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Topic Title: RCDs and TT
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Created On: 11 July 2013 09:40 PM
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 11 July 2013 09:40 PM
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drbreezy

Posts: 4
Joined: 30 April 2013

Hello people, in a domestic TT system is it acceptable to have a consumer unit populated with single pole rcbos only? Many thanks.
 11 July 2013 10:18 PM
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daveparry1

Posts: 6202
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Yes it is, the DP main switch will provide the required double pole isolation,

Dave.
 11 July 2013 10:30 PM
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drbreezy

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Joined: 30 April 2013

Thanks. Am I right in thinking there is more than one school of thought on this?
 11 July 2013 10:39 PM
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slittle

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Joined: 22 November 2007

Well, my school of thought follows Dave's providing it's a plastic consumer unit and you can assure yourself there is no chance of any CPC's or bonding conductors coming in contact with a live conductor before the RCBO's.


Stu
 11 July 2013 10:41 PM
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AJJewsbury

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Am I right in thinking there is more than one school of thought on this?

On this Forum? never!

SP RCBOs have the disadvantage that they don't disconnect a N-PE fault - which is a concern if there are upstream RCDs as they'll trip too, loosing discrimination.

I'm pretty sure the OSG lists an all-RCBO CU as an acceptable option for TT.

- Andy.
 11 July 2013 10:54 PM
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leckie

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Well I know some think that a grommet is enough to be considered sufficient protection from a set of tails not shorting to a metal-clad consumer unit/DB/MainSwitch. But we all have our own call on this. I personally would want an insulated enclosure prior to the RCD.

As Dave pointed out to me very recently, the regs only require the line conductor, to be rcd protected, I.e.. single pole protection, so SP RCBO's are compliant. Subject to fire risk assessment. For example, a petrol pump installation does require DP rcd/rcbo protection. Personally I fit DP rcbo's in a TT installation, but that's my preference. I keep DP bus-bar for that very reason.
 11 July 2013 11:13 PM
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redtoblackblewtopieces

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Just wwondering if the OP is coming from 531 and 531.2.1 ?
Kevin

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Safety through a Standard
Compliance by Approved Documents
 11 July 2013 11:29 PM
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AJJewsbury

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Just wwondering if the OP is coming from 531 and 531.2.1 ?

Only one line (as distinct from live) conductor in a (earthed neutral) single phase circuit.
- Andy.
 11 July 2013 11:35 PM
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redtoblackblewtopieces

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That would be bottem page 29 BGB , what I was trying to get at as to where the OP was getting the two schools of thought from as I've never heard this question before.
Kevin

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Safety through a Standard
Compliance by Approved Documents
 12 July 2013 09:31 AM
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daveparry1

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Kevin, it's because there seems to be a missunderstanding by some that each circuit has to have it's own individual DP isolation when in fact it's the installation as a whole that needs DP isolation,

Dave.
 12 July 2013 09:33 PM
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leckie

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The problem with SP RCBO'S is that if you have for example an S type 100ma rcd as an incomer on a TT system and 30mA SP RCBO'S on the outgoing final circuits, N-E faults will trip out the whole installation. Which may strictly comply......., but it's rubbish.
 12 July 2013 11:10 PM
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Skydog23

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The neutral is to be considered as a live conductor don't forget 😉
 12 July 2013 11:30 PM
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typiod

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 13 July 2013 09:27 AM
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daveparry1

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Skydog, Live conductor but not a "line conductor" which I think is the wording in the book,

Dave.
 13 July 2013 11:00 AM
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leckie

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Yes Dave, regs say "line" generally, but " live" if the installation is a particular fire hazard, eg flammable dust, etc.
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