Farm TT quandary

Coming back to the touch voltage on TT-TN mixes, it is not the N-CPC impedance in the loop divisor, it is the DNO side electrodes, so relative to the local terra-firma the L voltage will dip, by the amount that the neutral/PEN gets pulled away from terra-firmw earth.

Not perhaps in this case with several houses plumbing augmenting the DNO earthing of the neutral, but in many rural cases the DNO earth is not much better than the customer one, so during fault the L_N voltage does not depress, but if you have a 10 ohm electrode resitance and so does the DNO, the whole 3 phase triangle of voltages goes walkabout.

I'm also for option 2 by the way, and it does not have to be SWA if it is visible route.

Mike.

 

I'm also for option 2 by the way, and it does not have to be SWA if it is visible route.

Mike.

I would prefer S.W.A. cable as it offers protection from rodents in the barn, with possible shock/fire risks.

Z.

Zoomup: 
 

 

I'm also for option 2 by the way, and it does not have to be SWA if it is visible route.

Mike.

I would prefer S.W.A. cable as it offers protection from rodents in the barn, with possible shock/fire risks.

Z.

The brick built barn wiring is almost all T&E neatly clipped direct to wooden joists! 

“It wasn't me gov it was like that when I got here”.

I trust the man in the tele handler and I will be getting his son to do most of the work installing the cables, leaving me to do the technical bits sticking the bits of wire together?. I am getting too old for hard work and I no longer like working at heights. I find that as I have got older I have become far more risk adverse. 

Earlier this year my BIL, in his 50s,  fell off a short stepladder when one leg dropped into a mole run when he was cutting a hedge. He suffered ten broken ribs on one side with three of them broken twice, had a flail chest and was in serious danger of dying. He was put into an induced coma for over a week during which time he had an operation to plate the double broken ones together. It took about three months to recover but he still suffers from pain.

It has made me much more cautious especially when people who have survived serious accidents usually start the description “I was only…”

kfh: 
 

Zoomup: 
 

 

I'm also for option 2 by the way, and it does not have to be SWA if it is visible route.

Mike.

I would prefer S.W.A. cable as it offers protection from rodents in the barn, with possible shock/fire risks.

Z.

The brick built barn wiring is almost all T&E neatly clipped direct to wooden joists! 

“It wasn't me gov it was like that when I got here”.

I trust the man in the tele handler and I will be getting his son to do most of the work installing the cables, leaving me to do the technical bits sticking the bits of wire together?. I am getting too old for hard work and I no longer like working at heights. I find that as I have got older I have become far more risk adverse. 

Earlier this year my BIL, in his 50s,  fell off a short stepladder when one leg dropped into a mole run when he was cutting a hedge. He suffered ten broken ribs on one side with three of them broken twice, had a flail chest and was in serious danger of dying. He was put into an induced coma for over a week during which time he had an operation to plate the double broken ones together. It took about three months to recover but he still suffers from pain.

It has made me much more cautious especially when people who have survived serious accidents usually start the description “I was only…”

 

 

Educating experiences and wise words there.

Z.

As Mike and Graham correctly pointed out, I'd ignored Rb in my examples above.

That wasn't entirely accidental though. Ra (your electrode or posts (in parallel) resistance to Earth) and Rb (the source's electrode's resistance to Earth) are in series and so the voltage is indeed divided across both - so when an Earth fault current is flowing a  voltage will develop across the supply electrode and thus the voltage across Ra is thus reduced by the same amount - making the consumer's installation somewhat safer. It is however hard to put a safe value on Rb - it's maximum is normally 20 Ohms but is often far lower. On a system that supplies TN customers as well, the supply PE/PEN is very likely to be bonded to many extraneous-conductive-parts and PME electrodes which effectively reduces Rb to much much lower values - often well below 1 Ohm. Ignoring Rb (or rather presuming it's effectively zero) simply errs on the side of safety by working to the worst case.

   - Andy.