Exterior socket on PME system

[South Spark]

I'm installing a 32A socket on a PME system.

The socket itself is indoors and is on its own RCBO.

The intention is to use the socket for power for outdoor events.

My technical help line says that I should disconnect the earth at the socket then TT it. It seems to me that that would make it more dangerous.

 

[telectrix]

your help line is talking through it's corporate arse. personally I'd install a IP55 socket outside. saves leaving a door or window open. and use the TNC-S earth.

 

[South Spark]

It was a specific request to install indoors to avoid vandalism and unauthorised use.

 

[westward10]

What reason did they give.

 

[South Spark]

Their reasoning was to do with exporting PME. The difference between PME earth and true earth in the event of a failed neutral.

 

[westward10]

What would the socket supply.

 

[stevethesparks]

What scheme are you with?

 

[South Spark]

Niceic

What scheme are you with?

 

[South Spark]

What would the socket supply.

A PA system apparently. However I suppose I've got to consider multiple possibilities.

 

[westward10]

I can see why they are saying this and they could be taking Section 711 into consideration.

 

[Bobster]

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.

 

[South Spark]

I can see why they are saying this and they could be taking Section 711 into consideration.

Could you expand?

 

[westward10]

Could you expand?

It is a bit tenuous but Section 711 deals with shows and stands etc. where PME should only be used if supervised or its effectiveness has been confirmed. Probably way off the mark though.

 

[South Spark]

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?

 

[westward10]

Sounds like they are back tracking. One way you could look at it is you have been asked to install a socket, by good chance internally. Do this because how they choose to use the power from the socket is up to them. Some people lose hours of sleep worrying about lost neutrals, I do not however it is something that occurs.

 

[Octopus]

A 32 amp socket?

 

[happysteve]

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.

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).

 

[happysteve]

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?

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.

 

[South Spark]

A 32 amp socket?

 

[Bobster]

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).

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.

 

[happysteve]

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.

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.

 

[Bobster]

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.

 

[Midwest]

Are you both not saying the same thing?

 

[hightower]

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?

 

[Midwest]

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.

 

[South Spark]

Realistically the place has metal pipework disappearing underground that will have a lower resistance than any bit of copper I could bang into the ground.

 

[hightower]

Realistically the place has metal pipework disappearing underground that will have a lower resistance than any bit of copper I could bang into the ground.

Which you can't utilise in your design of this addition.

 

[soulman]

Hi, An Ra of 3.35 is not easily achievable. I have had 6 rods in the ground & managed 20 ohms

 

[Midwest]

It's remarkable how some threads go off on a

 

[soulman]

Apologies, I was just surprised at that comment.