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Topic Title: Poor advice or poor design.
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Created On: 12 September 2017 08:59 PM
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 12 September 2017 08:59 PM
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Gave advice over the phone for an electrical problem, electrical trip will not reset, where it is separate to the consumer unit or in the consumer unit. No separate, try turning off the switch on the consumer unit reset the RCD then turn on the consumer unit. Right that worked it not tripped again Ok will look what caused the trip next time. Well it was the garage with another RCD in the garage where the problem be. Both trips went at the same time, of course the problem went as the RCD in the garage stayed tripped and the fault was cleared. Now hundreds of premised are wired like this, 30mA RCD covering all the circuits in the consumer unit and another 30mA protects the garage or workshop with freezer.
Are these two RCDs in series? , No that and that makes a 1 which is an and gate (Series)
Are these two RCDs in parallel (or gate)? Maybe depending the way connected at the main source that or that makes a 1.
Well anyhow bad design, needs a comment in the regulations don't put RCDs with the same tripping current in series, (Sorry that should be parallel, defies the Boolean truth codes)
 12 September 2017 10:03 PM
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you mean in series.
Given the unreliability of RCDs, it is desirable to do this in high risk cases, so you find an RCD both at the post at the caravan site, and in the van itself.
Actually different ratings don't discriminate very well unless you have a very weak fault, so generally more useful is nested time delays.
Where earth protection relays are programmable, it is common to say have 300mA 1/3 second, feeding submains to distribution at 100mA, 1/10 second, in turn feeding final circuits feeding 30mA instant. It also only works if both L and N are interrupted at each stage, single poles opening devices like some RCBOs do not remove an NE fault for example.

regards Mike
 13 September 2017 04:38 PM
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Personally I think that having two 30mA RCDs in series as described is poor design. I accept the issue of reliability but that is where the time delay hierarchy as described by Mike should be part of the design if in an area of high risk.

One of my most frustrating customers, lovely woman but despite me saying I was very busy and would fit it in when I could and it would be a few weeks was chasing me and pleading on a very regular basis, had outside lights installed along with power and light in the garage, Two RCDs in series and very poorly installed outside lights with the consequence every time it rained and often when it did not the house RCD would trip but not the garage as the house RCD tripped much quicker then the garage RCD, neither was time delayed.

The original "electrician" had told the customer that if there was a problem the garage would trip first. No space to change or add anything in the house. It took me a couple of days to fix the lights which in addition to being poorly installed were from one of the sheds and in my view the chock block about 1/2 inch from the ground was not suitable for an outside light, all were rusty and most covered in soil, slugs, ants, earwigs etc. Not to mention the SWA mounted just under the surface of the soil, no glands on the SWA and no connections of the armour to earth other than via the slugs. Some of the lights were ELV with IP 66 transformers mounted in IPxx boxes with no waterproofing of the connections. and no drain holes. Aggghhhh.
 13 September 2017 04:56 PM
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Presumably the garage was added by the original sparky. Are you suggesting he should have changed the original consumer unit to have a time delay for the garage supply (which might have not been an option depending on cable route) I doubt the customer would pay for that when its not necessary.
If the garage supply was existing then down rating it from 30m/a protection is not an option) or just omit the rcd in the garage ( which would have had the same result only it would have taken longer to find the fault)
Sometimes you have to work with whats there.


Specialised Subject. The Bleedin Obvious. John Cleese
 13 September 2017 07:21 PM
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If I were designing it I would normally have SWA from the house CU, assuming detached garage, protected by a MCB and put the RCD in the garage CU. Or have the garage on a separate RCD or RCBO on the house CU with a main switch in the garage. I agree we have to work with what we have which constrains what we can install and I have often designed what I consider to be poor solutions because of limitations but I will always admit they are poor. On many occasions the electrician is given no choice as the cheapest option will have already been "designed" for him/her to install.

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