Friday, February 18, 2011

Killer Door Bells ?


Of all the things one would not think would be responsible for the cause of electrical based fires
the plain old Door bell would be on that list but this is but a myth , especially in older homes
with Knob and Tube type wiring.

Door bells originally existed in homes well before electric power wiring was ever introduced into homes. electric door bells were around when homes were lighted with natural gas ,it was a very simple system which used dry cell batteries and a bell that was 3-6 volts ac/dc and a simple button.
and small gauge cloth covered wire
However as Electrical power become available to the home owner the battery powered door bells were then switched to Alternating current thru means of a small step down transformer
which was either attached directly to the basement studs or put within a metal box with hinged lid. along with the transformer was a small Porcelain fuse block with 2 screw in type fuses that were same size used in the house fuse panel and this is were things started off on the wrong foot.

Unlike today’s modern doorbells which have built-in self restoring thermal breakers
the original door bell transformers had to have a separate fuse in each high voltage leg of 1-2 amps. but typically what would happen is the cloth wiring to the door bell would disintegrate
the wires would short together and one of several things happened. The transformer fuse would blowout or the door bell would latch on and stay on till short was removed or fuse blows.
but instead of most home owners or handymen replacing the fuse with the correct size typically they would grab a fuse the used in the main box which would way overload the door bell transformer system and then when the next time the wires would short out ,the bell would latch on and stay on to the point that the transformer gets so hot it would ignite the wooden timber to which it was attached.
Alas most investigators would blame the fire on the high voltage wiring along the beam, because usually the transformer completely disintegrates in the fire .if not mounted in a box.
this again is one of the reasons why point of origin in electrical fires can throw investigators off course and why Electrical fires are so hard to investigate.
Now just because your in a modern house with modern transformer does not let you off the hook from a door bell fire several situations can still and have caused fires.
one is a thermal overload in a modern transformer failing to operate as designed .
the other is the dangerous practice of not grounding the transformer and using open splice wiring
instead of wiring the transformer thru a box knockout.
many times I find transformers wired with out a box on modern wiring which is illegal by code
but homeowners see the mounting feet on the transformer and figure they don't need a box which is a dangerous practice the mounting feet are strictly for using the transformer on old style Knob and tube wiring. of course all transformers I have seen have proper wiring and installation instructions but who needs instructions??

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