which all conductive parts at the platform end are bonded together. There are other requirements for safe working such as gradient control devices, a means of preventing a vacuum in the hydraulic lines, etc.
- The worker can stand on an insulating ladder which is maneuvered to the line by means of non-conductive rope.
- The worker is lowered from a helicopter and transfers himself to the line.
- The worker is brought alongside the wire in a hovering helicopter and works from that position.
As the lineman approaches the wire, an arc will form between them as the worker is charged. This arc can be debilitating, and the worker must immediately bond himself electrically to the line to prevent further arcing. A worker may use a conducting wand during the approach to first make the connection. Once on the line, the worker is safe from shock as both the lineman and the wire are at the same electric potential, and no current passes through his body. This is the same principle that allows birds to safely alight on power lines.
When the work is completed, the process is reversed to remove the worker safely from the wire. Barehand working provides the lineman with greater dexterity than the hot stick method, and may be the preferred option if conditions permit it. With this technique, insulator strings, conductor spacers and vibration dampers can be replaced, or lines spliced, without any loss of supply.
The strong electric field surrounding charged equipment is enough to drive a current of approximately 15 ?A for each kV·m-1 through a human body. To prevent this, hot-hand workers are usually required to wear a Faraday suit. This is a set of overalls made from or woven throughout with conducting fibers. The suit is in effect a wearable Faraday cage, which equalizes the potential over the body, and ensures there is no through-tissue current. Conducting gloves, even conducting socks, are also necessary, leaving only the face uncovered.
There is little practical upper voltage limit for hot-hand working, and it has been successfully performed at some of the highest transmission operating voltages in the world, such as the Russian 1150 kV system.
Helicopter
A lineman wearing a Faraday suit can work on live, high-power lines by being transported to the lines in a helicopter. Wearing the suit, they can crawl down the wires.
General precautions
Calculation of minimum approach distances take account of switching surges and other transients. Transmission systems are often fitted with coordinated protection devices called autoreclosers, which are circuit breakers that automatically attempt to remake a circuit after a fault. In the event that a fault did occur it would be most undesirable for the autorecloser to re-energise the circuit because the limits of approach would be greatly reduced and the workers' position could be compromised. Hence, auto-reclosing equipment is rendered inoperative while live working takes place. Additional protection against unplanned overvoltage events (such as switching surges) can be provided by means of a surge diverter known as portable protective air gap.
An electric arc is extremely bright, including in the ultraviolet, and can cause arc eye, a painful and potentially blinding condition. Workers may be provided with appropriately tinted goggles that protect their vision in the event of a flash, and provide defence against debris ejected by an arc.
Government regulations may regulate conditions for live working conditions. For example, in United States, the Occupational Safety and Health Administration may require that more than one worker be present on site when working on live equipment above a specified voltage. The work may be postponed if adverse weather conditions such as lightning or rainfall are anticipated.
Source of the article : Wikipedia
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