3 units and potential earthing problems. Mixing PME and TN-S

[GaryMo]

Three light industrial units (units 1, 2 and 3), all knocked through to create one large unit.
Each unit has its own three phase supply.
Unit 1 has a PME supply, unit's 2 and 3 have TN-S supplies.
Currently there's no equipotential bonding to structural steel so this will be carried out from each MET at each supply.

Will there be a problem in that the earth of a TN-S supply will be connected (through structural steel) to the earth (and neutral) of a PME supply?

Also there's a distribution board for unit 1 (PME supply) mounted within unit 2 (PME supply). All cables are fed back entry.

Any potential problems?

 

[Click-Sure]

Unlikely. The earth, be it TN-C-S (PME) or TN-S is all grounded to the same earth (upon which we walk) and for arguments sake has has the same potential of zero. The only difference is that with TN-C-S the earth and neutral are shared, with TN-C the earth and neutral are seperate, but probably combine further down into the DNO's system anyway.

As long as you don't attempt converting the TN-S supplies into TN-C-S supplies you should be fine bonding structural steel into each MET on each supply.

 

[GaryMo]

That's what I'm thinking.
I've been advised to contact local DNO regarding this on another forum so may do that for piece of mind.

 

[Adam_151]

I reckon you need to consult the DNO over this, they'll probably have a viewpoint, certainly many TNCS supplies in a metal framed buildng raises issues and they sometimes don't like it, here you only have one TNC-S, but you would be connecting the earth of the TNS to the neutral of the other supply which could under some situations cause currents to flow, etc. and if anyone else has a TNS supply from the same streetmain... is it true TNS anymore if someone connects the earth with the earth of a TNCS system...

 

[plugwash]

If the building is to become a single equipotential zone (which it really should as it is almost certain that wiring/piping will go between the parts at some point) then all the supplies earths need to be bonded together and all bonding should be sized to suit the largest supply.

The DNO may wan't to convert all the supplies to PME to avoid a TN-S earth ending up connected to a PME earth or they may not be bothered, ask them.

 

[GaryMo]

... is it true TNS anymore if someone connects the earth with the earth of a TNCS system...

That's the bit which concerned me enough to post

 

[GaryMo]

The DNO may wan't to convert all the supplies to PME to avoid a TN-S earth ending up connected to a PME earth or they may not be bothered, ask them.

 

[Lectrician]

The DNO do not generally mix earthing types from a sub station. Old lead sheathed being converted to PME being an exception. And new supplies from an on lead sheathed service in the road being presented as PME.

I think the DNO will have issues. Their get out would be to TT you if you get awkward with them over there advice!

PME is fairly standard for smaller supplies in steel builds in my area.

 

[john1324]

The problem you have is the PME supply- return current from the pme supply will take the shortest route back to the LV TX. A small amount is likely to return via the TN-C-S main equipotential bonding to the TN-S supply earth etc. This value of current could get higher! as the TN-C-S demand goes up.
My thoughts are that the DNO normally like one of there supplies feeding one building. Can you install 2 no. sub mains from 1 no. TN-S supply and divorce the pme earthing and supply?. In therory bonding up all of the supplies would reduce the risk- but current could flow Removing the PME would get rid of the problems associated with your installation.

The joys of metal framed buildings and PME. I heard a rumour that some DNOS will not provide PME to metal framed units- All for the good in my opinion.

 

[ban-all-sheds]

The DNO have probably given their approval, or not, by now...

 

[ericmark]

I do note the date. When I wanted to use separate supplies from DNO they were very insistence that the two buildings should never join and that was a condition of giving us another supply.