"Ajax" wrote in message
...
On Tue, 31 Jan 2006 16:22:05 -0000, (Chris
Lewis) wrote:

According to Ajax :

When one of our neighbors came back from vacation they found that the
20 amp single pole breaker for their hot tub and the 20 amp single
pole breaker for their refrigerator in their detached garage had
tripped. Neither GFCI type outlet served by these breakers had
tripped.

then stuff about a subpanel breaker going.

Sounds to me that they have to go over the innards of the main,
and the feed line to the subpanel inch by inch. Wearing
eye protection and gloves, wiggle the cable on every staple and
box clamp. Check for wire exposure on the inside of every box,
gaps in tape covering of splice connectors etc.

It could be a piece of defective cable, or an over tightened
box clamp that only causes a short when the cable moves (thru
temperature changes, wind shake or whatever).

I have encountered defective cable where it's not possible to see the
defect, even after it's blown the breaker multiple times. In
my case, it was a solid short. In this case it sounds like an
externally-induced intermittent.

An intermittent short of this nature won't be detectable with any
kind of metering until its fried the insulation enough to provide
a carbonized conduction path. You don't want to let it go
that far...

Either that, or they have gremlins.

Run the thermal detector over the inside of the main panel
and subpanel too.

Chris, I think that you were very close to what we discovered was the
problem. After replacing the 60 amp double pole breaker with another
new breaker, it continued to trip.

Thinking that one of the Arc-Fault breakers in the subpanel could
somehow be at fault, all breakers were removed from that panel and the
60 amp breaker continued to trip after about ten minutes.

When the conductors were removed from the 60 amp breaker, it didn't
trip. A test of the resistance in the conductors (underground UF
cable) were within normal ranges.

Next step: excavate the UF cable.

Problem: One of the homeowners buddies who had helped him install a
PVC line for a sewage pump had nicked the UF cable with a saw while
cutting the existing PVC line. He just drove the saw into the ground
as he cut the pipe, even though he had been warned that there were
electric lines in that area.

The cut in the insulation was enough to cause some arcing between one
of the hot legs and ground. We suspect that the recent we weather
enhanced the ability of the conductors to arc. Part of the mystery is
that he did this work last Wednesday and it took until the following
Tuesday to start tripping the 60 amp breaker.

A few splice connectors with heat shrink insulation and the circuit
is purring like a kitten.

Lesson: Never trust a plumber with anything sharper than a can of PVC
glue.

In retrospect, what is really strange is that the saw cut never
tripped the breaker and he nicked both a hot leg and ground. I looked
at his saw and there was no sign of arcing that is so indicative of a
metal object hitting a live cable.

Anyhow, thanks to everyone for their input.

Maybe someone was smart enough to turn off the power before working around
electrical lines?
The fact that the lines were installed that close makes my head hurt.
Anyway, thanks for posting the solution. It isn't often we get to see that
in these groups. And we can all use the knowledge!