In
electrical engineeringElectrical engineering, sometimes referred to as electrical and electronic engineering, is a field of engineering that deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after...
,
live-line working is the maintenance of electrical equipment, often operating at
high voltageThe term high voltage characterizes electrical circuits, in which the voltage used is the cause of particular safety concerns and insulation requirements...
, while still energised, that is while it is "live" or "hot". The first techniques for live-line working were developed in the early years of the twentieth 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 very highest transmission voltages.
Electrical hazards
Electricity is hazardous: an
electric shockAn electric shock can occur upon contact of a human body with any source of voltage high enough to cause sufficient current through the muscles or hair. The minimum current a human can feel is thought to be about 1 milliampere ....
from a current as low as 35 milliamps is sufficient to cause
fibrillationFibrillation is the rapid, irregular, and unsynchronized contraction of muscle fibers. An important occurrence is with the heart.-Cardiology:There are two major classes of cardiac fibrillation: atrial fibrillation and ventricular 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 failureThe term respiratory failure, in medicine, is used to describe inadequate gas exchange by the respiratory system, with the result that arterial oxygen and/or carbon dioxide levels cannot be maintained within their normal ranges. A drop in blood oxygenation is known as hypoxemia; a rise in arterial...
and result in extensive and life-threatening burns. The first such human fatality occurred in 1879 and was of a stage carpenter in Lyons,
FranceFrance , officially the French Republic , is a country located in Western Europe, with several overseas islands and territories located on other continents. Metropolitan France extends from the Mediterranean Sea to the English Channel and the North Sea, and from the Rhine to the Atlantic Ocean...
, who 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 fieldIn physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field. This electric field exerts a force on other electrically charged objects...
surrounds all charged devices. Bringing a conducting object such as a human body into that field can intensify the field enough for
electrical breakdownThe term electrical breakdown has several similar but distinctly different meanings. The term can apply to the failure of an electric circuit....
of the air and an
arcAn electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. The phenomenon was first described by Vasily V. Petrov, a Russian scientist who discovered...
to jump from the equipment to
earthIn electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
via that person. Operating clearances are thus specified in order to maintain a minimum safe distance from conductors. 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 utilitiesAn electric utility is a company that engages in the generation, transmission, and distribution of electricity for sale generally in a regulated market. The electrical utility industry is a major provider of energy in most countries. It is indispensable to factories, commercial establishments,...
wish to avoid
loss of supplyA power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...
, 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
plant outage.
In a radially-supplied system, a plant outage necessarily results in the loss of supply to all more remote parts of the system, unless equipment is connected in parallel, back up supplies are available or the grid 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 generationEconomic dispatch is the method of determining the most efficient, low-cost and reliable operation of a power system by dispatching the available electricity generation resources to supply the load on the system...
may need to be ordered to maintain system security, which can be very expensive.
Methods
There thus can be an economic advantage to live-line working, but this comes with considerable hazards unless the proper precautions are strictly followed to ensure workers' safety.
In general, there are three methods of live-line working:
Hot stickA hot stick is an insulated pole, usually made of fiberglass, used by electric utility workers when engaged on live-line working on energized high-voltage electric power lines...
The work is carried out with insulated tools, such as long insulating poles.
Hot glove
The worker is protected by thick rubber gloves, often extending all the way up his arms, and sometimes wears a rubber apron as well.
Hot hand
The worker is transferred to an isolated platform, such as a heavily insulated
cherry pickerA cherry picker , is a type of aerial work platform that consists of a platform or bucket at the end of a hydraulic lifting system....
, or suspended from a helicopter, and brought into contact with the equipment.
Some organisations 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), 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 twentieth century, when insulating poles made from baked wood were used for tasks such as replacing
fusesIn electronics and electrical engineering a fuse is a type of sacrificial overcurrent protection device. Its essential component is a metal wire or strip that melts when too much current flows, which interrupts the circuit in which it is connected...
, swapping out post insulators, and transferring lines onto temporary supports. The sticks enabled the linemen to carry out the work without infringing 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 wrenchA socket wrench, more commonly referred to as a ratchet, is a type of wrench, or tightening tool, that uses separate, removable sockets to fit many different sizes of fittings and fasteners, most commonly nuts and bolts.-Description:...
es 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.
