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Topic Title: Testing RCD's wired in series
Topic Summary: Testing RCD
Created On: 25 June 2015 01:53 PM
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 25 June 2015 01:53 PM
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SpiderBazz

Posts: 4
Joined: 24 June 2015

I wish to test 30mA RCBO in garage, but it is supplied from the house which has its own 30mA RCD.
Is it acceptable to remove the CPC for garage circuit and put it in the neutral bar of the house CU to stop the house RCD from tripping during testing of the down stream garage RCBO?
My thinking is that the garage RCBO would see the supply as if it were a TNCS and the house RCD would receive balanced current in both live wires.
Many thanks,
 25 June 2015 02:24 PM
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mapj1

Posts: 9349
Joined: 22 July 2004

yes, it will test what you want - but please only do this if you can control that no class 1 equipment is being used in anger during testing, and keep folk out until its all put back. Also does not work well if there is bonding to plumbing or whatever in the outbuilding, as you create an N-E bypass path.
The other less contentious trick is to just take the tester earth to the neutral of the feeder into the second RCD, but leave the final circuit CPC as it is.
That way the RCD under test sees the tester as a current down one side of the RCD but not back up the other, like an earth fault, but any upstream RCD sees it as L_N balanced like a normal load. This is sometimes easier, and does not involve passing through a step where you undo the outbuilding earth.

-------------------------
regards Mike
 25 June 2015 05:27 PM
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phantom9

Posts: 1757
Joined: 16 December 2002

Well my question to you is why have two 30mA RCDs in series? Take the one out at the front end if there is one on the circuit. The point you have raised is not part of official testing procedure because you shouldn't have the situation presented to you.
 25 June 2015 08:02 PM
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AJJewsbury

Posts: 15823
Joined: 13 August 2003

The point you have raised is not part of official testing procedure because you shouldn't have the situation presented to you.

It would be usual for caravan situations where both the supply socket and local CU are required to have 30mA RCDs.

Doubled-up RCDs have the advantage of providing some defence against the relatively high rate of RCD failures (reputedly circa 7% of installed RCDs fail to trip as required when tested.)
- Andy.
 25 June 2015 08:13 PM
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Jamster

Posts: 22
Joined: 02 December 2014

If its a split board in the house? you could move the circuit on to the non RCD side for testing purposes and then reconnect on RCD side.
 25 June 2015 08:40 PM
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mossep

Posts: 377
Joined: 05 December 2010

Temporarily bridge out the RCD that you do not want to trip. Not ideal but easily done.

-------------------------
www.mossep.com
 25 June 2015 10:11 PM
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westfield6

Posts: 210
Joined: 12 October 2007

Originally posted by: Jamster

If its a split board in the house? you could move the circuit on to the non RCD side for testing purposes and then

leave it there.

You don't need 2 RCDs in series.
 26 June 2015 12:19 AM
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mapj1

Posts: 9349
Joined: 22 July 2004

You expect, and the regs do require, two instant trip RCDs cascaded for a boat mooring supply at a marina - one on shore one in the boat, a caravan supply, one at the pitch post and one in the caravan, and now coming soon at a car charging point, though the second one is going to be more of a voltage operated device.

You also expect a fast and slow RCD cascaded on a farm or building site with for example 300mA 1/3 second RCD fire cover or maybe 100mA feeding a selection of final circuits on 30mA RCDs.


In any case, its always best to not go moving wires you don't have to to test -it just introduces a risk of creating a fault not previously there. Just return the testing current of the downstream RCD to the neutral from the one before, instead of the CPC.

-------------------------
regards Mike
 26 June 2015 12:24 AM
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SpiderBazz

Posts: 4
Joined: 24 June 2015

Thanks for taking trouble to reply.
The cable supply to garage is routed underground, but its path is unknown. Hence RCD protection for the underground cable is necessary.

In fact my preferred option was to bypass the upstream RCD, but I was having trouble getting access to it. I was afraid I would end up breaking its enclosure as it would not open with reasonable force.

Edited: 26 June 2015 at 12:35 AM by SpiderBazz
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