This from a while ago but reading about everyone elses problems tonight made me remember this one. The original job was the lights in a recent cellar conversion weren't working althoug they did work for a while. Sure enough you switched the MCB on, the lights lit briefly and then the MCB tripped. Upon looking into it the cable from the 6A (old MK type 2 I think) was melted together, so too was the 3 core to the fan but not the cables to the lights though, obviously melted due to too much current running though them but there wasn't anything else connected to the circuit like a shower and the MCB seemed to trip off ok, although there was a spare MCB sat on top of the CU.
The wiring in the bathroom was a mixture of new wiring and original red and black and at the pullcord there was a black single wire which from what I remember was the live trigger for the fan. What I think had happened is the bathroom fitter come electrician had thought the single black was a neutral and connected it to the neutral part of the fan maybe. The pullcord was already in the on position and when he closed the MCB it caused a dead short across live and neutral, the MCB being old then welded itself shut instead of tripping back off and overloaded all the cable until he saw smoke and turned the main switch off. He then replaced the MCB for a different one, rectified his mistake, switched on and it all worked again - for a while.
Does that sound feasible, can an MCB weld itself shut in such a scenario. I was so pleased to get it working again I forgot to take the original MCB with me to open p and see.
The wiring in the bathroom was a mixture of new wiring and original red and black and at the pullcord there was a black single wire which from what I remember was the live trigger for the fan. What I think had happened is the bathroom fitter come electrician had thought the single black was a neutral and connected it to the neutral part of the fan maybe. The pullcord was already in the on position and when he closed the MCB it caused a dead short across live and neutral, the MCB being old then welded itself shut instead of tripping back off and overloaded all the cable until he saw smoke and turned the main switch off. He then replaced the MCB for a different one, rectified his mistake, switched on and it all worked again - for a while.
Does that sound feasible, can an MCB weld itself shut in such a scenario. I was so pleased to get it working again I forgot to take the original MCB with me to open p and see.
