I sat alone in a metal seat clutching the ‘hot-stick’ that would separate me from nearly half a million volts, and was slowly hoisted aloft 10 metres or so. As I drew near the big transformer bushing, sparks of up to 20 centimetres flashed and crackled to my hands and feet. My task was to ‘bond’ myself to the bushing with a giant clothes peg on a lead that was bolted to my worksuit. Once bonded I would be energised to the potential of the bushing and the pyrotechnics would subside to a slight hiss.
What prevented me from being crisped to a cinder was a suit costing over £600, covering all but my eyes. Its heavy fabric contains about 25 per cent metal fibres, fine enough to leave it reasonably flexible. In effect, I was inside what is known as a ‘Faraday cage’ that would short-circuit almost all current round rather than through me. It would leak no more than about 0.2 milliamperes into my body, or so I had been assured. This might be enough to induce a slight tingle but would be far too little to harm, they said.
The grid stays on
‘They’ were executives of the National Grid Company, the organisation that owns and operates the system for transmitting electricity in England and Wales. During July the company introduced a new practice called live-line working. It has trained a dozen of its best linesmen – the people who maintain and repair the high-voltage transmission system known as the grid – to work without first switching off the power. This select group have perfected the techniques required at the NGC’s high-voltage laboratory in Leatherhead, Surrey, using the same rig on which I tried live-line working first hand.
Advertisement
Spectacularly dangerous as it may sound, live-line working has a long history in electricity supply. Britain first showed an interest in the 1940s but the Americans were the pioneers. As early as 1913, engineers were using long wooden tools that they called ‘disconnect sticks’ to open live isolators – the switches through which overhead lines are turned on and off. By 1929, the Canadians were working on lines carrying 100 kilovolts; by 1954 they had reached 354 kV. Since then the range of maintenance and repair jobs that can be done without shutting off parts of the grid has increased vastly.
In the mid-1960s, Christopher Hinton, chairman of the Central Electricity Generating Board, forerunner of the NGC, recommended a study of practices in the US to help cope with increasing demands and costs of Britain’s newly introduced 400 kV ‘supergrid’. The first bare-hand contact to be made with a live 400 kV line in Europe is believed to have taken place at the CEGB’s Leatherhead laboratory in 1967. The CEGB then embarked on a joint study with its French counterpart, Electricite de France, to develop a standard safe working practice. The CEGB ended the partnership abruptly in the early 1970s – because, it is said, unions demanded a substantial premium for the practice which would make it uneconomic. The French pressed ahead and now use it extensively.
A decade or so later, when China became more open, Western observers were surprised to find women working on live lines. In the past couple of decades, as electricity companies have come to appreciate the savings of time and money to be made, the practice has become increasingly popular and is used widely in countries including Russia, Canada, Germany and France. Now, the NGC believes it is time to use it.
Since privatisation of the CEGB in 1990 the electricity transmission system has been owned and operated by the NGC, whose shareholders are the 12 regional electricity companies. It takes electricity made by the generating companies and carries it to the distributing companies. The original grid operates at 275 kV, but as higher voltage results in lower losses of power, a 400 kV system was added in the 1960s (see Diagram). This integrated network now interconnects with the transmission systems of Scotland and France.
Power broker
As well as controlling the grid, the NGC is also a trading company, managing the electricity marketplace, popularly known as the Pool. At 10 am each day all the generating companies intending to sell power the following day post their price and details of how much electricity each generator will supply. The NGC then sorts all these bids, and acts as a broker to ensure that its distributing companies receive a reliable supply of the cheapest electricity available.
The national grid is one of the biggest machines in Britain. It is also astonishingly flexible. Operating as a highly integrated, computerised system, it constantly has to balance supply with demand, for there is no way of storing electricity economically on a large scale. Typically, daytime demand is twice that at night; maximum winter demand is four times minimum summer demand; and there are unpredictable surges – such as the 10 per cent increase in demand following the end of the England against West Germany World Cup match in 1990. The power transmitted by the grid – 267.6 Terawatt-hours last year – also ebbs and flows in response to faults that may suddenly deprive it of a 660-megawatt generator, or a limb of transmission line should lightning strike a vulnerable point or a pylon collapse.
Altogether, the machine is valued at about £5 billion. Its infrastructure consists of at least 7000 kilometres of overhead high-voltage transmission lines carried by 20 000 pylons, over 500 kilometres of underground cables, and more than 280 substations, plus the interconnections through which it can trade power with Scotland and France. It also has about 2000 megawatts of storage capacity – power that it can call upon in an emergency. Since 1993, a national centre near Wokingham, Berkshire, has controlled the grid with the help of four regional centres in Birmingham, Bristol, Leeds and St Albans. And the NGC now plans to phase out all four area centres and rely on a single control centre.
David Jefferies, chairman of the NGC, says his company is still discovering what a remarkable machine it has taken over from the former CEGB. It is, he says, proving more flexible than the CEGB ever realised. With new technology – for example, to minimise losses through fluctuating voltage – the NGC is squeezing an extra 2 to 3 per cent capacity from the grid. This has resulted in the closure of some of the least efficient generators, but the system is able to adapt to these changes. Ironically, it is the old technology – the power cables themselves – that is creating the problems.
