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.

 

[Darkwood]

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?

No it doesn't become live par' se, it potential can rise up to mains voltage with reference to N or E, it would just complete a circuit if it touched down to earth thus lighting the fitting, unless rcd was covering the circuit that is, if the load was inductive in nature that was broken then inrush spiking as the loose N arced would easily take out the mcb.

 

[ianphilip007]

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?

Are you saying the 230v on the disconnected neutral that was in contact with the earthed metal case cause the MCB to trip

 

[Deleted member 9648]

Are you saying the 230v on the disconnected neutral that was in contact with the earthed metal case cause the MCB to trip

It wouldn't because the current neutral to earth would be limited by the resistance of the load.

 

[Lagentium]

I was wondering if it was the cause yes, having had some surprises from neutrals in my younger years!

 

[bmines67]

What about the possibility of reversed incoming polarity?

 

[R G]

What about the possibility of reversed incoming polarity?

Could even have been reversed polarity somewhere within the circuit at some point , if the op had seen the situation he described he should have tested at that point and then at the origin and correcting the fault before re energising that circuit