Adding sub board to main consumer unit on tt

[fatgreeny09]

bit stuck on this job got a consumer unit (in a barn) fed from a house (tns) 3 phase mcb 63 amp the zs at the barn is 0.76 but after adding a rod and making it TT managed to get zs down to 0.5. ive got to add fire alarm panel and another sub board. there will be 100ma rcd before the consumer unit but the bit im stuck at is should the fire alarm feed also be before the 100ma rcd and what about the sub sub board, its 25m away from the sub board in 16 mm armoured.... would the best thing to do to add 300ma main rcd off that add 100ma to both the sub boards then 30 ma from there ? any advice appreciated

just to be clear it goes like this..... house tns with main board> barn sub board fed from house (also fire alarm going in same place ) > sub sub board fed from sub board

 

[Deleted member 9648]

Why are you ditching a perfectly good TNS earth? Or is it TT with an Ra of 0.5 ohms.???
Confused

 

[Andy78]

Why are you ditching a perfectly good TNS earth?

I'm not sure he has. He says he has added a rod (singular) and achieved a Zs of 0.5 ohms.
I'm betting the TN-S path is still connected via the distribution circuit.

 

[Deleted member 9648]

So it's a TTTN-S then !
Next question.

So what's with the plethora of RCD's then?

 

[Andy78]

Maybe just pre-empting 18th edition requirements.

 

[fatgreeny09]

Disconnected the tns earth and just added the rod. The zs was 0.6 sorry not 0.5... I've left it as tns at the moment on a 40a mcb until I get my head round what to do with the fire alarm and sub board. Just had a thought though. The sub sub board I could split off 1 100ma rcd with the sub board but still stuck with the fire alarm should it go before or after the 100ma rcd ?

 

[Wilko]

Hey Greeny, don't mean to doubt you, but in my humble opinion no way "a rod" will give less than 1 Ohm. I'm betting there's still a connection to the house means of earthing

 

[fatgreeny09]

think it was still connected actually now mention it think i left the armoured connected hahahaha

 

[buzzlightyear]

but still stuck with the fire alarm should it go before or should I ring the Samaritans !

 

[Dave OCD]

That makes sense, to get an Ra that low on a TT you'd probably have to bury an aircraft carrier and use that as a rod.

 

[Hellmooth]

Hehe Dave

 

[fatgreeny09]

ok so heres what ive got,,,,,, ze in house is 0.47,,, sub board zs is 0.73,,, sub sub board is 0.82

house mcb needs to be at least 63 amp mcb to feed the sub board in the barn,,,barn sub board mcb needs to be at least 50 to then feed the sub sub board.....the customer is really objective to having a rcd inside the house incase it trips. any body see an option for me here ?

 

[buzzlightyear]

barn wood structure or metal or ring a friend !

 

[fatgreeny09]

barn wood structure or metal?

brick,, its getting converted to a shop,,, (always been brick too )

 

[bigspark17]

Couple of questions.
What is the purpose of this outbuilding isit a leisure building, why does it need a fire alarm?
Are there any extraneous conductive parts?
What size/core swa is feeding the outbuilding?