Is this a case of exporting PME earth??

[nozspark]

This is the set up.

3 phase incommer, each phase feeding single phase CU's via switch fuses. If one of these CU's is in an out building fed overhead (16mm 3 core SWA), would you class it as being an exported earth? and as such would you then make the CU into a TT install??

 

[Adam_151]

How far away is the outbuilding?
Is there anything that requires bonding in it?
what is it made from?
what is it used for?

The equipotential zone is exported if there is a MET in the outbuilding with bonds running from it, (in my mind), the green and yellow conductor then has to met requirements of 54H (which shouldn't be an issue in this case if you have 16mm² of copper), if there is nothing to bond then its just a supply taken outside of the equipotional zone (as an MK masterseal on a patio wall would be)

As to whether you do either, its a judgement call, but probably best to avoid using the PME earth where big distances, animals, large amounts of structeral metalwork, or water are involved, but the average garden shed should be ok

 

[nozspark]

Yeh, thought as much. Looks like I'll be TTing it then!! as the building is basically a mini house!!

 

[plugwash]

If it's going to be a house I really don't see any reason not to use the PME earth as long as the bonding in the new building is done up to PME standards.

 

[Lectrician]

PME can be exported. If there is any extraneous conductive parts, these need to be bonded. The main equipotential bonding conductor needs to be selected in accordance with table 54H - But is generally 10mm if you have a standard 60/80/100 amp supply with 16/25 tails.

The submain needs to also be sized to accommodate this main equipotential bonding conductor (it can share the same core as the submain - seen too many people run an extra core!).

 

[nozspark]

Lec,

As stated I am using 3 core SWA fed from a 60A switch fuse. During my first fix I did run 10mm meb's to the entry points for the gas & water.

So with my 3rd core and armour acting as an earth I should be OK then??

 

[Lectrician]

As stated, it depends on your incomming supply and tail size, not the rating of the submain!

 

[nozspark]

Incomming is 3 phase - 80A per phase - 25mm tails - 16mm main earth - 10mm meb's to water and gas in main house.

The 3 phase goes from the service isolator to 3 x single phase 60A switch fuses which supply - the main house, a mobile home and the new building.

 

[Lectrician]

Well, I certainly would not take the PME to the mobile home!

As for the other two, yes, and run your MEB's to either the MET at the origin, or the BEMT - The "Building Earth Marshalling Terminal".

 

[nozspark]

Cheers Lec,

I know that you should not export PME to a caravan, so I will TT that, although I can see the old "well it's worked ok for the lastxx years..."

 

[plugwash]

you can however use the PME earth to protect the cable running to the caravan and only switch to TT at the caravan. The advantage of doing it that way is that you can have the RCD in the caravan where it is easy to reset.

 

[nozspark]

Cheers guy's, will do as you suggest.

Would never use PME at a caravan/mobile home anyway, will probably have to sink a rod an put a RCD in - woul prefer the 100A 100mA TD main sw and a 63A 30mA for sockets etc, but I can see that cost will be a prob so might have to go with 63A 30mA as a main switch (although I do hate it when I see that set up!!)