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Sub CU, which end is the RCD?
I'm running power to a shed, not a long feed from an existing CU in
the garage. It will be fed from an MCB (or RCBO) in the garage CU and will have a small (2 circuit?) CU in the shed with one CB for power and one for light. So which end does one have the RCD protection? -- Chris Green · |
Sub CU, which end is the RCD?
On 26/01/2019 17:33, Chris Green wrote:
I'm running power to a shed, not a long feed from an existing CU in the garage. It will be fed from an MCB (or RCBO) in the garage CU and will have a small (2 circuit?) CU in the shed with one CB for power and one for light. So which end does one have the RCD protection? Assuming you don't need the RCD protection to disconnect an earth fault on the supply cable, then the RCD should be as close to the final circuit as possible. That way you make it easier to discriminate between faults, and stop for example losing the outbuilding lighting as a result of a trip on a power circuit. -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
Sub CU, which end is the RCD?
John Rumm wrote:
On 26/01/2019 17:33, Chris Green wrote: I'm running power to a shed, not a long feed from an existing CU in the garage. It will be fed from an MCB (or RCBO) in the garage CU and will have a small (2 circuit?) CU in the shed with one CB for power and one for light. So which end does one have the RCD protection? Assuming you don't need the RCD protection to disconnect an earth fault on the supply cable, then the RCD should be as close to the final circuit as possible. That way you make it easier to discriminate between faults, and stop for example losing the outbuilding lighting as a result of a trip on a power circuit. So two RCBOs in the garage/shed consumer unit in the 'destination' and an MCB in the 'source', yes? The whole installation is protected by a time delayed RCD as required by the rules. We have quite a 'long' installation as we have a large house and lots of outbuildings. At the 'source' after the company fuse is a time delayed RCD. This is in one of our garages and feeds a 'Henley Block' which connects to a local 'garage and rest of the site' consumer unit and a fuse switch which supplies the house. The house has a conventional CU with RCDs, RCBOs, etc. The 'garage and rest of the site' CU has RCD protected power circuits for our two garages, MCB protected circuits for the garages' lighting and one (soon to be two) MCB protected circuits for further outbuildings. The existing 'further' outbuildings comprise a small stable where there's a small CU fed by 2.5sq mm from the garage CU (20A MCB). So what I'm aiming to add is another MCB in the garage CU to feed another sub-CU with (as I said above) two RCBOs for power and light in the new garage/shed. OK, if it was all 'start from scratch' it might not be quite the same but it's not too bad! :-) -- Chris Green · |
Sub CU, which end is the RCD?
Logic would say at the start as you might get damage to the cable causing an
unsafe environment particularly if the cable run is outside part of the way. Brian -- ----- -- This newsgroup posting comes to you directly from... The Sofa of Brian Gaff... Blind user, so no pictures please Note this Signature is meaningless.! "Chris Green" wrote in message ... I'm running power to a shed, not a long feed from an existing CU in the garage. It will be fed from an MCB (or RCBO) in the garage CU and will have a small (2 circuit?) CU in the shed with one CB for power and one for light. So which end does one have the RCD protection? -- Chris Green · |
Sub CU, which end is the RCD?
