On 12/3/2015 2:26 PM, dpb wrote:
On 12/03/2015 3:08 PM, Don Y wrote:
....

E.g., note that when I unplugged the lamps this morning and re-plugged
them, they didn't trip the breaker. Yet, plugging them "cold" caused
an instant trip. So, modify the experiment -- wait *10* seconds
before re-plugging. Then, 2 minutes.

Ah, now I have new information to assist in formulating a theory
(what, in the circuit, can "account for time"?)

A thermal effect as postulated previously.

....

Agreed. I'll return to my original plan of diagnosis.

....

_IF_ you did still have high quality test gear, I'd tend to agree with your
attempt at more esoteric trouble-shooting. Lacking that, the likelihood of
placing a measurement at the right spot with the facility to catch the event is
approaching zero...

So you might as well eliminate the one common component from the problem before
continuing down all those various ratholes you've previously enumerated...

No, the logical approach is to imagine some agency (weather, infestation,
phase of the moon) that has asserted itself on the "system". Then,
think of what sorts of "changes" it could have made THAT WERE NOT
PRESENT, PREVIOUSLY.

E.g., leaf cutter wasps laying eggs *inside* receptacles -- given that
I know these exist, here; water infiltrating a fixture (nope, no water
sources); something chewing on insulation (unlikely); etc.

I.e., look for a simple explanation that ties all the observations
together, consistently. Yeah, it could be all my bulbs have
a problem; it could be all my extension cords; all my GFCI's;
etc. But, it's not LIKELY that all that stuff happened AT THE
SAME TIME. I'd buy it if I had a problem with one string
of lights last year, another the year before, etc.

OTOH, something affecting *one* of the receptacles would make
sense -- *if* a reason for the other observations ALSO made
sense!