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:uses, 2
:USES are fitted to vehicles in )rder to give the best possible irotection to the electrical ;ystem at the most reasonable :ost. The best protection which :ould be afforded is one fuse per :ircuit, but this carries cost and ;pace penalties.
The simplest solution is one arge fuse in the main electrical eed but this means that all the viring must be capable of :arrying the large current in the :vent of a fault serious enough o blow the fuse. This would :ause an unacceptable increase n harness thickness and cost. The vehicle designer tries to ichieve a compromise between hese two extremes by grouping ogether on the same fuse, ircuits having the same current atings. For instance, five oneimp circuits on on five-amp fuse equires five-amp cable to each circuit to prevent damage; whereas a one-amp courtesy lamp circuit on the same 15-amp fuse as the headlamps taking 14 amps would have to be wired using 15-amp cable.
The fuse arrangement is therefore designed to optimise space, cost and harness design.
The ratings and functions of the fuses on the Leyland Roadtrain, an excellent example of modern practice, ajipears below. It should be pointed out that this vehicle is also fitted with circuit breakers, another form of protection for the electrical system, which will be described in the next article.
I am indebted to Martin Jones and Trevor Longcroft, of Leyland Vehicles, for the information they have provided concerning this subject.
When a fuse blows it is essential to replace it with a fuse of identical value. If a fuse keeps 'failing it must not be replaced by one of a higher rating, but the fault in the circuit which is the cause of the trouble must be traced and rectified.
One shudders to hear stories of nails and bolts being used to replace a fuse — if this is done the consequences could be very serious, even causing the vehicle to burn out.
Glass cartridge type fuses are of two types, one with a cone at each end and the other with flat ends. Glass fuses are normally rated at their fusing value (Luca rating). Ceramic fuses are rated at their continuous value (BSS rating). Both types are colour coded as shown in Table 2.