Does a 32amp distribution circuit wired in SWA to a sub board need RCD protection ?

[WilliamAtkin]

For context i am in my 2nd year in college and my head is going round in circles a question in my project for which i need to select the right protective devices for :
A 32A distribution circuit from the main board to supply summerhouse will have a four-way SP-N distribution board fitted supplied by a 3-core thermoplastic SWA cable clipped direct. In the main board all the other circuits are on RCBOs.
I'm pretty sure I can't have a RCBO feeding another RCBO because of discrimination , and I think because the supply is wired in SWA that the distribution circuit doesn't need RCD protection .If I am correct in thinking this could someone give me the reason to why the circuit being wired in SWA means no rcd protection is needed
What I have done for this question is a 32A MCB in the main board for this distribution circuit feeding the summerhouse board and in the summerhouse I have put RCBOs in that four-way board- Is this correct ?
I also had another thought that I could do a 32A RCBO in the main board for this distribution circuit feeding the summerhouse board and then have just MCBs in the 4 way - board
I am quite confused about this and have been looking through section 5 of the regs but the words and the way the regs describe things don't make things crystal clear to me

 

[timhoward]

You have the right idea.
The basic thinking is
"Do I need an RCD for fault protection"
"Do I need an RCD due to the circuit's use (e.g. 411.3.3 411.3.4, or any of the section 7 special location rules"
"Do I need an RCD due to the wiring system and how the cable is run"

The last one is the one of interest here - and the journey goes:
Section 5 - selection and erection of equipment
Section 52 - " " wiring systems
Section 522 - " " in relation to external influences
finally arriving at section 522.6 Impact

If you read through that bit, remembering that SWA has an earthed metallic covering, you'll see the reason why your hunch is right.
If still confused feel free to fire questions back.

 

[WilliamAtkin]

You have the right idea.
The basic thinking is
"Do I need an RCD for fault protection"
"Do I need an RCD due to the circuit's use (e.g. 411.3.3 411.3.4, or any of the section 7 special location rules"
"Do I need an RCD due to the wiring system and how the cable is run"

The last one is the one of interest here - and the journey goes:
Section 5 - selection and erection of equipment
Section 52 - " " wiring systems
Section 522 - " " in relation to external influences
finally arriving at section 522.6 Impact

If you read through that bit, remembering that SWA has an earthed metallic covering, you'll see the reason why your hunch is right.
If still confused feel free to fire questions back.

so the circuit is protected by the earthed metallic covering in the armoured cable

 

[mainline]

so the circuit is protected by the earthed metallic covering in the armoured cable

What is the earthing system at the house ?
What is the size of the swa?
Is there any extraneous conductive ?

 

[timhoward]

so the circuit is protected by the earthed metallic covering in the armoured cable

I could just say "yes", but the definition of protected is a little ambiguous.

I know this sounds daft but you don't need an RCD unless you need one!

So thinking about fault protection (will it turn off under fault conditions) - as long as the Zs is under the limit for the protective device, the answer is yes. So if it's a B32 breaker, Zs of less than 1.1 ohms (on site guide value). If it isn't (e.g. the earthing system is TT) then you will need an RCD to meet disconnection times.

It isn't any of the things that the regs say need RCD protection (e.g. a final circuit for sockets, domestic lighting,) or a special location so no rules there apply.

The final consideration is Impact Protection. This section of the regs is one of the most confusing to wade through but it probably doesn't meet the criteria for 201, 202 or 203 and in any case you meet the "or else" 204 rule as it has an earthed metalic covering.