Hot glove
For medium or low voltages, work can be carried out if a worker wears insulating gloves of an adequate standard. The gloves often extend all the way to the shoulder to protect the worker's arms. Additional protection can be provided by a rubberised apron. To reduce the length of exposed conductor, insulating blankets and hoses can be draped over the equipment not being worked upon. An additional layer of protection can be provided by the use of insulated tools such as
pliersPliers are a type of hand tool used to hold objects firmly, or for cutting and bending tough materials such as wire. Generally, pliers consist of a pair of metal levers joined at a pivot positioned closer to one end of the levers, creating short jaws on one side of the pivot, and longer handles on...
. Linemen often work from an insulated platform or non-conducting ladder, however the primary protection is deemed to come from the gloves. This provides an ultimate limit on voltage for hot-glove working, perhaps 60 kV.
Hot hand
Hot-hand (also known as bare-hand, or contact) working involves placing the worker in direct electrical contact with an energised overhead line. He might work alongside the lines, from a platform that is suspended from them, or even sit or stand directly on the line itself. In all cases, the worker's body is maintained at exactly the same voltage as the line. It is imperative that no earthed equipment is brought within range, else a flashover would occur.
The first procedures for hot-hand working were developed in 1960 by Harold L. Rorden, a high-voltage engineer for
American Electric PowerAmerican Electric Power is a major investor-owner electric utility in various parts of the United States. AEP ranks among the nation's largest generators of electricity, owning nearly 38,000 megawatts of generating capacity in the U.S...
. Techniques were further refined following field and laboratory tests.
There are three general means of getting the worker to the line:
- The worker is placed in a heavily insulated platform, and raised to the line.
- He is lowered from a helicopter and transfers himself to the line.
- He 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 his body is charged. Although this arc carries no more than a few microamps, it is debilitating, and the worker must immediately bond himself electrically to the line to prevent further arcing. He may use a conducting wand during the approach to first make the connection. Once on the line, he is safe from shock as both the lineman and the wire are at the same
electric potentialAt a point in space, the electric potential is potential energy divided by charge that is associated with a static electric field. It is a scalar quantity, typically measured in volts....
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 him safely from the wire. Hot-hand 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 stringsAn insulator, also called a dielectric, is a material that resists the flow of electric current. An insulating material has atoms with tightly bonded valence electrons. These materials are used in parts of electrical equipment, also called insulators or insulation, intended to support or separate...
, conductor spacers and vibration dampers can be replaced, or lines
splicedRope splicing in ropework is the forming of a semi-permanent join between two ropes or two parts of the same rope by partly untwisting and then interweaving their strands. Splices can be used to form a stopper at the end of a line, to form a loop or an eye in a rope, or for joining two ropes together...
, without any loss of supply.
The strong electric field surrounding charged equipment is enough to drive a current of approximately 15 μA 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 fibres. The suit is in effect a wearable
Faraday cageA Faraday cage or Faraday shield is an enclosure formed by conducting material, or by a mesh of such material. Such an enclosure blocks out external static electrical fields....
, which equalises 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
RussiaRussia , officially known as both Russia and the Russian Federation , is a country in northern Eurasia . It is a semi-presidential republic, comprising 83 federal subjects...
n 1150 kV system.
General precautions
Transmission systems are often fitted with coordinated protection devices called
autorecloserIn electric power distribution, an autorecloser is a circuit breaker equipped with a mechanism that can automatically close the breaker after it has been opened due to a fault....
s, which are
circuit breakerA circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
s that automatically attempt to remake a circuit after a
faultA short circuit in an electrical circuit is one that allows a current to travel along a different path from the one originally intended....
. In the event that a fault did occur, shocking a worker, it would be most undesirable for the autorecloser to re-energise the circuit, shocking him again, perhaps repeatedly. Hence if reclosing equipment is fitted, it is normally switched out of service during live-line working. 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 arcAn electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. The phenomenon was first described by Vasily V. Petrov, a Russian scientist who discovered...
is extremely bright, including in the
ultravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
, and can cause
arc eyeArc eye, also known as welder's flash, bake eyes, corneal flash burns, or flash burns, is a painful ocular condition sometimes experienced by welders who have failed to use adequate eye protection. It is also referred to as arc flash, though this can also refer to an electrical explosion...
, a painful and potentially
blindingBlindness is the condition of lacking visual perception due to physiological or neurological factors.Various scales have been developed to describe the extent of vision loss and define blindness...
condition. Workers may be issued with appropriately tinted goggles that both protect their vision in the event of a flash and provide defence against debris ejected by an arc.
Regulations for live working are strict, and rigid adherence to protocols is essential. For example, it is usually required 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.