I get quite angry sometimes reading posts on TT systems on Forums. It always seems these days, that the TT system is not much more than an after thought, or some kind of supplementary back-up to an RCD or multiple RCBO devices. The most important aspect of any TT system is achieving as low a Ra figure as possible, and that may mean the installation of more than just ONE rod to achieve, as seems to be the norm these days, especially when working with unfavourable ground types or locations.
You really need a minimum value of 20ohms on any TT system to call it a ''functional'' system.( Personally i always will try and get below that value) The cable connection to the first or the single rod needs to be effectively ''Protected from the elements, and enclosed in a purpose made encloseure at ground level. The other rods of a multiple rod installation should ideally be braised or exothermically welded. If not, have the same protection afforded to them as the first rod. OH, ...and don't use the thinist and shortest rods available, you will always get better results from the bigger CSA area and longer or deeper driven rods, (sometimes achievable by rod connectors/extensions). Cable conductors should be mechanically protected by a good grade of PVC conduit or piping and be of an absolute minimum of 6mm. ....Never drive rods into the ground less than 1.5 to 2 metres from building walls, this is where the majority of the original buildings rubble will be located.
Get a TT system installed correctly, and there shouldn't be any worries of an RCD device failing, (and we all know that many DO!!) as the TT system will trip the MCB, on L-E and N-E faults maybe not as quickly as a DNO supplied earthing system, but quick enough!! Get it wrong and the installed TT system won't be capable of tripping anything....
The other thing, and a very important point is that the occupier needs to be instructed on periodic routine maintenance of the installed ground rods. So if any of you are installing TT systems you need to find out, or brush up on those maintenance procedures. Remember a well designed and installed system, should last a min of 30+ years and often much longer....
Basically, i've only scratched the surface here of installing a bona fidi TT system, there is much, much more to installing these systems than initially meets the eye!! It takes an amount of skill and experience to install a ''GOOD'' TT system...
As some have mentioned, if the supply can be converted to PME, so much the better, as TT systems for domestic installations should always be considered as a last choice, never a first choice.
In conclusion, Please never think that whacking a single rod into the ground and consider that getting under a 200ohm value is a good-un!! ...it isn't a TT system, and it certainly isn't a ''good-un''!!....