 

[WilliamAtkin]

What is the earthing system at the house ?
What is the size of the swa?
Is there any extraneous conductive ?

its a college project - TN-C-S system
there is no info in the spec on what size SWA and there isn't enough info to work it out (i don't have any info on the ambient temp or grouping factors like I did for other circuits ) and in the cable calculations part of the project I wasn't ask to work out the cable size of this circuit but i would assume a 2.5mm or 4mm swa would be used
there is no info in the spec about extraneous conductive
but yh I am going to stick with my initial design of an mcb from main board in hotel to feed the summerhouse which will have rcbos
and I have just noticed that quote at the bottom of what you wrote saying “Why would a spark do something so stupid”
And 1st of all I'm not a spark yet I am an apprentice in my 2nd year at college and prior to starting my apprenticeship I had no experience with electrics at all . Furthermore its rude to make a comment like that when I was just asking for help on this forum

 

[WilliamAtkin]

I could just say "yes", but the definition of protected is a little ambiguous.

I know this sounds daft but you don't need an RCD unless you need one!

So thinking about fault protection (will it turn off under fault conditions) - as long as the Zs is under the limit for the protective device, the answer is yes. So if it's a B32 breaker, Zs of less than 1.1 ohms (on site guide value). If it isn't (e.g. the earthing system is TT) then you will need an RCD to meet disconnection times.

It isn't any of the things that the regs say need RCD protection (e.g. a final circuit for sockets, domestic lighting,) or a special location so no rules there apply.

The final consideration is Impact Protection. This section of the regs is one of the most confusing to wade through but it probably doesn't meet the criteria for 201, 202 or 203 and in any case you meet the "or else" 204 rule as it has an earthed metalic covering.

thank you I think I got my head wrapped round it in the end

 

[timhoward]

and I have just noticed that quote at the bottom of what you wrote saying “Why would a spark do something so stupid”

Easy - that’s his signature that appears on every post - it refers to a YouTube video by Delroy (eastway electrical) that a lot of us have seen.
So it wasn’t a personal comment aimed at you.

 

[WilliamAtkin]

Easy - that’s his signature that appears on every post - it refers to a YouTube video by Delroy (eastway electrical) that a lot of us have seen.
So it wasn’t a personal comment aimed at you.

Oh ok that makes sense my bad I'm new to this forum and I just read that at the bottom and thought it was directed to me

 

[WilliamAtkin]

I could just say "yes", but the definition of protected is a little ambiguous.

I know this sounds daft but you don't need an RCD unless you need one!

So thinking about fault protection (will it turn off under fault conditions) - as long as the Zs is under the limit for the protective device, the answer is yes. So if it's a B32 breaker, Zs of less than 1.1 ohms (on site guide value). If it isn't (e.g. the earthing system is TT) then you will need an RCD to meet disconnection times.

It isn't any of the things that the regs say need RCD protection (e.g. a final circuit for sockets, domestic lighting,) or a special location so no rules there apply.

The final consideration is Impact Protection. This section of the regs is one of the most confusing to wade through but it probably doesn't meet the criteria for 201, 202 or 203 and in any case you meet the "or else" 204 rule as it has an earthed metalic covering.

And in the final bit of my project of this part is to pick protective devices for the hotel block.
There isn't that much info on the hotel block but the info I have been provided with is as follows: "The existing hotel building forms part of an 400V three-phase TN-C-S system...A new CCU will be installed to supply the distribution circuits. Ze is 0.09 and PFC is 5kA" - There are 8 distribution circuits to feed 8 sub boards, each board then feeds 2 rooms- each room has 1 socket circuit , 1 air con supply and I'm assuming one lighting circuit although the spec hasn't stated it. There is no information on what cables will be used and how they will be installed(as with the café block in this project I was provided with this info).I was going to use RCBOs in the sub board and then 3 pole MCBs to supply the sub boards and I'm confused to whether these distribution circuits need RCD protection. If they do need RCD protection then I will put a 100ma RCD to protect these distribution circuits as I know I would need the 3:1 ratio with the other RCD devices(RCBOs in sub boards) and the RCBOs in the sub boards are going to be rated 30mA so if a fault happens in any of the circuits in the sub boards it then doesn't trip the RCD protecting the distribution circuits. But if no RCD protection is needed for these distribution circuits then I will just have the 3 pole MCBs on their own. Am I right in thinking this ?