Hi all, Can anyone help explain this phenomenon? 


In a singles in conduit install, one of my team noticed that in a switch room, when energising the MCCB to a 63A socket, an arcing noise could be heard in the trunking above her head. It sounded quite localised. She immediately switched it off, and we made arrangements to use a different 63A outlet for the equipment being powered from this circuit. 


On disconnecting the equipment, (a Final Distribution Unit in 7909 speak) it was clear that in the plug, either the L1 pin had been overloaded or had been arcing as a result of a loose termination, which is sad, as the unit had been recently maintained. The socket had suffered damage to it's L1 pin too, and the internal wiring (L1) between isolator and socket had suffered thermal damage. 


The installed wiring checks out fine on continuity and IR, and we haven't yet been able to find any damage in the trunking where the arcing sound was heard. - no smell, but to be honest it's hard to visual as the trunking is absolutely packed, and larger circuits have clearly been put in first, at the back. Socket outlet will be replaced.


Question is; is there any reason the arcing would present itself audibly, some 60m away from where there was clearly a fault, or could there be a second fault where the arcing was heard? The load on the FDU was around 16A a phase, and would have been mostly SMPS in LED luminaires, (so quite reactive?) but these luminaires don't light illuminate on power-up, hence using the MCCB as a means of switching-on. 


This all happened before lock-down, but thankfully we're heading back in to work, so it will be one of the first jobs to sort out. I'm thinking we might be able to get one of those inspection cameras with a flexible neck into the mass of cables, and hopefully trace a good length of the larger cores to rule out damage where the arcing was heard. 


Thanks, 


Dave