MCB tripping help

[dtec dave]

Afternoon. I have a problem on a lighting circuit where the MCB is tripping randomly. It is when just 1 of the lights on the circuit is turned on, I have changed the MCB, the switch and the light fitting, I have also done an IR test which was clear. It will trip at times of anything between 30 secs and 30 mins. Can anyone point me in the right direction?

 

[jackhammerJIM]

what light fitting is it ?

 

[dtec dave]

It was a chrome fitting with 4 gu10 halogens but I have swapped it to a pvc pendant, but it is still tripping. it only trips when the lamps are in and not when there are no lamps in but turned on.

 

[Plonker 3]

What were you IR results? Did you carry the IR test with all switches on the circuit in the closed position?

 

[dtec dave]

I have IR tested the whole circuit with all switches open and closed.

 

[Plonker 3]

But what readings were you getting?

have you carried out continuity tests on all the conductors?

 

[dtec dave]

>1000 M ohms @ 500v. I haven't checked for continuity on all the conductors yet, would a high resistance reading be causing this?

 

[Pete999]

Tried changing or removing the lamps and then testing?

 

[Plonker 3]

>1000 M ohms @ 500v. I haven't checked for continuity on all the conductors yet, would a high resistance reading be causing this?

Do you remember the VIR triangle from college?

 

[dtec dave]

Of course, just need a bit of help using it to solve my problem!!

Do you remember the VIR triangle from college?

 

[Plonker 3]

Well yes high resistance could cause the circuit to trip.

have you checked all the other accessories on the circuit?

 

[polo1]

If it's the ground floor lighting circuit, have someone walk about upstairs as you check the resistances; very common to have cables damaged by flooring!

 

[dtec dave]

I was only there a couple of hours this afternoon, I came away to try and arm myself with some information before I went back, so I haven't yet checked all the other fittings on the circuit. As all the other fittings are working fine I assumed it would have been a problem with fitting that is tripping the circuit.

Well yes high resistance could cause the circuit to trip.

have you checked all the other accessories on the circuit?

 

[Plonker 3]

Fault finding is a art, could be solved in half a hour could take several hours. You need too be through and keep notes of all readings as you go to help spot any discrepancies. No right or wrong way, just work logically when you strip the circuit down.

 

[i=p/u]

Every time you find the fault you say if I'd carried out the tests taught at college it would of found this