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

[davesparks]

3 pole MCBs to supply the sub boards and I'm confused to whether these distribution circuits need RCD protection.

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.

 

[DPG]

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 ?

If they do need RCD protection then a 100mA device may not be suitable.

Also, for selectivity consider time delayed devices rather than 100mA devices.

Note, I'm not saying any of this is required in this case though. Do your SWA sub mains need RCD protection?

 

[nicebutdim]

If I've got this right there's 3ph supply to main db, then 8 distribution circuits to boards that each serve two rooms. Rooms have an aircon unit, socket and lighting.

Given that selection of materials will be a factor in any college assignment, I can't help wondering why 3 pole MCBs are being considered for protection of distribution circuits?

 

[WilliamAtkin]

If I've got this right there's 3ph supply to main db, then 8 distribution circuits to boards that each serve two rooms. Rooms have an aircon unit, socket and lighting.

Given that selection of materials will be a factor in any college assignment, I can't help wondering why 3 pole MCBs are being considered for protection of distribution circuits?

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 and due to discrimination I didn't think I could have a rcbo feeding an rcbo but idk I'm finding this quite confusing

 

[WilliamAtkin]

If they do need RCD protection then a 100mA device may not be suitable.

Also, for selectivity consider time delayed devices rather than 100mA devices.

Note, I'm not saying any of this is required in this case though. Do your SWA sub mains need RCD protection?

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

 

[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.