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Topic Title: TT Front end RCD
Topic Summary: Acceptable configurations?
Created On: 03 July 2013 07:45 AM
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 03 July 2013 07:45 AM
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SherlockOhms

Posts: 320
Joined: 05 April 2011

I came across a TT installation yesterday without any front end RCD protection.

CU is a split load single RCD. Unprotected ccts are 2xlighting and 1xsmokes.

Fairly easy to rectify but it did make me think.

It's acceptable to install a 17th ed board on a TT supply.
Considering the second RCD in such a board or, the one furthest from the main switch, then we have the links to the main switch then potentially up to 3m of tails upwind of the RCD.

So in the board in question I have very short meter tails (less than 500mm) straight into the main switch. Immediately after the main switch are the three non RCD MCB's.

I could move one of the three to a spare way on the RCD side of the board leaving one lighting cct and the smokes unprotected.

The sensible option here is to simply install 2xRCBO's in place of the remaining MCB's. This could be costly and tricky due to availablity.
Alternatively an RCD in the tails (very little space to accomodate).

If I were to install a RCD FCU immediatly after the CU and route the lighting cct and smokes (converting to one MCB) via that, would this give a similar level of comliance?

S.
 03 July 2013 08:01 AM
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leckie

Posts: 1758
Joined: 21 November 2008

I would try either:
1/ fit an S type 100mA rcd before the consumer unit.
or
2/ replace the main switch in the consumer unit with the same.
 03 July 2013 08:13 AM
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SherlockOhms

Posts: 320
Joined: 05 April 2011

Originally posted by: leckie

I would try either:

1/ fit an S type 100mA rcd before the consumer unit.

or

2/ replace the main switch in the consumer unit with the same.


But that wouldn't give me 30mA protection for the lighting cables (burried). It would I agree make the TT better.
Still a costly solution.?
 03 July 2013 09:29 AM
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daveparry1

Posts: 6150
Joined: 04 July 2007

If there's no other practicable way of doing it put a 30m/a rcd up-front (or main switch). I know it's not compliant but it will be safe and quite often it's not practicable to do things "by the book". Of course we wouldn't think of doing it on a new installation or re-wire but when dealing with existing installations we often have to do what's sensible in the circumstances!

Dave.
 03 July 2013 09:45 AM
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leckie

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Well, as you said, you could replace with a dual rcd board. If you can fit it in. That's probably the lowest cost option.

Or do as you suggested and fit 30mA rcbo's if you need to protect circuit lighting circuit, special locations, etc. However, you still need to protect against N-E faults for TT. So either the RCBO's will want to be double pole, probably not practicable within the existing board, or space permitting, fit another small board for the lighting etc with a 30mA incomer.

I did a TT con unit swap a few weeks back. I fitted double pole rcbo's for each circuit, now that is expensive! But thats the best job
 03 July 2013 10:03 AM
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AJJewsbury

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Swap the switch and RCCB?

So either the RCBO's will want to be double pole, probably not practicable within the existing board, or space permitting, fit another small board for the lighting etc with a 30mA incomer.

While I wholeheartedly agree it's a sound idea to use DP RCBOs, I'm not sure that the regs actually require that as even a SP RCBO will disconnect L and so remove the immediate shock hazard. If you had another RCD upstream and needed reliable discrimination then you may well need DP RCDs downstream - but neither seem to apply in this case.

- Andy.
 03 July 2013 10:08 AM
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leckie

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Why I was saying DP rcbo's was because if you just fitted sp rcbo's and a standard main switch, a N-E fault could still be present. So as it TT you could have a current flow N-E that could be a fire risk.
 03 July 2013 10:42 AM
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AJJewsbury

Posts: 11349
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So as it TT you could have a current flow N-E that could be a fire risk.

True, but do the regs require us to protect against that? It would be similar on a TN-S system (SP disconnection and voltage difference between PE and N) and between PE and true earth on PME systems, but we don't use all-pole disconnection on them. Likewise a conductor fault in series with a load (e.g. high resistance joint , loose connection or damaged conductor) would be a fire risk, but we (unlike the Americans) don't install arc fault breakers.
- Andy.
 03 July 2013 10:43 AM
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leckie

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Have a look at page 35 of this.
http://www05.abb.com/global/sc...04B0201_RCDs%20EN.pdf

Quite an interesting guide, rcd's made in the 1950's, I can't remember seeing any until the latter part of the seventies.
 03 July 2013 11:46 AM
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daveparry1

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There is no requirement for double pole rcbo's even if it is a TT installation. What's said is that there must be double pole isolation, this is provided by the DP main switch. Nothing says that the cpd's have to be DP,

Dave.
 03 July 2013 12:12 PM
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leckie

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Sorry, that is correct Dave, 531.2.1 does say "disconnectinmg all the line conductor..."

I was getting a bit mixed up thinking of protection against the risk of fire

reg 532.1(i)

"where in accordance with the requirements of 422.3.9...."
"the rcd shall switch all live conductors"

422.3 refers to locations with risks of fire due to the nature of processed or stored materials.

So in fact you could fit SP rcbo's and a DP mains switch in a TT and it would comply. Even though there would still be a risk of fire due to N-E faults. And as Andy pointed out, this risk also applies to TNS.

So there is an arguement that PME can be the safest system - providing the neutral doesn't break!

Im still going to carry on fitting DP devices though

Just noticed I have broken the 1000 post mark today, soon be catching up OMS
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