Repairing wall warts - test-drb@ccmp.vtda.org

21 Feb 2006

...
   I don't
understand how a receptacle can trip a circuit breaker.  At  worst,
 > it would seem that it would fail to make contact or have high resistance
...
   contacts.

 Well, if the insulation carbonised and became slightly conductive, it
 could pass enough leakage current to trip an RCD/RCCB/ELCB/whatever it's
...
  called thise week.

 I once witnessed a metalclad socket outlet fail in the UK. Apparently,
 repeated unplging of high-power devices without turning off the built-in
...
  switch (UK socket outlets often contain a double-pole
switch) had either
...
  depostied enough metal on the insulator to become
conductive, or had
 carbonised the insulator. Anyway, the resulting failure caused a
 spectacular arc that lasted for several minutes. 
I've seen many (240V) plugs, sockets and lighting fixtures fail
similarly, in several cases becoming conductive enough to blow the
circuit fuse, considerably more difficult than unbalancing a leakage
trip. It's not at all uncommon in damp environments and another cause
(though uncommon nowadays) is where an old-fashioned piece of fuse-wire
has previously vapourised to produce a slightly conductive film.

Bob Adamson