Intermitten fault, its tripped the mcb twice in the last week. I dont think its the starter it happens once the lights have been on about ten hours, im just not sure which light it is there are six light fittings.
Sounds like something is getting hot then maybe the insulation is breaking down. Is there a way you could isolate a couple of fittings and leave the remainder on and see if it still trips.
If not split the circuit out and test each fitting comparing the results of each.
If the power factor correction capacitor is breaking down (slight smell of burning) then too much current will be drawn and this increased current could trip the thermal part of the mcb after a few hours.
Get the insulation resistance tester out. These things sometimes work until they get hot as an earlier poster stated.
In a factory I used to do maintenance for there was an RCD covering several lights with magnetic ballasts and occasionally one of them would cause tripping. Testing live or neutral to earth usually pinpointed the culprit.
You could fit high frequency ballast(s) but those babies leak current to earth anyway (or they don't work) so be careful how many you use on an RCD
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