MCB tripping

[ianphilip007]

called out as a light had come loose from its ceiling bracket. customer reported that since the light had loosened, when ever they switched on a light switch the mcb tripped. No problems or trips before.

upon visual inspection it showed that a neutral ( one of two had come loose and was now in contact with earthed metal casing) all other connections secure and as expected.

no damage to cabling

reconnected loose neutral and circuit worked as normal.

As this was a MCB and not RCD or RCBO I'm baffled as to why the mcb tripped at all. can any one shed any light on this.

 

[Octopus]

Was the loose neutral in the fitting come into contact with the live in the fitting? Or was the casing live?

 

[ianphilip007]

No live conductor was secure, in its correct terminal. Earthing conductor secure, the only cable that was in contact with the casing was one of the two neutrals.

regards live conductors ( looped at switches, i assume as only one switch wire at the light fitting no loop)

 

[snowhead]

Was it really a Neutral or was it a switch wire coming back from the switch?

 

[ianphilip007]

Definitely a neutral, loop was at switches so just a single switch wire to light fitting, neutrals + earth looped at light fitting

 

[Darkwood]

It was probably arcing thus sending transients through the mcb and causing it to trip.

 

[tomspark93]

Did you carry out any tests? R1+R2, insulation tests and an rcd test would have been my first thought after that scenario.

 

[ianphilip007]

Did you carry out any tests? R1+R2, insulation tests and an rcd test would have been my first thought after that scenario.

no unfortunately it was a very quick call and i was time limited, the fault appeared obvious at the time it was only later that i thought hang on a n-e fault shouldn't trip the breaker.

arcing sounds like a good possibility one i hadn't thought of.

 

[davesparks]

Did you carry out any tests? R1+R2, insulation tests and an rcd test would have been my first thought after that scenario.

Why would you carry out an RCD test on a circuit which only has an mcb protecting it?

 

[tomspark93]

I wouldn't. But thanks for pointing it out to me that I misread the op.

 

[Pete999]

called out as a light had come loose from its ceiling bracket. customer reported that since the light had loosened, when ever they switched on a light switch the mcb tripped. No problems or trips before.

upon visual inspection it showed that a neutral ( one of two had come loose and was now in contact with earthed metal casing) all other connections secure and as expected.

no damage to cabling

reconnected loose neutral and circuit worked as normal.

As this was a MCB and not RCD or RCBO I'm baffled as to why the mcb tripped at all. can any one shed any light on this.

Are you sure the neutral black wire wasn't the switched live??

 

[ianphilip007]

Are you sure the neutral black wire wasn't the switched live??

yes i am, if it was the switch wire as you state when it was reconnected it would of just tripped as soon as the switch was turned on.The switch wire (red single) had not moved from its terminal in the light fitting. As stated neutral loop and earth loop at light fitting, lives looped at switch. Circuit wired in singles not t+E.

To add Mcb was tripping when any switch further down the radial than the light in question was switched on.

 

[Pete999]

yes i am, if it was the switch wire as you state when it was reconnected it would of just tripped as soon as the switch was turned on.The switch wire (red single) had not moved from its terminal in the light fitting. As stated neutral loop and earth loop at light fitting, lives looped at switch. Circuit wired in singles not t+E.

To add Mcb was tripping when any switch further down the radial than the light in question was switched on.

Ah there you go then not enough info

 

[ianphilip007]

Definitely a neutral, loop was at switches so just a single switch wire to light fitting, neutrals + earth looped at light fitting

it was there

 

[Lagentium]

If it was the neutral from the fitting itself that had come loose and touched the casing then wouldn't it become live due to losing its neutral connection?