Earthing on a domestic TT supply system with submain, advice needed please

vde-electrician · Dec 14, 2013

advice needed please , what is the correct thing to do. The scenario:

-meter position and switch fuse @ front of house , with 100mA time delay protecting submain (TT SYSTEM)
-the above submain supplies a 17th ED split load DB at back of house (2 x 30mA rcds etc etc)
-submain is only 10mm T+E (4mm CPC within cable)
-@ the split load db it has 6mm cpc to earth rod close to this db (at back of house)
-should i connect 4mm cpc within twin and earth at switch fuse position to a 2nd rod via an addition 6mm earth?
(at front of house). ie front rod ---6mm---sw fuse ---4mm in T+E-----17ed db--------6mm-------back rod
-i believe the min for TT is 6mm and therefore the 4mm within the 10mm t+E submain throws me a little.
do i keep rods seperate, and only connect one of them to the sub main??

i guess ideal scenario would be to run an addition 6mm between db's or increase submain size, but this isn't viable really.
please can i have other opinions on his matter. thank you very much. ps new to this forum
merry xmas

jaydee81 · Dec 14, 2013

earth rod should be at the origin of the supply so install new rod and connect it to switch fuse, then connect both rods together will give you a better overall Ra

Richard Burns · Dec 14, 2013

Welcome to the forum.

The minimum size for a main earthing conductor (protected against damage and not buried) on a TT system is 2.5mm² (subject to complying with the adiabatic calculation).
Effectively you have your means of earthing connected to the origin by the submain.
There would be no problem with installing an extra rod at the front of the house and it could only reduce your Ra which would be good.

Engineer54 · Dec 15, 2013

Makes a change to see an up front S type RCD backing up the 2 30mA units!!

Cable size connecting Earth electrodes to the MET are directly related to the Ra values obtained, the closer you get to 1 ohm or TN values the closer those main earthing cables need to be to match those required by TN-S. Not forgetting of course minimum requirements relating to method of installation as laid out in BS 7671....