In electrical engineering

Electrical engineering

Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...

, live-line working is the maintenance of electrical equipment, often operating at high voltage

High voltage

The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...

, 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.

Electrical hazards

Electricity is hazardous: an electric shock

Electric shock

Electric Shock of a body with any source of electricity that causes a sufficient current through the skin, muscles or hair. Typically, the expression is used to denote an unwanted exposure to electricity, hence the effects are considered undesirable....

 from a current as low as 35 milliamps is sufficient to cause fibrillation

Fibrillation

Fibrillation is the rapid, irregular, and unsynchronized contraction of muscle fibers. An important occurrence is with regards to 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 failure

Respiratory failure

The 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 Lyon

Lyon

Lyon , is a city in east-central France in the Rhône-Alpes region, situated between Paris and Marseille. Lyon is located at from Paris, from Marseille, from Geneva, from Turin, and from Barcelona. The residents of the city are called Lyonnais....

, France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

, 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 field

Electric field

In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

 surrounds all charged devices. Bringing a conducting object such as a human body into that field can intensify the field enough for electrical breakdown

Electrical breakdown

The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....

 of the air and an arc

Electric arc

An 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. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

 to jump from the equipment to earth

Ground (electricity)

In 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 utilities

Electric utility

An 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 supply

Power outage

A 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 planned 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 generation

Economic dispatch

Economic dispatch is the short-term determination of the optimal output of a number of electricity generation facilities, to meet the system load, at the lowest possible cost, while serving power to the public in a robust and reliable manner...

 may need to be ordered to maintain system security, which can be expensive.

Methods

Thus, there 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 stick / live line tool (IEV 651-01-09
live working carried out according to a method whereby the worker remains at a specified distance from the live parts and carries out the work by means of insulating sticks

Insulating glove / rubber glove (IEV 651-01-10)
live working carried out according to a method whereby the worker is electrically protected by insulating gloves and other insulating equipment, and carries out the work in direct mechanical contact with live parts.

Barehand / potential (IEV 651-01-11)
live working carried out according to a method whereby the worker carries out the work in electric contact with live parts, having the potential of the worker's body raised to the voltage of the live parts by electric connection, and suitably isolated from the surroundings at different potentials .

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 20th century, when insulating poles made from baked wood were used for tasks such as replacing fuses

Fuse (electrical)

In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...

, replacing 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 wrench

Socket wrench

A socket wrench is a wrench with interchangeable heads called sockets that attach to a fitting on the wrench, allowing it to turn different sized bolts and other fasteners. The most common form is a hand tool popularly called a ratchet consisting of a handle with a ratcheting mechanism built in,...

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.

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.

Most utilities require work to be performed from an insulating platform to provide isolation from earth/ground potential hence the term "insulate and isolate".

Bare hand

Bare-hand, or potential 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 the same voltage as the line. It is imperative that the worker maintains 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

American Electric Power

American 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 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, means of preventing a vacuum in the hydraulic lines.
• the worker can stand on a insulating ladder which is manoeuvred to the line by means of non conductive rope
• the worker 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 potential

Electric potential

In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...

 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. 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

Rope splicing

Rope splicing in ropework is the forming of a semi-permanent joint 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...

, 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 cage

Faraday cage

A 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 and non-static electric 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 Russia

Russia

Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

n 1150 kV system.

General precautions

Calculaltion of minimum approach distances take account of switching surges and other transients. Transmission systems are often fitted with coordinated protection devices called autorecloser

Autorecloser

In electric power distribution, a recloser, or autorecloser, is a circuit breaker equipped with a mechanism that can automatically close the breaker after it has been opened due to a fault. Reclosers are used on overhead distribution systems to detect and interrupt momentary faults...

s, which are circuit breaker

Circuit breaker

A 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 fault

Short circuit

A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....

. 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 for transmission work auto-reclosing equipment is rendered inoperative whilst 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

Electric arc

An 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. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

 is extremely bright, including in the ultraviolet

Ultraviolet

Ultraviolet 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 eye, a painful and potentially blinding

Blindness

Blindness 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 provided with appropriately tinted goggles that protect their vision in the event of a flas, 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.