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