Similar readings to those you have are often the result of insects or
other small creatures across connections. Spiders will cause this but
more likley is worms or slugs. Corrosion or degredation of the
insulation on its own will not cause this, but a hot spot as caused by
a poor connection is a magnet to little creatures who try to get as
close to the heat as possilbe and then .... ZAP! Often there is
enough current flow to burn out light gauge cables, particularly where
conventional fuses are used (mcbs will almost certainly trip). The
trouble is that when you pull the wire out you might not see any
evidence of the intruder as it may well get left behind in the tube,
although is now harmless!


SteveR wrote:
Anybody ever run into this? I'm trying to figure out what's causing it.

I've got a 40-year old house in which much of the outlet wiring is in steel
conduit under the slab (or in it, for all I know). The wires are individual
#12, with alternate phases sometimes sharing a neutral.

In the first incident I suddenly lost one branch circuit completely. The
wire had no continuity from the panel to where it comes out of the slab. Two
other branch circuits in the same conduit started tripping their breakers
intermittently. I discovered that randomly the hot wires would have low and
variable resistance to neutral. (And yes, I measured with the breaker off
and absolutely nothing connected to the problem runs.)

I sort of shrugged off the first incident, but now it's happened again. This
time another run of conduit carrying just one branch circuit developed the
same intermittent low resistance to neutral problem, causing breaker trips.

The resistance measurements are particularly puzzling. Sometimes the meter
shows a few hundred ohms, gradually creeping up over minutes as if some
large capacitance is being charged. And then suddenly the resistance will
drop to 20 or 30 ohms or jump up to a few thousand ohms.

I've worked out fixes for both problems, but I'd really like to hear if
anyone has any idea what's going on. Could it be the slab settling on the
conduit? The floors are flat and level and I see no foundation cracks.