There's a Unimog in the hillsides
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Brian Weatherley visits the Bwrdd Canolog Cychyrchu Trydan (CEGB), Rhanbarth Y De Orllewin (SW Region) to see these crosscountry vehicles in use on power line maintenance
NO WELCOME in any Welsh hillside would be very warm without electricity, whether for heating, cooking or lighting. Most of us take electricity for granted, particularly during the summer when demand drops. However, for the South Wales Transmission District of the Central Electricity Generating Board summer is the time to carry out vital maintenance work on overland power lines carrying electricity generated at the Pembroke power station.
A major part of this programme is the replacement and checking of the string of ceramic insulators which insulate the steel pylons from the 400,000-v high-tension power lines, or conductors as they correctly are known. With most of these lines extending across country rather than conveniently following main roads, the CEGB has at times to operate in difficult terrain that would tax a Welsh mountain pony let alone a conventional road vehicle.
In order to help the line gangs in their work, the South Wales Transmission District operates two Mercedes-Benz Unimog 4x4s — a U900 and U1000 — from its Swansea depot. Bought to replace a conventional tractor, the little all-wheel-drive vehicles, however, are not used simply to get to remote sites, for the replacing of insulator strings (fitted two to each conductor) requires them being lifted up the 60m (200ft) high towers and held in place while they are attached by linesmen who have climbed up the steel framework.
Consisting of twenty four 16kg (371b) individual insulators, the whole string weights around 384kg (7.5cwt) and obviously cannot be pulled up by men and pulleys. The two Unimogs, therefore, are each fitted with a four-ton capacity hydraulic winch equipped with a 304m (1,000ft) line which can raise and lower each insulator string up to the cable arms.
To see the two Unimogs in action, CM spent a day with two line gangs working on power lines near Pontardulais and Llannon some 12 miles outside Swansea. On average, four steel towers are used per mile on the 440kv conductor runs, although this distance can vary according to terrain.
Whatever the terrain, a minimum ground clearance of 7.1m (25ft) must be maintained on the lowest line.
Conductor tension is maintained by tensioning towers which are used at every change of direction in the cable run or in the middle of long, straight sections. On these towers the insulator sets hang horizontally instead of vertically. As well as lifting the insulators, the winches on the two CEGB Unimogs are also used with tensioning blocks for stringing conductors to the pylons.
Even in the height of summer, the steep and narrow Welsh country lanes make vehicle operations difficult. Although most maintenance work is carried out during this period, the line gangs are expected to work in all weathers, consequently nearly all vehicles are fourwheel drive.
As well as the two Unimogs, the Swansea depot operates a number of long-wheelbase 2.5litre petrol-engine Land Rovers and a Dodge S66 4x4 fitted with a demountable personnel body, converted by NAM Special Vehicles. (CM Tech News, May 1, 1982). This vehicle is used by the line gang when working in remote areas. Ascending into the hills, the bright orange procession appears somewhat like a military convoy, turning the heads of many locals.
Restricted to such specialist work, the 12-year-old J registra
tion CEGB U9 0 0 Unimog operated from Swansea has a mere 7,296 miles on the clock. By comparison to that of the latest A-registration Unimog 1000, the interior is positively Spartan, although its driver, Jack James, had few complaints. Although initially fitted with a canvas top, the roof has been replaced by a more practical one-piece grp unit.
The floor of the old U900 model is festooned with various levers: there are ones for forward and reverse selection, two and four-wheel drive, power take-off engagement as well as the normal six-speed gearchange lever. And the air handbrake in the Unimog is supplemented by an umbrellatype ratchet below the dash.
Starting off up the narrow road, Jack James had to work hard with the cold gearbox despite its being fully synchromesh, although changes became easier as it warmed up. Overall ride on the 2.38m (94in) short wheelbase Unimog, which is powered by the 62kW (84bhp) 0M352 diesel engine, is surprisingly good, being helped by the weight of winch, although the high body tends to roll when driving around the switchback roads.
The pylons near Pontardulais designated for insulator replacement are situated high on a bluff of waste land and give a panoramic view of "Wild Wales". The heathland hill-top is served by a narrow, metalled road, but the immediate area beneath the first tower is rough ground.
Before Jack set up the Unimog for winching operations he cleared a fern covered area by driving over it with the rear hydraulically operated ground anchor blade lowered. This pto operated blade is activated by twin hydraulic rams and is placed firmly on the ground when the winch gear is operated.
While it is only necessary to shut off one set of conductor lines on one side of the tower when maintenance work is carried out, to prevent any induced current from the live side affecting the metal tower, the de-activated power lines are well earthed by line men who have climbed up the tower. The position of the Unimog winch vehicle is also important and to prevent any problems caused by induced voltage while working beneath the live conductors, an earthing spike which is attached to the chassis is also driven into the ground.
The two line-gang workers who have climbed the tower then fix pulleys to the conductor arms. Equipment can then be pulled up to them, including the winch wire from the Unimog. Two insulator sets are used on each conductor. The line men replace them one at a time, supporting the weight of the power line by a 4.5-ton capacity ratchet lift. Once the insulator set has been attached to the winch cable, it is disconnected from the conductor cable at the bottom and from the cable arm at the top, leaving the Unimog free to winch it down slowly through the steel tower framework to the ground where a new insulator will be attached and raised up. Once the highest insulators have been replaced, the process is repeated on the middle and bottom conductor arms.
With work progressing rapidly at Pontardulais, we moved down the line at Llannon, some six miles away, where the latest U1000 was working. Unlike the power lines, which are sited almost as straight as the crow flies, our road journey following line foreman John Watkins required a more circuitous route.
Despite carrying out the same basic duties as the older Unimog, the A registered U1000 has a longer — 2,650mm (104in) — wheelbase and an all-steel, new generation Unimog cab.
The level, grass-covered pylon site outside of Llannon was in marked contrast to Pontardulais and presented no real access problem for the U1000. When operating in rougher terrain, however, the Unimogs need all of their impressive 440mm (17.3in) ground clearance.
Both vehicles operate within a 75-mile radius of Swansea. Although capable of a top speed of more than 40mph on good roads, to reduce unnecessary travelling time the vehicles are carried by a low-loader on long journeys.
Used solely as winch vehicles, the U900 and U1000 are able to be taxed as agricultural rather than commercial vehicles by the CEGB.
The long-serving U90Ci will shortly be replaced by another U1000 which will also be based at Swansea. It is unlikely, however, that householders in the area will notice any difference. As long as their house lights go on at night, they'll continue to take the CEGB for granted. Thanks to the South Wales Transmission District's Unimogs, they can.