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?

 

[fatgreeny09]

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?

going to be a shop
no extraneous conductive parts all plastic
its all 16mm but upgrading will be as bad as getting a new main in. so customer said no to upgrade

 

[buzzlightyear]

what toffee shop !

 

[fatgreeny09]

what toffee shop !

no,, cafe,,,meat shop and coffee shop

 

[buzzlightyear]

why are you trying to make it to a TT system in the barn when their is no extraneous parts in the barn?

 

[buzzlightyear]

all answers on a postage stamp, and post it to father xmas north pole.

 

[Deleted member 9648]

I think the gyst of the thread is that the Zdb in the barn is too high for the 63a MCB although I don't have a BYB to check it. If that's the case I'd be looking to change the OCPD to a type which meets the disconnection time rather than adding RCD's where they are not otherwise required

 

[fatgreeny09]

why are you trying to make it to a TT system in the barn when their is no extraneous parts in the barn?

because customer doesnt want rcd in the house so i was going to make the barn tt but while im typing this just realised my mistake, board in the house would still need rcd for fault protection. how do i get a fire alarm in the barn and get sufficient fault protection on a 63 amp breaker in the house. can i put 300ma rcd in the house after the main board then put the fire alarm on before the sub board and before 100ma rcd or can the fire alarm not go on any rcd whatsoever ?

 

[fatgreeny09]

I think the gyst of the thread is that the Zdb in the barn is too high for the 63a MCB although I don't have a BYB to check it. If that's the case I'd be looking to change the OCPD to a type which meets the disconnection time rather than adding RCD's where they are not otherwise required

i would need to take it to 45 amp and wouldnt be sufficient for the shop

 

[Wilko]

Perhaps service connector blocks and a 63a switch fuse in the house, feeding the barn shop via swa ?

 

[Deleted member 9648]

Can you not feed it from a separate switch fuse with a 63a HRC? I don't have the table to hand but I'd guess that would meet the disconnection time with your Zdb and a 5s disconnection time.

Edit...Wilko types quicker lol.

 

[fatgreeny09]

Perhaps service connector blocks and a 63a switch fuse in the house, feeding the barn shop via swa ?

i was thinking this, is the max zs not less than a 60898 ?

 

[Deleted member 9648]

i was thinking this, is the max zs not less than a 60898 ?

Pretty sure it'll be higher, check it in the BYB for a 5s disconnection time.

 

[buzzlightyear]

have you got any pics of the job before we design it for you ?

 

[fatgreeny09]

have you got any pics of the job before we design it for you ?

its 0.82 at 100% but only 0.66 on the 80 % table

 

[bigspark17]

No extraneous structuall steel? Is the swa 16mm 3c? How far is the run. Soz for all questions but vital info if you want help

 

[fatgreeny09]

No extraneous structuall steel? Is the swa 16mm 3c? How far is the run. Soz for all questions but vital info if you want help

i really appreciate the help haven't been qualified long.

no extraneous steel

sub board and sub sub board both fed in 16mm 4 core, steel as the earth

 

[Wilko]

From OSG table B3, for BS88-2 fuse 63A with 5sec dis we need 0.62 Ohms at ambient, leaving 0.15 to get you to the barn shop

 

[fatgreeny09]

From OSG table B3, for BS88-2 fuse 63A with 5sec dis we need 0.62 Ohms at ambient, leaving 0.15 to get you to the barn shop

zs in the barn shop is 0.73 :/

 

[fatgreeny09]

zs in the barn shop is 0.73 :/

this is exactly why im stuck

 

[Wilko]

Ok, I bet you've measured the armour as it's the cpc. If you can use a 16mm core for cpc you will get there, I think.

 

[Wilko]

Do you need to take 3 phase over to the shop? The 50A fuse is 0.79, from the same table.

 

[fatgreeny09]

Ok, I bet you've measured the armour as it's the cpc. If you can use a 16mm core for cpc you will get there, I think.

the cable is already in and connected ( i didnt put it in)

i need the breaker to be 63amp feeding the barn, i cant upgrade the cable, so im looking at this as putting rcd on everything( which the customer is most happiest with) but if i put rcd in the house i cant put a fire alarm in the barn shop ( <---- thats right isnt it ) is my only option to bring a new feed for the fire alarm feed from the house?

 

[Wilko]

Understood. I'm still at trying to avoid extra RCDs etc ... I think you said it's a 4 core cable, do you need 3phase 63A in the barn or is the barn single phase? If it's single phase you can reuse a spare core for additional cpc and get the impedance down.

 

[fatgreeny09]

Do you need to take 3 phase over to the shop? The 50A fuse is 0.79, from the same table.

yes need 3 phase and 50a might not be sufficiant

Understood. I'm still at trying to avoid extra RCDs etc ... I think you said it's a 4 core cable, do you need 3phase 63A in the shop or is the shop single phase? If it's single phase you can reuse a spare core for additional cpc and get the impedance down.

needs to be 3 phase got 3 phase oven in barn and it feeds some 3 phase slicers

 

[Wilko]

Oh well, that idea didn't fly

 

[davesparks]

Why do you want to protect submains with MCBs? This is very poor design practice.
Why do you want the submains on RCDs?

 

[davesparks]

If the existing submain is not suitable for the increased load then the answer would be to replace it, not to bodge around it with poor design choices

 

[buzzlightyear]

you took the words right out of my mouth dave sparks

 

[davesparks]

you took the words right out of my mouth dave sparks

There's a song in there somewhere!

 

[buzzlightyear]

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 !

 

[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.