Several groups have attempted to show that electromagnetic radiation emanating from overhead wires can cause diseases including leukaemia. Although the jury is still out, the National Radiological Protection Board, watchdog of public exposure to radiation, reports that overhead cables generate more public inquiries than radiation from nuclear activities. But the NGC believes that working close to power lines is safe. It points to the records of power companies internationally. The NGC also has the backing of the Health and Safety Executive, which imposes no limits on occupational exposure to electromagnetic radiation, and approved NGC plans to go ahead with live-line working starting this month. ‘We’re satisfied, from studies done by EdF in France and Quebec Hydro in Canada, that we are not exposing our people to any problems,’ says Jefferies.
Leon Sparrow, senior engineer responsible for developing safe practice for NGC, believes that even in Britain, live-line working is already well-established. ‘If you use an insulated screwdriver on a live circuit – that’s live-line working.’ And he has no worries about safety provided the right precautions are taken and workers are kept far enough from live cables to allow for fault conditions that can produce huge surges in the normal working voltage. ‘We’ve been pushing our knowledge to the limit on what voltages actually arise, how insulators behave and what happens when something shorts,’ he says.
Despite the considerable body of experience that exists worldwide in live-line working, there is still no international standard of practice. ‘We wanted to understand the scientific basis,’ says Sparrow. ‘We were not entirely happy to take other people’s findings.’ But he is encouraged by the American experience, which suggests that live-line working is safer than dead-line working – because the linesmen take greater care.
The techniques used fall under two broad approaches – hot-stick and bare-hand work. Hot-stick work is done with tools at the tip of insulating poles which are long enough to separate the user safely from electricity. Bare-hand work – where the linesman is in direct contact with a power line – was made possible by the development of conductive suits. Workers may be hoisted aloft in a cradle, or lowered onto pylons from a helicopter, allowing them to do similar jobs but much more quickly.
The NGC intends to use both approaches. It has imported hot sticks from the US, which consist of a resin-bonded glass-fibre tube packed with plastic foam to ensure that no moisture can get inside. For 400 kV, the safe length is about 0.7 metre. But to safeguard linesmen against surges in voltage that can occur when a line is turned on or off, the company intends to use hot sticks about 3 metres long – a wider safety margin than the Americans use on their 765 kV system. The result is a bit like trying to unlock a door with a key at the end of a fishing rod.
Pleasing tingle
Bare-hand working also originated in the US. The principle is simply that a person in contact with a high-voltage conductor but isolated from earth need be no more at risk than if at earth potential but isolated from the high voltage. The tricky bit is to place the person safely on the live conductor, without allowing harmful amounts of current to flow through the body. This is where the cumbersome insulating suits come in. Although they have been used for decades, they are not greatly in demand and as a result design remains purely functional. Sparrow describes the suits as ‘a bit Quatermassy’. He hopes that future developments will produce a more comfortable fabric that also permits the linesman to wear a hard hat.
The effects of electricity on the human body are well documented. According to Anthony Barker, principal physicist at the Royal Hallamshire Hospital in Sheffield, currents of between 1 and 6 milliamperes at ordinary frequencies are harmless, resulting in a pleasing tingle that can be tolerated indefinitely. As current increases up to 24 mA, what Barker calls the ‘let-go’ value is reached. This varies, but is 16 mA for an average male and as high as 22 mA for 1 in 200 people. At the ‘let-go’ value, when a live conductor is touched the current causes muscle contractions making the hand involuntarily clamp down on the conductor. Currents above about 25 mA flowing from limb to limb – across the chest – and lasting a few seconds or more are liable to interfere with the heartbeat. Then the heart stops and fails to restart when the electric shock is removed.
The NGC has set a limit of 0.2 mA for its linesmen, and is confident its conducting suits will keep leakage to this level. Also each operation will be meticulously planned and rehearsed at its pylon training facility at Eakring in Nottinghamshire, before the linesmen go live. The first jobs they tackle will be those accessible from the pylons. These include routine repair and cleaning of insulators, each weighing 0.75 tonnes, that carry the overhead cables. In the autumn, the NGC hopes to introduce live-line working from helicopters which will allow linesmen to replace the neoprene spacers that keep power cables apart. These tend to become hardened as they age, so reducing their ability to absorb shock. As a result, constant wind vibration causes individual lines in the cables to snap.
‘The linesmen are all very enthusiastic – they really see them-selves as a crack team,’ says Jefferies. And NGC’s managers are keen to see them succeed. They are anticipating significant economic benefits as a result of faster repairs and maintenance and constant electricity output from the cheapest sources. There are no figures to indicate just how substantial these might be, but with countries already operating live-line working increasing the scope of these practices, the trend is clear. Savings will flow first to the NGC, says Jefferies, but within a few years they should have passed on to the consumer.
David Fishlock writes and publishes R&D Efficiency for research managers.