Pme TT in same farm building

[Rkidcd]

Metal farm building currently has single phase TT supply with all the required bonding in place.

Client requires a 3 phase supply for a piece of equipment in this building - is it possible to bring a 3 phase supply from the farms house which is PME?

 

[westward10]

That is a high risk strategy considering TN-C-S is not permitted in farm buildings.

 

[davesparks]

Two different earthing systems in one building sounds like a bad plan to me.

 

[Richard Burns]

Earth the 3 phase with current outbuilding's earthing system but only so long as you know that the single phase and 3 phase supplies are from the same source.

 

[Rkidcd]

Sorry I should have said it's a storage building not used for livestock. Anybody got any ideas to a solution?

 

[westward10]

Sorry I should have said it's a storage building not used for livestock. Anybody got any ideas to a solution?

Doesn't matter. There is a high shock risk should you introduce a derived earth into a true earth (TT) environment. Why can't you TT it?

 

[telectrix]

TT the bugger and be done.

 

[Rkidcd]

Sorry to bring this back up but I would like thoughts on my plan based on the above advise.

I'm going to bring 3phase PME supply across from main building to metal outbuilding but isolate the earth externally.

This means that I'll be TTing this new supply but what I need advise on is whether I sink a new Rod for this or would it be possible to use the Rod already installed for the current single phase TT supply the building uses (see original post for background info)

Thanks in advance

 

[Richard Burns]

So long as the single phase supply is coming from the same supply transformer then you can use the existing earthing method. If the three phase supply and the single phase supply are from different transformers then you would be providing an earth link between supply transformers which could be a very bad idea. Circulating currents, and possibilities of transferring faults to other systems spring to mind.

 

[Rkidcd]

Thanks for advise - would the distributer be the only source of this info?

 

[Rkidcd]

Or is it easier to sink a new rod to avoid any doubt?

 

[Richard Burns]

If the supply cables are overhead follow them to see where they come from; if the single phase supply is from the same service head as the three phase supply they are the same;start with the basics and if there is no luck there then contact the DNO and hope.
You also need to consider the effects if there are two different supplies in the same building, but find out where they are fed from first.

 

[bigspark17]

That is a high risk strategy considering TN-C-S is not permitted in farm buildings.

Pme is allowed if there is a metal earthing grid embedded in the sgreed with multiple earth connection points.

That said i love TT and loath exporting pme.. cant you do away with the single phase supply and upgrade 3 phase into whole building and TT it?

 

[Timothy P Booth]

If the power is coming from a transformer up a pole then it will be TT

 

[davesparks]

If the power is coming from a transformer up a pole then it will be TT

Not necessarily, it's perfectly possible to have TNCS or TNS from a pole mounted transformer

 

[davesparks]

Or is it easier to sink a new rod to avoid any doubt?

You'll need to consider what to do about having the two different earthing systems in installation and so be no further forwards.

 

[davesparks]

Sorry to bring this back up but I would like thoughts on my plan based on the above advise.

I'm going to bring 3phase PME supply across from main building to metal outbuilding but isolate the earth externally.

This means that I'll be TTing this new supply but what I need advise on is whether I sink a new Rod for this or would it be possible to use the Rod already installed for the current single phase TT supply the building uses (see original post for background info)

Thanks in advance

its possible to use the existing electrode if you have confirmed that it is suitable for the job.
I'd probably look at augmenting it with additional new electrodes if possible to get the best earthing connection possible

 

[Timothy P Booth]

Not necessarily, it's perfectly possible to have TNCS or TNS from a pole mounted transformer

Yes you're right had time to think about it , thay normally have a earthing mat at the bottom of the pole / transformer

 

[Rkidcd]

You'll need to consider what to do about having the two different earthing systems in installation and so be no further forwards.

As both earthing systems would be TT is the risk not sufficiently reduced? (as apposed to a TT earth and PME earth)

If not it sounds like I either:

1.confirm both supplies are from same source and use one TT earthing system (possibly using multiple rods)

2.get rid of single phase supply and supply entire building from the new 3 phase PME supply (with TT earthing)

 

[Rkidcd]

Visited site again today and it isn't really practical to lose the existing single phase TT supply so it's looking like option 1

Any more thoughts on the issue?

 

[DPG]

Did you confirm whether they are both from the same source yet?

 

[Rkidcd]

I'm onsite now having a look and it's a strange one to trace to be honest, the supply cable for the TT supply actually comes from underground with a PME sticker at the cutout but no earth block has been supplied - does this mean it has the potential to be a pme system however can't be because of the buildings use and therefore has been TT'd?

 

[Ian1981]

Sorry to bring this back up but I would like thoughts on my plan based on the above advise.

I'm going to bring 3phase PME supply across from main building to metal outbuilding but isolate the earth externally.

This means that I'll be TTing this new supply but what I need advise on is whether I sink a new Rod for this or would it be possible to use the Rod already installed for the current single phase TT supply the building uses (see original post for background info)

Thanks in advance

Can you not use the structural steel already in place as the electrode in the metal building?
Or isn't it imbedded in the ground?
It is permitted.

 

[Wilko]

I'm onsite now having a look and it's a strange one to trace to be honest, the supply cable for the TT supply actually comes from underground with a PME sticker at the cutout but no earth block has been supplied - does this mean it has the potential to be a pme system however can't be because of the buildings use and therefore has been TT'd?

Without seeing it ... May be the original DNO supply required TT installation and at some point DNO did an upgrade and it became PME, but the installation has not been updated to use it as means of earthing (may require new bonding etc).