gkenyon:

When PME was introduced, we were still in the "post-war" period.


Now we are in a position where, in existing urban areas, PME is here to stay.


I understand some DNOs are offering TN-S for new-build - of course, there is a slight cost increase for the extra core.


With TN-S, you only need one failure after broken PE, on circuits where RCDs are not used. Where RCDs are used, I thought you said you weren't happy with reliance on the RCD?


Your point regarding earthing is very valid ... for all system types. The impact on the effectiveness of main bonding due to the change to plastic service pipes can't be underestimated. I have measured the effective combined earth electrode resistance of my water supply pipe, which is still lead from the street, and it's well under 4 ohms.


How should we deal with that? Germany insist on foundation earth electrodes, which achieve a similar result ... but that idea keeps being shot down here in the UK. I'd like to see some alternative approaches considered. Don't forget, the loss of plastic service piping is not a fault of the DNO, it's a result of health & safety (replacement of corroding metal gas mains for plastic) and public health (replacement of iron and lead water pipes for plastic) and far outside the DNO's control.

Two failures for TN-S without RCD, Three failures with RCD. RCDs aren't 100% reliable, hence why I advocate a foundation of low loop impedance and rapid disconnection. As has been said in this thread by others, if you have say 0.5% probability of failure, 1.5% and 5% chance summed together the odds of all 3 failing at the same time are much more slim vs relying on only one.   


Personally, I think there needs to be less focus on earthing and bonding, and more on loop impedance. I've run the numbers and a lot of time having an earth grid below one's feet bounded to the MET makes little difference when in circuits 32 amps and below 99% of the VD during a fault is on the circuit itself and not the supply between the transformer and consumer unit.

gkenyon:

When PME was introduced, we were still in the "post-war" period.


Now we are in a position where, in existing urban areas, PME is here to stay.


I understand some DNOs are offering TN-S for new-build - of course, there is a slight cost increase for the extra core.


With TN-S, you only need one failure after broken PE, on circuits where RCDs are not used. Where RCDs are used, I thought you said you weren't happy with reliance on the RCD?


Your point regarding earthing is very valid ... for all system types. The impact on the effectiveness of main bonding due to the change to plastic service pipes can't be underestimated. I have measured the effective combined earth electrode resistance of my water supply pipe, which is still lead from the street, and it's well under 4 ohms.


How should we deal with that? Germany insist on foundation earth electrodes, which achieve a similar result ... but that idea keeps being shot down here in the UK. I'd like to see some alternative approaches considered. Don't forget, the loss of plastic service piping is not a fault of the DNO, it's a result of health & safety (replacement of corroding metal gas mains for plastic) and public health (replacement of iron and lead water pipes for plastic) and far outside the DNO's control.

Two failures for TN-S without RCD, Three failures with RCD. RCDs aren't 100% reliable, hence why I advocate a foundation of low loop impedance and rapid disconnection. As has been said in this thread by others, if you have say 0.5% probability of failure, 1.5% and 5% chance summed together the odds of all 3 failing at the same time are much more slim vs relying on only one.   


Personally, I think there needs to be less focus on earthing and bonding, and more on loop impedance. I've run the numbers and a lot of time having an earth grid below one's feet bounded to the MET makes little difference when in circuits 32 amps and below 99% of the VD during a fault is on the circuit itself and not the supply between the transformer and consumer unit.