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Topic Title: TT and double pole isolation
Topic Summary: Where is it in the regs?
Created On: 28 June 2012 06:07 PM
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 28 June 2012 06:07 PM
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Grumpy

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TT. Just run an SWA sub main to an annexe. Split the tails into an insulated enclosure, 100mA RCD-S and then, at great expense, a Merlin 80A double pole MCB as I have it in my head that double pole is required for TT. However, I can't find a reference to it in the BGB. Any pointers or am I barking? Thanks. Woof!
 28 June 2012 06:37 PM
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spinlondon

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537.2.2.1 "A device for isolation shall isolate all live supply conductors from the circuit concerned, subject to the provisions of Regulation 537.1.2."
 28 June 2012 06:47 PM
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OMS

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Originally posted by: Grumpy

TT. Just run an SWA sub main to an annexe. Split the tails into an insulated enclosure, 100mA RCD-S and then, at great expense, a Merlin 80A double pole MCB as I have it in my head that double pole is required for TT. However, I can't find a reference to it in the BGB. Any pointers or am I barking? Thanks. Woof!


Not barking - just that you could have relied on the slugged RCD for isolation (as it is double pole) and used a single pole MCB for overload and short circuit protection

Regards

OMS

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Failure is always an option
 28 June 2012 11:23 PM
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spinlondon

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Depends whether the MCB is intended to provide isolation.
 28 June 2012 11:39 PM
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OMS

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Well if the MCB is intended to provide isolation in addition to the RCD (sort of double blocking) then yes it will need to be double pole

For compliance with BS 7671 it doesn't need to be though - the RCD can perform that function - as could any double pole switch essentially - including the installation main switch, which by virtue of this 9probably being domesytic or similar) needs to be double pole regardless of earthing type.

Regards

OMS

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Failure is always an option
 29 June 2012 12:00 AM
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typiod

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So to work on one circuit in a TT installation the whole installation would need to be turned off if all you had were single pole rcbo's and a double pole main switch?
 29 June 2012 08:22 AM
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Grumpy

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Thanks for your replies. Who wants to make a profit anyway!
 29 June 2012 08:24 AM
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daveparry1

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That's about the strength of it Typiod but it would comply,

Dave.
 29 June 2012 08:26 AM
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davezawadi

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The MCB may not be a good idea on discrimination grounds either! Old fashioned BS88s are a better bet.

-------------------------
David
CEng etc, don't ask, its a result not a question!
 29 June 2012 08:41 AM
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OMS

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Originally posted by: typiod

So to work on one circuit in a TT installation the whole installation would need to be turned off if all you had were single pole rcbo's and a double pole main switch?


Effectively yes - but resilience and operational durability aren't really considerations of BS 7671 are they.

i was simply pointing out that there should be DP isolation - if that is a single device at the origin or multiple devices controlling individual or smaller groups of circuits then that's a case of engineering judgement.

In practice, wouldn't most sparks isolate the submain in the OP by just popping the RCD test button ? (and probably giving the RCD to only exercise it ever gets - )

regards

OMS

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Failure is always an option
 29 June 2012 10:20 AM
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Boyobach

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OMS - if only we lived in Electrical Utopia!
 29 June 2012 11:01 AM
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OMS

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LoL- then we'd have a double pole RCD and a double pole MCB for double blocking isolation of the sub main - in a domestic - utopia indeed -

Regards

OMS

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Failure is always an option
 29 June 2012 12:50 PM
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Boyobach

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But would we earth heating radiators with plastic pipes????
 29 June 2012 01:10 PM
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OMS

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Well I guess you or I wouldn't - I can think of one guy who just might though !!

OMS

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 30 June 2012 12:28 PM
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sparkingchip

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Which came first the chicken or the egg?

Or in this case the RCD or the MCB?

Does it matter?

Andy
 30 June 2012 01:07 PM
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ebee

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The Egg of course! Simples

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 02 July 2012 01:12 PM
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AJJewsbury

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Or in this case the RCD or the MCB?

Does it matter?

Ideally have the DP RCD upstream of the SP MCB - if you get a N-PE fault simultaneously with or after a L-N fault, you want the RCD to trip to remove the N-PE fault (especially if you have more RCDs upstream) - but if the MCB trips first and is upstream of the RCD, the RCD will be deprived of power and so won't trip. OK, the chances are small and the consequences usually not dangerous, but ideally...

- Andy.
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