So where did the .35 and 0.8 come from

There's a few factors involved. For TN-C-S where the L-PE loop is also the L-N loop, there's voltage drop in the supplier's lines to consider - with a maximum permitted drop from 253V (230V+10%) to 216.2V (230V-6%) they must loose less than 36.8V - and with a 100A supply - then by Ohm's Law in the simplest case the L-N impedance must be less than 0.368Ω. Rounding to a nicer number with a just a small margin for safety would give you 0.35Ω It also meets the Zs requirement for a 100A fuse for 5s disconnection times.

0.8Ω for TN-S seems curious since it doesn't match at all the Zs required for a 100A (or even 60A) fuse for 5s disconnection time. But go back a few editions of the regulations - before we had 5s and 0.4s - and disconnection times were deemed to be met if the earth fault current was 3x the fuse rating -  and in 240V days 0.8Ω gives exactly 300A. Perhaps one to think about if you're installing a new metal CU on an existing TN-S supply...

   - Andy.

Thanks Andy, 

makes a lot of sense along with other info from Graham. Wallywombat does ask a good question though. Many times in our more remote areas in the SW it does go a fair bit over .35. Mind you WPD are getting reticent to even give an earth terminal in a lot of cases. It's T.T. mate sort it out yourself. Also not that easy on Dartmoor = Granite

Many thanks

Marc

Agreed it's not easy. An earth electrode doesn't have to be one (or two or three) driven rods. Sometimes, buried bare copper wire/tape, or an earth mat, is much easier, provided you can get it buried 600 mm (roughly two spades' depth), and can often (but not always) help.

Agreed it's not easy. An earth electrode doesn't have to be one (or two or three) driven rods. Sometimes, buried bare copper wire/tape, or an earth mat, is much easier, provided you can get it buried 600 mm (roughly two spades' depth), and can often (but not always) help.