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

 

[davesparks]

this farm shop you saying ,calculate the existing load before the extra load .
you need to put some pics on here to so we can see the existing supply.
candy man !

Calculate? What on earth would you want to do that for?
Just pick an MCB rating that sounds good and add a generous quantity of RCDs 'just in case'

 

[bigspark17]

Sounds if you pulled a 16mm cpc in with the sub then this would get you too your max zs value. Is this doable? Stay away from rcds & mcbs on large subs if possible for discrimination of ocpd in series during fault condition. Even if you end up TT the barn the sub would still need to meet TN disconnection times up to the isolator. Understand your in a pickle but dont get led by the customers "tight budge" as you will still need to cert your work.

 

[davesparks]

What size is the existing sub-main? Is it ok for the increase to 63A, what sort of volt-drop is there going to be on it?

 

[fatgreeny09]

What size is the existing sub-main? Is it ok for the increase to 63A, what sort of volt-drop is there going to be on it?

It's a 16mm 4 core armoured. Not much volt drop at all. It's about 30 meters but that's just from r1 and rn test and rough distance

 

[davesparks]

It's a 16mm 4 core armoured. Not much volt drop at all. It's about 30 meters but that's just from r1 and rn test and rough distance

Not much at all? That's a great answer to a technical question.

A quick smoke packet calculation based on the full ratings of the ocpds suggests that your looking at 4.5V drop on the 63A submain and 3V drop on the 50A submain after that, giving a total of 7.5V drop on the submains alone.
That leaves you very little left for your final power circuits and nothing at all for any final lighting circuits.

 

[marc8]

Make sure that RCD up front (100ma) is time delayed, if you install a 30ma rcd down stream the 100ma rcd could still trip unless time delayed.