Farm TT quandary

3- TT the barn lighting circuit with a separate earth. It will very very difficult  to get a Ra lower than the barn supports due to the sandstone around most of the farm so potential for step voltages again and problem of finding an accessible place away from animals;

In any pure TT system (i.e. not one that's been made TN-like by bonding to shared extraneous-conductive-parts that are bonded to a TN Earth elsewhere), exposed (earthed/bonded) metalwork will always be pretty close to 230V during a fault - and no (practical) amount of electrodes will make much difference to that. What you have is a potential divider with a L conductor and the traditional “fault of negligible impedance” on one side, and the means of Earthing on the other. It doesn't really matter if the earthing gives 500 Ohms, 50 Ohms or 5 Ohms - it's going to do very little against a fault of negligible impedance to pull the touch voltage down. (The best I can imagine was if the fault was on the end of a long thin line, so R1 was perhaps as high as 0.5 Ohms and Ra was really low - say 5 Ohms - in which case you might shave perhaps 20V off the touch voltage - but 200V+ is still almost as lethal as 230V)

The point is that in TT systems protection against shock is provided by disconnection time rather than limiting touch (or step) voltages. 0.2s for 230V provides a similar level of protection as 0.4s for 115V (i.e. the typical TN situation) - and even a S-type RCD should easily open within 0.15s for an earth fault - so there should still be a quite reasonable level of protection for both humans and animals.

For sure, low Ra is good - especially for dealing with protective conductor currents (earth leakage) that's just too low to trip the RCD  - and burying the tops of rods (e.g. at the bottom of a chamber) can much reduce step voltages at the surface of the ground.  But I wouldn't loose too much sleep over the step voltages during a fault scenario. Farms have been TT'd for years (along with fully bonded steel framed barns) and the piles of electrocuted cattle are pretty conspicuous by their absence.

   - Andy.

3- TT the barn lighting circuit with a separate earth. It will very very difficult  to get a Ra lower than the barn supports due to the sandstone around most of the farm so potential for step voltages again and problem of finding an accessible place away from animals;

In any pure TT system (i.e. not one that's been made TN-like by bonding to shared extraneous-conductive-parts that are bonded to a TN Earth elsewhere), exposed (earthed/bonded) metalwork will always be pretty close to 230V during a fault - and no (practical) amount of electrodes will make much difference to that. What you have is a potential divider with a L conductor and the traditional “fault of negligible impedance” on one side, and the means of Earthing on the other. It doesn't really matter if the earthing gives 500 Ohms, 50 Ohms or 5 Ohms - it's going to do very little against a fault of negligible impedance to pull the touch voltage down. (The best I can imagine was if the fault was on the end of a long thin line, so R1 was perhaps as high as 0.5 Ohms and Ra was really low - say 5 Ohms - in which case you might shave perhaps 20V off the touch voltage - but 200V+ is still almost as lethal as 230V)

The point is that in TT systems protection against shock is provided by disconnection time rather than limiting touch (or step) voltages. 0.2s for 230V provides a similar level of protection as 0.4s for 115V (i.e. the typical TN situation) - and even a S-type RCD should easily open within 0.15s for an earth fault - so there should still be a quite reasonable level of protection for both humans and animals.

For sure, low Ra is good - especially for dealing with protective conductor currents (earth leakage) that's just too low to trip the RCD  - and burying the tops of rods (e.g. at the bottom of a chamber) can much reduce step voltages at the surface of the ground.  But I wouldn't loose too much sleep over the step voltages during a fault scenario. Farms have been TT'd for years (along with fully bonded steel framed barns) and the piles of electrocuted cattle are pretty conspicuous by their absence.

   - Andy.