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

 

[DPG]

I don't know if they are SWA in the hotel area as no info is given
On a separate board in the café block I had one distribution circuit to a summerhouse which was stated as SWA
But for these distribution circuits there is no info on how they are installed and what cable type- I think I'm just meant to select the correct protective devices for the sub boards and distribution circuits so that they are eligible

It does seem like maybe a little more info should have been given you.

 

[timhoward]

Tbh with you I only work in domestic premises and I have never come across any 3 phase installations at work so I generally don't know. I thought that for 3 phase circuits you would need a 3 pole circuit breaker

If I'm reading it right you have a 3ph distribution board, 8 distribution circuits, and 8 single phase boards.
i.e. there is nothing that needs 3 phase for the pair of hotel rooms?

If so you can pretty much consider the 3 phase board as you would a single phase board, it just happens to be the case that every protective device will be using any one of the three phases.

Do you have plans or a means of working out the distance of the cable runs?
If you are then you should be able to work backwards working out the max demand of a pair of rooms, choose a likely cable size that can carry this, calculate the Zs for that cable size and the length, and then calculate the volt drop. If all that comes out ok for a given cable size then you won't need RCD protection to achieve fault protection.

 

[nicebutdim]

Better answer from Tim than I could have given.

Without knowing full details of the project I don't want to lead you in a wrong direction, but everything points to 8 single phase circuits.

 

[WilliamAtkin]

MCB's are rarely suitable for protection of distribution circuits as they don't afford much selectivity with the downstream devices.
Generally it is better to protect distribution circuits with MCCBs or fuses.

Would these MCCBs need RCD protection ?

 

[pc1966]

Would these MCCBs need RCD protection ?

Same issue as for MCB or fuse design - it depends on the end of circuit Zs being acceptable to meet distribution circuit disconnection times (typically 5s for TN systems) for the chosen OCPD, and assuming SWA or similar cable so no need for RCD "additional protection" against shock on cable damage.

 

[Penguin]

It’s obviously frustrating NOT to be issued with enough information for your paperwork test scenario.! But that’s probably as it’s not what their look for from you.
Start thinking about different value types of RCBOs ??? What does that ‘Letter’ before the amp rated number mean. or B32 C32 D32.

 

[mainline]

calculate the Zs for that cable size and the length, and then calculate the volt drop. If all that comes out ok for a given cable size then you won't need RCD protection to achieve fault protection.

That's still going to be dependent on cable type or installation method.

 

[timhoward]

That's still going to be dependent on cable type or installation method.

Agreed, read post #1 !