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]

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