If the conduits are under the slab, water in the conduit can be causing the
problem, and if they're poured in the slab, concrete could have seeped in
and deteriorated the conductors insulation. Either way, if they're in
conduit, pull out the offending conductors and pull in new ones



"SteveR" wrote in message
...
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.