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.