Just a quick question. I have a fault on a lighting circuit which was added to about 5 years ago. The fault only appears when the lights are switched on first thing in the morning, causing the mcb to operate. When the mcb is reset everything is fine. The building is an 1930`s wood building with stranded cable installation.

Surely you have done IR test? For a start?

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Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!

An IR test gives me 0.02 rising to clear and then when retested all clear.

Look for water/damp ingress.

Edited to add -

Chech ceiling extract or fan or online fans for water through ducting.

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Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!

This is obviously a commercial premises as the lights are switched on in the morning, if they are fluorescents then I would start checking for a faulty ballast or even an overloaded mcb or wrong type(b instead of a c) if of course obvious water ingress is not a problem as mentioned

This is likely to be your test meter charging suppression or power factor correction capacitors in the circuit. It is likely that your meter leaves the circuit charged at the test voltage after the test is finished. A new test reads infinity, as the capacitors are still charged.

Regards,

Alan.

Interesting questions come to mind.
1) Is this a new phenomenon, or has this always happened? Fittings with PFC capacitors often need circuits with type C or even D breakers to prevent the charge current tripping them. If new then something has changed, probably the capacitors have been replaced?
2) If this doesn't happen every time the circuit is energised, why not? Has the MCB been replaced or anything else?
3) It is likely that there is a fault present which clears after powering up, and reappears after a period. This may be polyester (self healing) PFC capacitors, just disconnect each PFC cap in turn, and the fault will go away.

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David
CEng etc, don't ask, its a result not a question!

Give the last man a medal. The was a small spike to earth. This was after a period of time turned off. Also should of mentioned this is a pre- war wooden hut that has had little or no maintenance since being built. I later found out this had been a problem since the install of the new lighting.
Thanks for all the help.