According to bitzah bitzah@invalid:
On Wed, 4 Aug 2004 10:07:51 -0500, "Dan Kuechle"
wrote:

I have a dedicated 20 amp circuit that feeds 4 outdoor recepticals spaced
around my yard. There is probably a total of 300 feet of 12 AWG Romax
underground wire used, burried about a foot deep in our (very) sandy soil.
This setup has been working fine for years, until the other day when the
circuit breaker popped. We reset it, and things were fine until the middle

I am going to go with ground water rising as a first choice.

A trifle unlikely for tripping a 20A breaker. But...

But check these other two for 'strange events' that I have had to
investigate in the past.
An old house which was originally wired in VRI (a rubber based
insul) and 'modernised' with CB's and RCD's (I think equivalent
to those GFI devices that USA guys mention) kept having 'ghost
trips' of a CB . Turned out it was the cable stretched over a gal
iron water pipe that was being stressed as the house moved from
AM to PM on +30°C days.

Given that the OP's situation seems very simple with nothing exotic,
a temperature dependency like you suggest and a very worn cable sheath
seems most likely to me.

Ie: when you install a buried cable, where it exits the house
(or enters receptacle "standards" buried in the yard), there should
be a few inches of slack. Ie: a small loop of cable before
penetrating the house foundation.

If you don't provide such slack, repeated expansion/contraction cycles
could cause the cable insulation to degrade (ie: pressure from the rim
of the hole in the concrete), and the conductors touch at certain times of
day.

That said, I'm going to throw out another - do you have any groundhogs
or squirrels digging holes nearby? Could be something like a a chewed
chunk of cable shorting periodically (due to temperature cycling, or
another animal brushing the cable nearby on their nightly rounds).

And another: a wire inside a receptacle so close to shorting that
temperature cycling (or brushing past by above animal ;-) causes it
to ground short.

I'd start with killing the power, and inspecting the inside of every
box on the circuit very carefully. Then, if that doesn't yield anything,
carefully dig out where the circuit exits the house and check the
condition of the cable.

In any event, I believe you should be changing the circuit over to GFCI
protection (breaker or upstream outlet). The circuit is already dubious,
and the GFCI will probably help you find the problem faster while making
it safer in the meanwhile.

And of course, the breaker may simply be just about kaput.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.