Rcbo and rcd in series

[martinh89]

Good evening, it's been a while since I've posted on here but I'm looking for a little advice.

I'm doing a job this week, it's a tt supply and I'm bringing a sub main to an out building sub c/u which will supply lights sockets, and also a leg that will run to the end of the garden to a decking area where an outdoor socket will go.

I'm going to protect the sub main with a 25a rcbo, my question is will I be fine with sub c/u not containing an rcd main switch, as this will be in series I didn't want any nusance tripping.

I've done my cable calculations and volt drop ect and sized the cable at 4mm/1.5mm swa because of the long run. The ze is 50ohms

 

[Pete999]

What is the earthing system where you intend to take the supply from?

 

[martinh89]

It's a tt earthing system, I know I don't have to make a separate tt system at the submain, just wondering is the rcbo will be enough for the circuits on the submain aswell as the submain it's self

 

[Pete999]

It's a tt earthing system, I know I don't have to make a separate tt system at the submain, just wondering is the rcbo will be enough for the circuits on the submain aswell as the submain it's self

Add an RCD on the end of the CU covering al the circuit fed from it, or are you intending to supply a socket prior the CU ie run a cable from your house to a socket and then to the CU???, or two circuit one feeding the CU and the other feeding the deck area??

 

[Leesparkykent]

Unless the RCBO is single pole switched neutral or double pole then you will also trip the time delayed RCD as the neutral will remain connected in the event of a fault. You will struggle to find a SPSN RCBO or DP RCBO in single module form...only one I know of off the top of my head is SBS.

 

[buzzlightyear]

delete

 

[Octopus]

Where are you buying 4mm/1.5mm swa from - that sounds a bit non standard!

 

[buzzlightyear]

I've done my cable calculations and volt drop ect and sized the cable at 4mm/1.5mm swa because of the long run. The ze is 50ohms

and the length of the cable is?

 

[HandySparks]

.. then you will also trip the time delayed RCD ...

Do we know that there is one?

 

[Leesparkykent]

Do we know that there is one?

Good point I had just assumed

 

[martinh89]

No there's no socket prior to the sub c/u and sorry the size of the cable is 4/2.5 lol fat fingers on a small phone, the length of the run in its entirety is 65 meters, I'm also aware the time delayed rcd tripping if not double poled, and in response, it's not, it's an old moeller board, this is why I was wondering if the rcbo at the main board will protect the sockets ect after the sub main

 

[buzzlightyear]

how did you come to 4mm 65 meters 25amp rcbo?

 

[Pete999]

No there's no socket prior to the sub c/u and sorry the size of the cable is 4/2.5 lol fat fingers on a small phone, the length of the run in its entirety is 65 meters, I'm also aware the time delayed rcd tripping if not double poled, and in response, it's not, it's an old moeller board, this is why I was wondering if the rcbo at the main board will protect the sockets ect after the sub main

I really thought you intended to put a socket prior to the CU ap,ologies if I misunderstood, how do you intend to supply the supply to the deck area?

 

[martinh89]

There will be further decking and a longer run, I measured the garden at 89meters but I'll be ending the run at a out door socket at 65, just future proofed it, and yes there will be a socket but not prior to the board, it will be at the end.

 

[buzzlightyear]

Would protecting the circuits at the sub c/u with rcbo be acceptable instead of a

what?

 

[buzzlightyear]

a clue for him mv/ a/ m

 

[martinh89]

OK I think I'll go with separating it from the main tt, and creating its own tt and protect it from the submain, think this will be the easier option and prevent any inadvertent tripping.
Cheers for the replys

 

[buzzlightyear]

I've done my cable calculations and volt drop ect and sized the cable at 4mm/1.5mm swa because of the long run. The ze is 50ohms

There will be further decking and a longer run, I measured the garden at 89meters but I'll be ending the run at a out door socket at 65, just future proofed it, and yes there will be a socket but not prior to the board, it will be at the end.

this confuse me now 25amp fuse ?

 

[Pete999]

this confuse me now 25amp fuse ?

I have a head ache now

 

[westward10]

Unless the RCBO is single pole switched neutral or double pole then you will also trip the time delayed RCD as the neutral will remain connected in the event of a fault. You will struggle to find a SPSN RCBO or DP RCBO in single module form...only one I know of off the top of my head is SBS.

What time delayed rcd.

 

[telectrix]

even going up to 6.0mm SWA your VD @65m will be approx. 10V. assuming a Ib of 20A, so i think 4.0mm is a bit of wishful thinking.

 

[Leesparkykent]

What time delayed rcd.

Read post 9 and 10

 

[westward10]

Read post 9 and 10

Sorry Lee I jumped ahead

 

[HandySparks]

OK I think I'll go with separating it from the main tt, and creating its own tt and protect it from the submain, think this will be the easier option and prevent any inadvertent tripping.
Cheers for the replys

Separating the earthing systems won't do anything to prevent inadvertent tripping.

 

[suffolkspark]

I STILL DONT GET WHAT 4MM/2.5MM SWA IS?

 

[Pete999]

I STILL DONT GET WHAT 4MM/2.5MM SWA IS?

A new one on me as well