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Many hundreds of ingenious proposals have been put forward for avoiding the use of overhead contact wires by using a contact system on or near the road surface. To avoid the possibility of electrocuting unwary road users, two basic principles have been tried:
a) The use of low voltage supplies.
The inherent difficulty of this system is the heavy current needed in order to supply the required amount of power. This, in turn, results in an inordinate voltage drop, where little can be spared, and considerable losses in contacts and conductors. To avoid further losses, the power distribution system usually works at a higher voltage with closely-spaced electrical sub-stations supplying low voltage as near the track as possible.
The proliferation of trackside apparatus is wasteful and very expensive.
b) Making the surface contact area 'live' only
when the vehicle is over it.
This was the basis for the only two successful surface contact systems in Britain, the 'Lorain' and the 'Dolter' Systems, which operated in Wolverhampton and Hastings until replaced by overhead wires.
Magnets under the tram were used to operate switches under the road, making metal studs live, the current was then picked up by a long metal skid hanging from the underside of the vehicle and sliding over the studs. As soon as the tram had passed, the magnetism ceased and the switch was supposed to open, disconnecting the stud. The progress of the vehicles was accompanied by spectacular sparks and flashes because the contacts were usually intermittent, being covered in road dirt.
In the event that a switch stuck in the 'on' position, leaving the stud live, a trailing metal contact on the rear of the tram either rang a bell (in response to which, the conductor was supposed to alight and stike the offending stud with an insulated hammer) or short-circuited the stud through the vehicle frame and wheels to the earthed track and blew the fuse for that track section.
Even with these precautions, live studs were sometimes discovered and their whereabouts, so it is said, were rapidly communicated to local horse owners. The less scrupulous would then drive their most broken-down or aging nag over the stud and claim handsome compensation from the tramway company for the 'accident'.
A combination of both switched studs and low voltage was used in Paignton for a very short period. The low voltage made it safe for street urchins to detect live studs with their bare toes, which practice was encouraged by payment. A warning had to be issued not to attempt this in Wolverhampton, where similar studs carried 550 volts.
A recent variation of surface contact system claims to be safe because it uses only 'Low Voltage'. In fact it operates at 750 volts, which is defined as low voltage by the electricity supply industry, but this could be misunderstood with fatal results.
All surface contact systems are liable to excessive leakage currents in wet weather or when salt is used in snowy conditions. Stray pieces of metal or foil can cause short-circuits and stones and grit can disconnect the circuit. There is one commercial tramway service operating on the surface contact system in Bordeaux and this has already experienced safety problems with water getting into the equipment
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