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

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.