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Topic Title: double pole isolation on TT installations
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Created On: 22 March 2013 07:34 PM
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 22 March 2013 07:34 PM
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MAXMIRA

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Joined: 25 January 2011

Can anyone tell me where i can find in the regs whether or not an RCBO can be used on a TT installation.

A few threads i've seen have indicated that a TT installation requires double pole isolation. Can anyone expand?
 22 March 2013 07:48 PM
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slittle

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It does indeed need double pole isolation but not necessarily double pole disconnection under fault conditions.

You need a double pole mainswitch, but you can use RCBO's on a TT installation. Personally I'd have a S type RCD mainswitch but that's because of the sort of sites I work on.


Stu
 22 March 2013 08:22 PM
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MAXMIRA

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Thanks, the board does have a double pole rcd feeding it although it's not selective its just a standard MG Multi 9 100mA RCCB.

Do you know where i can find details about this in the regs.
 22 March 2013 08:42 PM
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weirdbeard

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Have a read in this previous topic:

http://www.theiet.org/forums/f...AR_FORUMVIEWTMP=Single
 22 March 2013 09:49 PM
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leckie

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I've just fitted double pole rcbo's today to a TT system. The reason? Because I didn't want a neutral to earth fault to trip the rcd main switch. An S type will still detect a N-E fault and trip the entire installation. DP rcbo offers discrimination. Haven't got a regs book to hand but this is mentioned I'm sure. Or at least in a guidance note. It's certainly a good design choice.
 22 March 2013 09:56 PM
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AJJewsbury

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Haven't got a regs book to hand but this is mentioned I'm sure.

It is indeed - 531.2.9 for one.
- Andy.
 22 March 2013 10:18 PM
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Parsley

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Originally posted by: leckie

I've just fitted double pole rcbo's today to a TT system. The reason? Because I didn't want a neutral to earth fault to trip the rcd main switch. An S type will still detect a N-E fault and trip the entire installation. DP rcbo offers discrimination. Haven't got a regs book to hand but this is mentioned I'm sure. Or at least in a guidance note. It's certainly a good design choice.


I did the same a few months ago with external CCTV on an industrial estate to avoid any unwanted tripping.

Regards
 23 March 2013 10:29 AM
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MAXMIRA

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Can anyone advise on a suitable double pole RCBO for an old merlin gering metal clad board
 23 March 2013 12:01 PM
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slittle

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Merlin used to make them, not sure if they still do


Stu
 25 March 2013 01:46 PM
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weirdbeard

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Originally posted by: MAXMIRA

Thanks, the board does have a double pole rcd feeding it although it's not selective its just a standard MG Multi 9 100mA RCCB.





Can anyone advise on a suitable double pole RCBO for an old merlin gering metal clad board



Hi Max,

If you don't have a time delayed main RCD then you won't get discrimination with a 30mA RCBO whether its one or 2 pole -

When theres a fault the RCBO won't limit the current, only the time before disconnection, so the main RCD may still trip.
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