Percival P. Cassidy wrote:
On 08/26/08 07:33 am John Grabowski wrote:

Pulled a switch to replace it and noticed that
(c) there was a charred black mess that seemed to be "cooked"
insulation tape with a small area of something red and hard at the end
where the wires went in and the end of a screw sticking out of the
other end of this mess.

As I cut away the insulation tape, it became clear that this was a
"fried" wirenut, and that what had looked like screw threads was the
wire spiral that was gripping (or was supposed to be gripping) the
conductors.

The first time I ever saw a wirenut, I thought "What kind of a Rube
Goldberg kludge is this?" but I now understand that they are NEC
approved. So what would cause a wirenut to "fry" like this? Note: I
had never noticed any problems with the lights that the switch was
controlling, or with any other circuits fed through the wires being
held by that wirenut.

Most likely a loose connection caused the conductors to arc which
generated the heat.

Shouldn't the arc-fault breaker have caught
this? And wouldn't the lights and other devices fed from that circuit
have behaved erratically?

AFCIs until this year would only detect a "parallel" arc - H-N, H-G.
Starting 1-1-2008 they were required to detect a "series" arc you describe.

If there is an arc you will probably see flickering lights. Some other
loads there may be no indication.

As others have said, a loose connection can also produce damage (with no
flickering). An arc should give you more heat.

If there was a lot of tape I suspect workmanship. There should be no
need of tape with a wirenut. May also have been the wrong size wirenut
or a junk wirenut that was not UL listed.

There are a lot of wirenuts I won't use. They are hard plastic outside
the spring. The kind I like allow the spring to expand over the wires
and have more contact area.

There is no requirement I know of (manufacturer/UL/code) to twist the
wires before using a wirenut. It may be a good idea, particularly with
the springs that don't expand.

--
bud--