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Topic Title: Not latch
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Created On: 16 November 2012 04:27 PM
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 16 November 2012 04:27 PM
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Any theory's as to why a front end 30mA RCD will not latch on long run of underground cable roughly 60meters, with the consumer unit double pole main switch off at the other end. My theory is the capacitance between live and earth will conduct current, while there is no capacitance between neutral and earth, therefore an unbalance. Of course, 60 amps fused main switch also fitted at the front end, switch that off first, and then switch on RCD then main switch again latches ok. While this is not a usual occurrence at a consumer unit, I put this down to maybe one pole making before the other or the difference in the neutrals having less core area than the live in installations therefore an unbalanced current at switch on.

Caught out this week by RCD tripping (well it was sequence of events rather than lack of knowledge). Got phone call could you help whole house is in darkness. Farmhouse with the meter board in a farm shed, about 60meters from the house, TT with supply 30mA RCD at the meter board would not reset? Normal procedure for quickness switched off double pole main switch in the consumer unit. Pulled all re-wire able fuses at the house consumer unit, intending to insert one at a time to find which circuit was causing the tripping, Sent someone over with torch though the puddles and muck to reset the RCD, nothing, came back the RCD will not reset. Thinking it can't be the underground cable at this time of night had look my self, Sure enough the RCD would not latch, however switched off the local 60amp main switch at the meter board first then switched on the RCD then the main switch was the only way to get power. It was a earth fault on a lighting circuit, water from bathroom above leaking into a ceiling rose.

There again all theory, telling people we are in trouble, it is the underground cable before checking What would I know anyhow with outdated grandfather rights, and all for: I know you will not take money, would you take a cup of tea.

 16 November 2012 04:35 PM
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Silly Questions first:

Did you IR test the 60m of cable?
Did you test the RCD independantly - with no cable connected?

I'm sorry if I'm not reading the post correctly...
 16 November 2012 05:12 PM
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My gut feel is that 60m of cable in good condition shouldn't leak >15mA - regardless of how many conductors are (temporarily) live.

One pole closing before the other sounds like a good theory...

If there was poor insulation N-PE on the underground cable, it might survive OK when N is close to 0V, but leak more if the L pole closed first and some load still connected N to L, so temporarily bringing N up to close to line voltage.

As there was a fault downstream, I wonder if that was contributing somehow. I'm puzzling over how it might happen that it trips with one pole connected but not two. So far I can only make it work if there was some load still connected (e.g. not all the fuses pulled out) - even if it was one on a sound circuit and perhaps a dodgy isolator at the house end....
- Andy.

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