Additinal protection to TT submain

[Blacktail]

Feeding two outbuildings by overhead cable from TNCSsupply. Building one, steel framed barn,earth floor etc. Building two, stableblock, concrete floor/block walls and steel roof frames. Approx 20 and 30m respectively from supply.

For all obvious reasons going to TT each building and fiteach with its own DB. Sub main will run fromsupply to outside of first building to junction box. Here it will split to feed DB-1 and feed, via overhead, to stable block and DB-2.

Do i need to consider the “ daisy chained ” feed from buildingone to building two, to now be a TTsupply and therefore require additional protection. It won’t be supplied from DB-1 as it willsplit at the junction box.

Armour/cpc continuity will be maintained through junctionbox to downstream of DB-2. Bounced thisof my “Approver” and they say second feed requires additional protection as itis now TT. Not sure.:crazy:

 

[Engineer54]

Use a 300mA S type RCD for outbuildings submain supply, that should cover you for the additional protection. I wouldn't be tempted to install a standard 300mA RCD, as you will stand a pretty high risk of not achieving discrimination with the downstream 30mA RCD devices within the two outbuildings....

 

[D Skelton]

As Eng said, you need the 300mA RCD as it's an agricultural premises, but for discrimination purposes make it an s-type. One thing I will add though is that the second feed in the case of the installation you are describing in no different to the first feed. It requires no more protection that the first either as they are both one and the same daisy chained or not. Also, whoever told you it needed additional protection is wrong, additional protection can only be provided by a 30mA RCD or supplementary bonding.

 

[Richard Burns]

Each building should be it's own TT system, maintain the supply earthing for the submains and divorce each building separately to it's own TT earth.
I suppose you could have a long earth conductor from a rod at building 1 running to building 2 and use a single earth point, but this would not be good practice.

 

[Engineer54]

Think i must of misread that the original supply was PME, I was taking it as all being TT!!
So as R Burns states, maintain the TNC-S earthing for the submain supply and divorce at each of the two buildings in favour of a TT system(s)

There is however no need to keep the two buildings TT systems separate. Each building should have it's own earth electrode postion(s) but there is nothing stopping you from linking those positions. In fact, it will probably be advantageous and resulting in a much improved overall lower Ra value to do so.

 

[wozz]

Sorry it's not constructive but what's a "approver"?

 

[PEG]

Sorry it's not constructive but what's a "approver"?

It's the natural born enemy of the "Dismisser" ...

 

[Blacktail]

Thanks all. Seem to be having a lot of problem posting reply, also sorry for all the typo's. Had to cut and paste to get anything in the dialogue box and transposed v.badly. So as I was thinking really.

"The Approver"? The NICEIC helpline of course. I actually contacted them to confirm if Ex dairy farm, redundant farm building and a goat in the horse house falls under 705.

Thanks again