On 26/01/2019 22:05, Chris Green wrote:
John Rumm wrote: On 26/01/2019 17:33, Chris Green wrote: I'm running power to a shed, not a long feed from an existing CU in the garage. It will be fed from an MCB (or RCBO) in the garage CU and will have a small (2 circuit?) CU in the shed with one CB for power and one for light. So which end does one have the RCD protection? Assuming you don't need the RCD protection to disconnect an earth fault on the supply cable, then the RCD should be as close to the final circuit as possible. That way you make it easier to discriminate between faults, and stop for example losing the outbuilding lighting as a result of a trip on a power circuit. So two RCBOs in the garage/shed consumer unit in the 'destination' and an MCB in the 'source', yes? Sounds ok The whole installation is protected by a time delayed RCD as required by the rules. Is this a TT earthing install then? In which case you will need RCD protection for the submain as well. A type S (time delay) RCD would then be appropriate for the head end, and normal RCDs/RCBO for the shed. We have quite a 'long' installation as we have a large house and lots of outbuildings. At the 'source' after the company fuse is a time delayed RCD. This is in one of our garages and feeds a 'Henley Block' which connects to a local 'garage and rest of the site' consumer unit and a fuse switch which supplies the house. ok so that gives you the earth fault protection on the submains. With a very large install you may find some advantage with more than one time delay RCD feeding the install in parts rather than as a whole (just to get a bit more total leakage budget). The house has a conventional CU with RCDs, RCBOs, etc. The 'garage and rest of the site' CU has RCD protected power circuits for our two garages, MCB protected circuits for the garages' lighting and one (soon to be two) MCB protected circuits for further outbuildings. The existing 'further' outbuildings comprise a small stable where there's a small CU fed by 2.5sq mm from the garage CU (20A MCB). So what I'm aiming to add is another MCB in the garage CU to feed another sub-CU with (as I said above) two RCBOs for power and light in the new garage/shed. OK, if it was all 'start from scratch' it might not be quite the same but it's not too bad! :-) -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
Sub CU, which end is the RCD?
On 27/01/2019 10:38, Brian Gaff wrote:
Hmm, not convinced. would you not wish to know that the cable has an issue? Define issue. If the head end is TN-S or TN-C-S, then any fault on the submain could trip the head end MCB if required. Having a fault trip your lights while working in an outbuilding may well indicate you have an issue, but not in a particularly useful way! -- Cheers, John. /================================================== ===============\ | Internode Ltd - http://www.internode.co.uk | |-----------------------------------------------------------------| | John Rumm - john(at)internode(dot)co(dot)uk | \================================================= ================/ |
Sub CU, which end is the RCD?
John Rumm wrote:
On 26/01/2019 22:05, Chris Green wrote: John Rumm wrote: On 26/01/2019 17:33, Chris Green wrote: I'm running power to a shed, not a long feed from an existing CU in the garage. It will be fed from an MCB (or RCBO) in the garage CU and will have a small (2 circuit?) CU in the shed with one CB for power and one for light. So which end does one have the RCD protection? Assuming you don't need the RCD protection to disconnect an earth fault on the supply cable, then the RCD should be as close to the final circuit as possible. That way you make it easier to discriminate between faults, and stop for example losing the outbuilding lighting as a result of a trip on a power circuit. So two RCBOs in the garage/shed consumer unit in the 'destination' and an MCB in the 'source', yes? Sounds ok The whole installation is protected by a time delayed RCD as required by the rules. Is this a TT earthing install then? Yes. In which case you will need RCD protection for the submain as well. A type S (time delay) RCD would then be appropriate for the head end, and normal RCDs/RCBO for the shed. Is that in addition to the one protecting the whole system then? How does it discriminate with the 'main' one? We have quite a 'long' installation as we have a large house and lots of outbuildings. At the 'source' after the company fuse is a time delayed RCD. This is in one of our garages and feeds a 'Henley Block' which connects to a local 'garage and rest of the site' consumer unit and a fuse switch which supplies the house. ok so that gives you the earth fault protection on the submains. Ah, OK, so that answers my quesion above. With a very large install you may find some advantage with more than one time delay RCD feeding the install in parts rather than as a whole (just to get a bit more total leakage budget). Yes, but this doesn't seem to be an issue at the moment. -- Chris Green · |
Sub CU, which end is the RCD?
On 26/01/2019 17:33, Chris Green wrote:
I'm running power to a shed, not a long feed from an existing CU in the garage. It will be fed from an MCB (or RCBO) in the garage CU and will have a small (2 circuit?) CU in the shed with one CB for power and one for light. So which end does one have the RCD protection? I put mine in the head end CU so the distribution cable was protected - then MCBs in the sub CU for outside circuits (I do not care if one circuit blacks out all outside circuits, none are critical). YMMV -- Email does not work |
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