Three RCD's in one ring main.


Postby Dadwood » Mon Mar 02, 2009 5:01 pm

The instructions that came with these RCD's state that when an earth leakage is detected there is no way of telling which of the three RCD's installed in the same ring main will trip first. I suppose this means that if, say, my washing machine were to develop a fault, it's RCD would not necessarily be the one that tripped first. In such a case, would the w/machine RCD still trip, albeit a split second later?
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Postby stoneyboy » Mon Mar 02, 2009 10:34 pm

Dadwood,
Assuming they are all in the same circuit and all rated the same it is most likely that the one detecting the highest leakage would trip first, the others will do nothing.
end
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Postby Dadwood » Tue Mar 03, 2009 8:34 am

Hi stoneyboy, thank you for your kind reply. Now that I've read my original question again, I'm not sure I've explained it properly. So let me try again. All the socket outlets on the ground floor of my house are wired on the same ring circuit. In the kitchen, one of the 2 gang sockets has an in-built RCD. A spur has been taken off of the back of that socket and wired to another RCD which serves my washing machine. So that makes 2 RCDs off the same ring circuit. Now, Volex say that when more than one RCD is connected on the same ring circuit there is no way to tell which will trip first if earth leakage occurs. So, if my washing machine develops an earth leakage fault but its RCD doesn't trip first, will it still trip, albeit shortly afterwards? (If it don't trip the earth leak problem will continue.)
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Postby kbrownie » Tue Mar 03, 2009 12:52 pm

Hi Dadwood,
If both RCD's are of the same rating say 30ma and are both heathly and functioning correctly (not faulty or tired due to tripping many times)
And the faults is at your washing machine that RCD should pick up phase-neutral ballances and function.
In therory this could trip the second RCD as a phase current has been delivered but no neutral has returned.
What's your problem?
KB
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Postby Dadwood » Tue Mar 03, 2009 8:06 pm

Thank you for explaining that kbrownie. You asked what my problem is. Well, it's the fact that in my ignorance about all things electrical I thought that if the RCD not protecting the washing machine was the one to trip first, the second RCD might not then trip thus leaving the machine connected and still with the ongoing fault. For thick people like me, it just shows how very valuable this forum is. Again, many thanks.
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Postby kbrownie » Wed Mar 04, 2009 8:08 am

This would only happen if the RCD at the washing machine was faulty as mentioned in previous post, If a fault occured in between these two rcds it would trip the rcd that the spur was from.
KB
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Postby sparx » Wed Mar 04, 2009 8:37 pm

Hi Dadwood et al,
just to give an easy eg.
if say you have a ring circuit and a lighting circuit plus a cooker on a consumer unit with a main 30mA rcd as main switch, plus a local 30mA rcd socket or spur on the ring.
the rcd socket / spur will see just the leakage from the downstream device (washing mc. say) but the main one will also see leakage from washing mc. plus all other circuits, so in practice the upstream device will always trip first, sometimes without the local one tripping at all.
EG, lights leakage 5mA, cooker 10mA, so already approaching main rcd's limit (which dispite its 30mA rating usually trip around 19-27mA), therefore washing mc leakage of say 10mA takes main rcd over limit (5+10+10 = 25mA) but local rcd only sees 10mA.
regards SPARX
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Postby Dadwood » Thu Mar 05, 2009 4:59 pm

Thank you sparx for your clear explanation - in fact, thank you to all who gave replies. I can now proceed in the knowledge that provided all RCDs are in good working order, the required protection will be met. Many thanks again.
Dadwood
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Joined: Tue Aug 12, 2008 4:17 pm


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