On net wrote:
Our house upstairs lighting circuit has decided to start tripping it's
circuit breakers - with no obvious cause. First suspect was that a wire
had come loose in the last light fitting installed which was a tight fit
for all the connections when it was replaced. I've removed the light
fitting and the dimmer switch, but still the problem persists.

Secondly, we have an alarm system whose power is derived from this
lighting circuit and is doing some rather strange things (spurious
alarms and modes). This alarm has been installed for years and hasn't
been seen by the installer for years either and I suspect the original
installers are no more. I know alarms have a lead-acid battery that will
eventually deteriorate.

It could be a malfunction in the alarm triggering the circuit breaker,
so calling out an electrician could be a complete waste, or it could
just be the alarm is spooked by a circuit fault.

Generally, the circuit breaker can be set, but after a while shuts off,
so I guess that would indicate some form of leakage rather than a direct
contact short.

Current plan is to disconnect the alarm from the lighting circuit and
see if the breaker still has a problem.

Any other thoughts?

Paul

Update:

Disconnecting the Alarm and security light had no effect on stopping the
circuit breaker and left me scratching my head. Even with all lights off
the circuit would eventually trip.

Looking at the consumer unit I remembered that we used to have an
electrical feed to the garage that had been connected to one of the
lighting circuits (before we moved in) but that had been disconnected.
On inspection I found that the cabling had been disconnected from the
garage but still lay in a covered walkway with some exposure to the
elements - this would have been the case for a few years now.

I disconnected that cabling completely and now the upstairs circuit
hasn't tripped since about 6:30PM yesterday.

I guess the disconnected cable had been tripping the circuit with the
effects on the cable from the weather. One of those cases where the job
should have been finished properly.

Thanks for all the great suggestions - much appreciated - I'll be back
if it acts up again!

Paul