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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

    is it a measure 0,35 ohs or is that the DNO quoted. If the latter it could well be less in practice. But making provision for an earth fault relay or a 100 or 300mA delay RCD is very sensible. There are very few situations where it would ever pose a problem, and several where it makes things a lot safer.

    Mike.

  • 0 in reply to mapj1

    Thanks, the 0.35 is from the DNO, so yes I will check to confirm. The more I think about it a 300ma time delay RCD seems the way to go to be as sure as possible especially as both houses have full RCD protection on all circuits.

  • 0 in reply to Will Elec

    300mA RCD with time delay at origin ok but, unless you are using double pole RCDs / RCBOs downstream then neutral to earth faults might cause the 300mA RCD to operate.

  • 0 in reply to lyledunn

    Good point about the both-house blackouts that could come from NE fault with single-pole RCBOs. 

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  • 0 in reply to lyledunn

    Good point about the both-house blackouts that could come from NE fault with single-pole RCBOs. 

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  • 0 in reply to Metal Clad

    I must admit I had overlooked this, both houses have boards with single pole RCBO's in them so maybe an upfront 4 pole 100ma time delay RCD on the incoming supply to each house could be a solution.. although diiscrimination could be an issue...

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