UK RCD protection to Meter Tails?

[Weezy]

About 20Meters of 25mm meter tails are routed though the floor boards feeding a sub main.
To protect the tails under the floor i was going to fit a Mini Metal Unit with RCD protection Fused at 50A at the mains position, Just wanted to check to ensure it’s a 30ma ‘S Type’ RCD i need? As the c.u being fitted at the other end is a dual rcd board and want to minimise the tails rcd tripping.

(unfortunately cables are already in place so running the feed in swa for example isn’t possible)

Thanks.

 

[HappyHippyDad]

I don't think there is a 30mA S type.
If the tails are in the wall they need mechanical protection. SWA would be better to use, but I guess it's already in place.

 

[Dartlec]

About 20Meters of 25mm meter tails are routed though the floor boards feeding a sub main.
To protect the tails under the floor i was going to fit a Mini Metal Unit with RCD protection Fused at 50A at the mains position, Just wanted to check to ensure it’s a 30ma ‘S Type’ RCD i need? As the c.u being fitted at the other end is a dual rcd board and want to minimise the tails rcd tripping.

(unfortunately cables are already in place so running the feed in swa for example isn’t possible)

Thanks.

If they are 50mm from top and bottom of joists then that is sufficient and you only need an MCB at the mains position.

If they are not then it's tricky, because in theory they need mechanical protection, or would have to be protected with 30mA at source, which then means you can't meet selectivity requirements if there are multiple circuits at the other end.

A Type S RCD wouldn't meet the requirements of BS7671 (though would provide some protection in addition to a mcb)

If you can access the top of the joists then mechanical protection could be added at each joist.

If it was existing cabling that you couldn't verify, like a similar situation I had recently, then you could exclude it from the scope of your installation and it would (according to NICEIC) be acceptable to connect it to an MCB assuming it was 'safe to energise'.

However, sounds like this is all part of a new install?

Ask all the newish build flats that have submains in T&E how they seem to get away without worrying about such things - (several threads on this in the past)

 

[Mike Johnson]

I am unsure of the UK regs on this, but in France they fit a 100 mah S type at the consumer end of the tails, this is mainly to prevent someone going over the tariff they have paid for and the protection is secondary to that, even though the supply may be 20Ams/phase.

Just for information anything beyond the Circuit breaker (Disconjoncteur de branchement type S 100Mah) is solely the responsibility of the consumer, including supplying a suitable earth.