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Its TN-C-S with 100amp cutout fuse (Although might only be a 60 or 80 amp fuse in the carrier but says 100amp so have to presume). Tails look really small, have not taken it apart but looked at next doors that had been cut off and I would say they are 10mm tails so really need upgrading. I thought the regs say they only need RCD protection if more than 3 meters, cable length will not be more than 3 meters, but thought there had to be some mechanical protections.
How about a large wiska box with a couple of henley blocks inside to make connection between tails and swa?
Saying that trying to get even 2 core 25mm SWA though the cavity could be a problem so may not have a lot of choice.
On a TNCS or TNS system 16mm tails are ok with a 100A cut out fuse providing the installation meets the following 2 criteria:
1. Max demand of the installation (taking into account diversity) should not exceed the CCC of the 16mm tails.
2. Regulation 434.5.2 has to be met (i.e protection of the tails against fault current).
This regulation will always be met on a TNCS system as long as Ze<0.35Ω which it should be.
It will be met on a TNS system is Ze<0.48Ω as at that value K²S²>I²t (just!)