[VAL] Re: Tricky shorts

["Al G." <waymark1@xxxxxxxx>]

This type of short is most likely with bulbs that have an almost circular
filament. If the filament breaks near one end and the short piece falls
across to the support that supplies power to the other end of the
filament the short piece of filament has so little resistance that it
pops the breaker. It likely is vaporized while it is popping the breaker,
so the "evidence" may disappear.

Bulbs that have only two filament supports, both of which are also the
power supply wires to the filament, cannot have a loose end of a filament
fall across to the other side - it isn't long enough.
Unfortunately the bulbs most suited to RV use have multiple supports,
minimizing filament breakage due to vibration, which can occur even to a
cold filament (the 120V bulbs are of course not on during travel). 
"Rough Service" bulbs are the strongest example of this type of
construction.
All bulbs will eventually suffer a broken filament when they burn out, so
this fault can happen to even a "Rough Service" bulb.

Perhaps this fault is more common than one might think as I have had it
happen several times, not to 120V bulbs in an RV but to bulbs in a house.

All loads in houses and RVs and other vehicles are connected in parallel.
They all are supplied the same voltage. If one fails the others stay
energized unless a fault causes a breaker or fuse to blow. 
Convenience outlets and light fixtures are wired from one to the next in
a given circuit. That's why every receptacle has two screws on each side
(never use the push-in sockets on the back of a receptacle in RV
service). From the breaker the wires go to the first receptacle or light
fixture, then to the next in the same circuit. With light fixtures the
wire nuts are used to connect three wire ends each, one from the breaker
or the fixture ahead of it, one to the next fixture in the circuit, and
one to the fixture itself or to its wall switch. The last fixture or
receptacle in the circuit of course has no wires going on to another.

Al