Arcing on neutral at main switch

[Rockomuir]

Hello all, Been to look at a fault today which consisted of the lighting circuit tripping the mcb. When looking into it further no obvious fault became apparent. When I took a look at the d/b I found some scorch marks and melting of the plastic on the neutral rod from the load side of the main switch. The main switch is a 30ma RCD.Also when looking at one if the loops into a ceiling Rose I could see the start of some heat damage on the neutral there as well.I spent all day investigating all my dead test came back okay, ze values and RCD times okay. I do not think it is overload issue as it is a basement flat not many circuits, no major loads. The only other advice they gave was that they sometimes used the shower and the trip went and the could smell that nasty hot plastic smell.I have let it safe tonight but going back tomorrow hoping that the occupant in the flat above accommodating to let me investigate through her floor. Has anyone had a similar problem or any suggestions why the scor h marks. I have checked the terminations in the board and they Are okay , neutral nice and tight .Any input would be greatly appreciated

 

[truckster]

Sounds like a loose neutral, have you wiggled the connection? Sometimes it can feel tight with a screwdriver but its not biting the cable.

 

[Rockomuir]

Thanks I will have another look, but I checked that as that is what I thought the problem to be

 

[Marvo]

The common causes of heat at terminations is loose screws causing arcing or overtightened screws causing the wire to be damaged and the resistance of the damaged section causes heat. Corrosion and wiring that was previously annealed by overheating could also be a factor as well as general overloading or harmonics which is unlikely in domestic premises.. Either way I would remove the wiring, cut back and re-strip if possible and remake the terminations.

 

[GLENNSPARK]

I am just guessing here but what type of lamps are they? If non incandecsants (flourescent lamps) it may be inductance. I may be wrong of course but.....
Is it wired in singles? Are the neutrals in the same conduit as the line conductors? It may be just a breakdown in insulation as you mentioned it was the breaker that wasn`t holding. It may be someone more experienced than myself on here can help.......

 

[Blowcat]

I agree with Marvo, sounds like a poor connection best to reterminate.
Another thing to check is the continuity through the actual RCD/where the plastic is melting. I have replaced several D.P switches or MCB's due to a very high resistance through one of the poles.

 

[Rockomuir]

Thanks for the response, no it is not wires in singles and I believe they have had the RCD changed before due to the same problem.