Exterior socket on PME system | Page 2 | on ElectriciansForums

Exterior socket on PME system

Discuss Exterior socket on PME system in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

Rob said:
Personally I would just fit one and have done.

However you could please everyone by installing a decent rod with a good Ra and connect it to the MET of the house.
Click to expand...
It would have to be a very good Ra. :)

If max demand (for the installation, not just the outside socket) is, say, 30A, then you need an Ra less than 3.35Ω if you want a touch voltage less than 70V (using the formula in A722.3 for EV charging in single phase installations, which is just the potential divider formula rejigged).
 
South Spark said:
Speaking to them again , they also talked about 711. However they're now suggesting that a risk assessment with a warning that only CLass 2 equipment should be used and a non standard socket fixed.

How likely is it that the supply neutral would ever be lost?
Click to expand...

Regarding the first suggestions (Class 2 only), this is a complete non-starter, unless there's a specifc item of equipment to be used. For general PA... no chance.

Regarding the second question: there was a study done, "Charging of Electric Vehicles at Domestic Dwellings Using Protective Multiple Earthing (PME) - Risk Analysis PE01417/2012/1" (Commentary on the Wiring Regs mentions it). Increase in individual risk (seriously injured or killed) was 1.1x10^{-6} to 7.8x10^{-9} (sorry, not sure how to get superscript!) - worst case just more than 1 in a million. That's only for people in contact with EV charging systems (not the whole population). Even with risks that low, extra measures are recommended to reduce the risk "if reasonably practicable to do so" such as putting in an earth rod with low Ra.

In the DPC of the 18th Edition, they are proposing to remove the "if reasonably practicable" option in 722.411.4.1.
 
happysteve said:
It would have to be a very good Ra. :)

If max demand (for the installation, not just the outside socket) is, say, 30A, then you need an Ra less than 3.35Ω if you want a touch voltage less than 70V (using the formula in A722.3 for EV charging in single phase installations, which is just the potential divider formula rejigged).
Click to expand...

It's RCD protected...

It's solving the problem of a lost neutral on the DNO side.

Even then an Ra of 3.35 isn't a huge effort.
 
Rob said:
It's RCD protected...

It's solving the problem of a lost neutral on the DNO side.

Even then an Ra of 3.35 isn't a huge effort.
Click to expand...

I'm confused. Surely an RCD won't help for additional (shock) protection when there's a missing PEN (on the DNO side)? Current flows out through the L, through the RCD, through the load, back through the N, same current back through the RCD (so, balanced), to the PME block (where earthing conductor is connected to PEN), to MET and through CPC to exposed conductive parts, through body of person, to earth.

I agree that having an earth electrode with good Ra is better than not having one (for PME) to reduce the touch voltage in the event of loss of PEN on the supply side. To remove the shock risk requires a very low Ra, and something like 3.35Ω, depending on the local ground conditions, may be... challenging. :)
 
Draw it out Steve, the N>E link is before the RCD, so current flowing down the PE will not go through the RCD causing an imbalance.
 
On quickly reading and passing through, I've got to agree with happy steve.

Scenario: Class 1 appliance, no faults present anywhere on installation except broken neutral on PEN conductor. Current is balanced through RCD, earthed conductors rise to a dangerous voltage, link between Class 1 appliance and earth made via a person, RCD is going to say "hey, this looks alright to me"

What am I missing?
 
Last edited by a moderator:
Don't know, my understanding, limited as it is, is broken PEN conductor, is your worst case scenario (don't have nightmares folks) RCD or no RCD.
 

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