We were looking at a piece of kit yesterday, where there had been a fairly decent flashover across a set of terminals. No harm done, nobody hurt and virtually no damage apart from the melted ends of the input terminals.
The terminals were Klippon ST5, and the circuit was fed from the distribution board through a 100A MCB and 100A RCD.
The original cause of the failure was a short between one of the incoming leads and a chassis screw, but what surprised us was the subsequent flashover which actually went over the tops of all terminals in the group, presumably through the ionised metal vapour following the first initial short circuit. The terminals are rated for more than the applied voltage, there was no cover over the terninals as it is inside a cabinet with bolted on covers and suitable warnings about isolation. We don't think a cover would have helped in this instance.
The load had its own isolator and 32A MCB (which didn't trip) the distribution board RCD didn't trip, or rather it tried to but failed halfway, although the contacts did open, the 100A MCB didn't trip but one of the fuses (100A HRC) in the feeder switch fuse did blow.
We are now looking into why the RCD didn't open in time to stop the subsequent flashover, as the original fault was a straightforward connection between one phase and earth.
Conductors were 10mm tri-rated tails from the feeder cable to the appliance inlet, about 24" long, 16mm 4-core TRS feeder and 35mm in the distribution box.
Customer has asked us to make recommendations about replacement parts, but as far as we can see, all the parts seem to be fine as far as ratings go. The use of an MCB following HRC fuses is a bit of overkill, and if the 100A HRC fuses were to be replaced by motor-rated types, that would probably improve the discrimination between the MCB and the fuses.
The load is a 10kW output 110V charger.
Peter
-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web: