what am I missing?

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Got a call from a hotel about a room where the lights have tripped, checked it out not good,bedside lights wired from L2 on bathroom light switch etc.Changed to permenant live replaced fuse everything good.
10 days later got a recall,powers off again checked found bathroom light had loop to redundant light left above ceiling in shower.I figured steam going into void and shorting out.Removed connection tested ok .IR over 1 megohm ,all lights running using 0.5A .Got to go back tomorrow its gone .Any ideas?
 
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You've only got 1Mohm?

I work by 2Mohm minimum, but even that is low, it should be a lot higher, into the hundreds really. Unless it is RCD protected, I doubt the steam is tripping the breaker, unless it is reforming into water and logging a JB or similar.

You should be dropping all fittings and switches on the circuit that's failing and meggering each leg to find where it's down.
 
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Wouldn't expect 1MOhm to blow a fuse, but perhaps indicative of some carbon buildup where there's a short appearing. Had this annoying type of fault many times. Nearly every time it's down to a screw having nicked a cable, and sometimes it took months before a problem developed.

Electric shower randomly blowing the fuse in the old Wylex board every few weeks: Changed shower, IR the cable, etc., still happened. Eventually it shorted completely and stayed that way. Bashing a hole in the plasterboard and pulling cable out showed a screw holding shower in place had just pierced cable. Pressure against the shower wall when someone leant against it pushed it in far enough to make contact with the conductors.

Random RCD tripping: Found high resistance neutral to earth reading. Eventually found ring main cable squashed between metal upright and plasterboard with a plasterboard retaining screw just catching it.

Random blowing of lighting circuit 5A fuse. Would go days or more working fine, then blow. Eventually found cable running under floorboards of apartment above had been overlaid with ply and screwed down. Screw straight through cable, just stripping a bit of insulation off the live while nicely touching the earth (t+e). IR initially showed >200MOhm on the loop. Few blown fuses later, just a slightly low reading but still many MOhms. Carbon had been building up from the occasional contacts until that part of the cable was thoroughly cooked and charred, at which point the short became obvious when IR'ed.

With most of these faults, we isolated each run of cable on the circuit and independently IR'ed it like the person above says. Took ages but narrowed it down.
 
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Sorry ,should've said greater than 1 meg.
Its a 10amp breaker controling all electrics going into a large juction box which then goes into a fused spur for the lighting circuit.Replace the fuse it works fine.
Put a customer in the room and it still works but randomly over the next few days it will blow,sometimes just the 3a fuse other times the 10a breaker.There is no pattern
 
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At my last job we had an A/C unit that kept tripping it's supply randomly. The sparky we use knew just where to look.

As I mention SWA I assume many a seasoned sparky will guess ...

.. the installer had nicked the insulation while sawing the wire armouring, and it was arcing between live and metalwork - but hidden behind the cable.
 
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