Hi Jim:
> I'm curious: given that North American plug blades are >1/2" apart, there
> must have been substantial contamination to aid in 120Vac jumping that far
> (arcing). Did you identify any sort of contamination or moisture?
I don't believe contamination is a significant factor
in events such as this one.
I believe such events start with a loose connection
between the plug and the socket (or between the wire
and the socket parts). A loose connection means
that the contact area is relatively small. In turn,
this means high current density at the point of
contact.
The smaller the contact area, the greater the
resistance of the contact.
The smaller the contact area, the greater the current
density at the point of contact.
These two factors contribute to heating of the two
parts, the plug blade and the socket. Heating tends
to reduce the "springiness" of the socket part, and
of the connection between the supply wire and the
socket (because they are thermally connected).
The heating also tends to degrade the surface of the
insulating material in which the conductors are mounted.
Heating also enhances oxidation of the plating on the
parts, which further increases the resistance of the
connections.
If the plug-connected appliance is "ON," arcing can
occur as the parts expand due to heating and make
various intermittant connections. Arc temperatures
are very high, and can "burn" the surface of nearby
insulating materials via radiation.
As the surface degrades, leakages occur across the
surfaces. At this point, whether or not the appliance
is on or even connected is not a factor. There is a
current path between the two poles along the surface
of the insulator. This can either be between the
socket parts, or between the wired parts. The leakage
current causes further heating and micro-arcs where
the leakage path opens due to current density. The
micro-arcs further damage the insulator until there
is nearly continuous micro-arcing. I suggest this
is the source of the noise. The heat from the micro-
arcing and the resistance of the carbonized surface
of the insulator eventually lead to ignition and
flames.
I admit that this is a hypothesis. I believe that
the process is more-or-less correct, but the details
may not be correct.
Best regards,
Rich
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