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Topic Title: Tt inside and out, rcd tripping ?
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Created On: 09 October 2012 03:47 PM
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 09 October 2012 03:47 PM
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goldenboy1818

Posts: 492
Joined: 22 February 2011

Hi, tt inside at main board, supply out to shed, with separate earth rod for shed, earth from inside not connected into shed board. However rcd will trip inside and outside, both rcds trip, shouldn't just the inside the shed one trip ????
 09 October 2012 03:57 PM
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CPC

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What ratings are they both rated at 30 milliamps ? Does the rcd fitted in the house have a time delay?
 09 October 2012 03:59 PM
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AJJewsbury

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Nope. RCD's aren't like the old voltage-operated ELCBs. RCDs compare the outgoing (L) and returning (N) current - and trip if the difference exceeds the trip level (somewhere between 15mA and 30mA for a 30mA device). It doesn't matter which rod the current escapes to, it's the fact that it's left the L-N circuit that's the trigger. Both RCDs see that the same current has gone missing, so both trip.

If you need RCDs to discriminate with each other, the upstream one will need to be a higher rating and include a time-delay (S-type).

- Andy.
 09 October 2012 04:01 PM
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OMS

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How does a RCD work - imbalance between Line and Neutral which it assumes is a leakage going "somewhere" perhaps

draw it out on a bit of paper - stick in some numbers - will imbalance on the down stream part of the system also be an imbalance on the upstream part of the system - do the devices have similar (or identical) characteristics when presented with imbalance

Regards

OMS

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Failure is always an option
 09 October 2012 04:11 PM
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daveparry1

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It would be better to do away with the rcd in the shed and connect the shed earth to the supply earth, leave the earth rod at the shed in place if you want to but there's little point really,

Dave.
 09 October 2012 06:52 PM
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goldenboy1818

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Why better to connect into the supply earth ?
 09 October 2012 06:56 PM
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OMS

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What dave is saying is you have a source end RCD and electrode - just use the earth from the submain for the shed circuits - the second RCD and electrode are pointless.

Regards

OMS

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Failure is always an option
 09 October 2012 07:20 PM
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goldenboy1818

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Yea I understand but it's fine as it is ? I know overkill but if both tt perfectly fine ??
 09 October 2012 07:28 PM
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Legh

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Originally posted by: goldenboy1818

Yea I understand but it's fine as it is ? I know overkill but if both tt perfectly fine ??


Well its better to have RCD protection at either one end or the other. Having both, in the likely event of an earth fault, excessive leakage, voltage transients in the garden, house or shed is then likely to arbitrarily trip one, or the other or both.
My preference would be to mechanically protect the supply cable and RCBO the circuits as close to the point of utilization.

Legh

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 09 October 2012 07:33 PM
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peteTLM

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last time i done something like this, i made the feed end of the submain have a 100ma TD at the supply end, and then a 30ma inside the shed or garage on the DB. I only had one electride at the supply end, not at the far end, but dont see a problem if either the far one is omitted in favour of the supply one or the 2 tied together.

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 09 October 2012 07:41 PM
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goldenboy1818

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As it stands the earth from inside is not connected to the board, appears to be just earthing the armoured cable. Then the rod at the shed end is the earth for the shed. So a total separate earth zone.
 09 October 2012 07:43 PM
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OMS

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Originally posted by: goldenboy1818

Yea I understand but it's fine as it is ? I know overkill but if both tt perfectly fine ??


How is it fine as it is ? - it's tripping at either end and it's a lottery as to which one does trip anyway.

How do you get the idea that the two TT systems are perfectly fine.

It's not unsafe for sure - but it's a bloody daft design by any measure - and if it was based on some crazy notion of making the outbuilding TT, then the designer clearly didn't recognise the whole installation was TT - an exercise in futility.

If you want to keep the RCD the shed end, then the source end protection needs to seperated out from the house (which probably needs to keep a 30mA RCD for at least part of it) and you need a time delayed RCD of 100mA or above characteristics on that sub main- ie a type S at the supply end

regards

OMS

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 09 October 2012 07:48 PM
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OMS

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So a total separate earth zone.


It might be a "seperate earth zone" - that's not the problem - it's the through fault on identical RCD's - as we mentioned above, if it's leaking anywhere the RCD's don't care - if they are out of balance then they will trip - draw it out as I suggested

regards

OMS

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Failure is always an option
 09 October 2012 07:51 PM
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goldenboy1818

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Yes they must not of realised inside was tt, or maybe it was gonna be changed over earthing system? Like you say it's not unsafe. And with it being new cable and just a few sockets can't see too much trouble, customer wants minimum done
 09 October 2012 07:53 PM
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goldenboy1818

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How do you mean if it's leaking any where the rcds don't care ?
 09 October 2012 08:04 PM
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goldenboy1818

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I mean a fault inside could only trip internal rcd, but fault outside could trip either ?
 09 October 2012 08:34 PM
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DolbySurroundSystem

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Hi Goldenboy,
It seems like you have a electrode & source end rcd. Use the submain's earth 4 shed circuits & forget the 2nd rcd.
Good Luck Natasha
 10 October 2012 08:46 AM
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OMS

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How do you mean if it's leaking any where the rcds don't care ?


As I said, draw it out - put on a few faults, assign a few electrode values, work out the fault currents and map those currents right around the earth fault loop. At that point you should be able to see clearly that an earth fault in the shed will trip both the shed RCD and the house RCD (unless you provide discrimination by time - ie an S type at source - but be careful you aren't changing the protection for the rest of the house)

If the customer isn't bothered then all you have is a discrimination (or selectivity) faulure in design - no one will die, it's just a nuisance issue - if you want a reg number look at 314.1 (and in part 314.2) - if applicable.

I'm not sure of your role in this - are you fault finding, providing an EICR or just reporting to your client on what/why has happened to cause both RCD's to trip

Regards

OMS

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Failure is always an option
 10 October 2012 09:26 AM
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AJJewsbury

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I suppose two RCDs in series does give a little extra protection in terms of RCD reliability - which after all seem to have failure rates in the several percent range. While RCD redundancy isn't called for by the regs, it doesn't worth going to any effort to remove it if it's already there. Discrimination-wise it's no different to omitting the downstream RCD. It's what usually happens with caravans after all (one 30mA RCD at the pitch socket, another as the incomer inside the caravan itself). Like OMS says, it's really down to whether discrimination is actually required.
- Andy.
 10 October 2012 09:50 AM
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jcm256

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I like Natasha's solution, (trust a woman to come up with a simple answer just like that advertisement for oatabix) . In my short experiments have found that a neutral to earth fault could cause both RCDs in series to trip more so than a live to earth fault. RCDs are sometimes living on the edge, (Maybe the upstream one more so) where just a small amount of extra leakage upsets the balance and both trip. TT systems have no connection between Neutral and Earth making them exceptionally vulnerable to nuisance tripping through common-mode noise (unwanted voltage between Live & Neutral as a pair and Earth).
IET » Wiring and the regulations » Tt inside and out, rcd tripping ?

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