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Single submain to multiple buildings disconnection times

Will Elec

Hi all, got an interesting project, that I'm in the planning stages for.

There is a 3 phase supply in a small outbuilding that is to be exported to 2 houses (same owner on 1 site), the supply is 100A PME (0.35 ohms).

The PME will be exported via SWA armouring and a separate 35mm 6491x cable (to both houses for bonding and CPC)

The supply head, the first house and the second house are in a straight line so in effect an SWA cable can come from a switch fuse (100A) in the outbuilding, go to the first house and then loop out into the second. Due to the location of the buildings it wouldn't be practicle or economical for the DNO to bring the service head closer or more centralised.

The first house needs a 45m run of SWA and then there is a further 40m to the next house. I intend on using a 95mm 4 core SWA cable for the first 45m then terminate it in an enclosure and split off in 25mm 4 core into the first house and 50mm 4 core for the next 40m to the second house. The 50mm cable would be in a duct that is already buried and is the largest cable size that could be used and still realistically be pulled through.

Now the above works for the voltage drop requirements but I am having trouble with disconnection times as the first house would meet the required 5 seconds (just) but the second would not, as I can't increase the 50mm cable in size I'm thinking a time delay RCD protecting the whole instalation is the way to go? I hope that all vaguely makes sense ish! Thoughts would be appreciated, thanks!

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  • 0

    What's your calculation for the pefc at the second house? Did you assume max operating temperature of the phase?  In reality with a very oversized cable it could not be that hot so it will have less resistance.  What about the swa resistance.

    From table 41.4 in 2018 regs it looks as if BS88-2 gG 100 A needs 0.42 ohm loop. Are you sure you're above this?  Or are you using a different fuse.  Change the fuse type?

    The main trouble is the external loop which is so near the threshold. Even doubling your cable area would only make a small change to the total loop.

    As you're space-limited you could better increase the 35mm2 parallel wire to reduce earth loop Z.

    Two smaller cables with 63A or 80A fuses?  Or further fuses in the enclosure on the first house, to protect the cable to the second?  It's not likely that a house really needs even 63 A three-phase. Nice to have some separate protection too.

    Worrying about a tiny difference like 0.416 vs 0.420 is a bit silly for technical reasons, but perhaps understandable for contract/legal reasons.

    After all the above, a time-delay RCD does sound a reasonable way to avoid the issue as long as both houses have RCD protection of all circuits so they don't risk tripping the main one.  And don't forget the AFDD.

  • 0 in reply to Metal Clad

    Thanks for the very detailed reply, much appreciated! I think as you say the time delay RCD route is the way to go as both houses have full RCD protection on all circuits. Going back through my calculations I'm right on the edge of complying (helped by a 50mm parellel earth wire) so again the RCD with give a belt and braces approach. Thanks for the idea of further fuses in the enclosure, that is something I hadn't considered and could be useful for extra protection - I guess din rail holders would do the job?

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  • 0 in reply to Metal Clad

    Thanks for the very detailed reply, much appreciated! I think as you say the time delay RCD route is the way to go as both houses have full RCD protection on all circuits. Going back through my calculations I'm right on the edge of complying (helped by a 50mm parellel earth wire) so again the RCD with give a belt and braces approach. Thanks for the idea of further fuses in the enclosure, that is something I hadn't considered and could be useful for extra protection - I guess din rail holders would do the job?

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  • 0 in reply to Will Elec

    Thanks. I must admit I'm not sure what products there are for 63+A fuses in din-mounting for UK standard fuses. Anyone else know of one?   But the RCD could leave it a non-issue, with decent backup from the 100 A fuses if that failed. 

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