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Am carrying out a job soon a sub main to a out building in someone’s garden.
The house is tncs earthing so was planning on doing tt at the shed.
There’s no extraneous metal work, but it is a concrete base and they are planning on putting some fridges or freezers in there.
Would others do the same and tt it due to the class 1 equipment and concrete base or use the pme cause there no extraneous metal like water pipes.
 
What about if theirs a difference between the ground earth and the pme earth and you get a small current when touching the fridge or if a neutral is lost and the fridge metal work becomes live and your standing on the concrete base.
or would you need be too worried cause the risk is quite small of it happening
 
What about if theirs a difference between the ground earth and the pme earth and you get a small current when touching the fridge or if a neutral is lost and the fridge metal work becomes live and your standing on the concrete base.
or would you need be too worried cause the risk is quite small of it happening
Tin hat on!
It's rather unlikely there will be people doing laundry in bare feet standing in a soggy puddle, and this is concrete not earth.
Lost Neutral chances are tiny - there are about 400 a year with 40 of those causing electric shock according to HSE stats. The chances of this installation being one of them is too low to attempt to design for in my view.

It's statistically more likely that the RCD will fail through never being tested, in which case having a decent Zs for ADS becomes the more desirable design, especially with metal appliances.
 
Thinking through the risks is an essential aspect of design!

All engineering is the art of compromise, making something as reliable and safe as practical within the available budget and space. Both TN-C-S and TT have their advantages and disadvantages, and here is as good a place as any to go through what is best for any given situation.

TT has:
  • No open PEN risk
  • No need for >= 10mm CPC if bonding to extraneous
  • However, RCD fault is often single point of failure (unless using 100mA delay and 30mA) and probably more likely than open PEN as hardly anyone actually tests them as they should
  • Needs a local earth rod, risk of damaging buried services when installing
TN-C-S has the opposite:
  • Reliable ADS on either RCD, or OCPD (in most cases) within 0.4s
  • Open PEN means large CPC for extraneous bonding as fault current could be sustained for minutes or hours, may require going up to 10mm 3-core SWA or running supplementary CPC to meet this.
  • Where contact with true Earth is possible either tingling on normal PEN drop voltages when wet, or serious risk if open PEN fault (hence restrictions for caravans, EV charge, livestock areas, ship supplies, etc).
 
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This page from Stroma has some advice, but does veer towards TT when there is a concrete floor, whereas I think I agree with more with the other comments about the benefits of PME compared to TT in a situation like this.

I know there are some open PEN detector devices available (mostly for EV) - perhaps they will end up at a size that can be easily added to setup like this in the future to get the best of both worlds....
 
This page from Stroma has some advice, but does veer towards TT when there is a concrete floor, whereas I think I agree with more with the other comments about the benefits of PME compared to TT in a situation like this.

I know there are some open PEN detector devices available (mostly for EV) - perhaps they will end up at a size that can be easily added to setup like this in the future to get the best of both worlds....
The biggest problem with that Stroma advice is the title "Exporting a PME supply" because the one thing you are not doing is exporting PME supply. They are not the only ones using incorrect terminology and it is probably a symptom of the mass ignorance among them which leads others to blindly quote their words. PME stops at the service head, you cannot export it you are merely using a cpc derived from a PME supply. It is no wonder I have never once contacted a scheme for advice.
 

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