Converting TNC-S to TNS

There are many 230/400 volt 3phase systems wired TNC-S, some even with Neutral-Earth links in the downstream distribution boards. This gives rise to serious circulating currents in the Earth system through the N-E links (in one email this was reported to be in the order of anywhere from 5 to 50A).

TNC-S is a terrible system and gives rise to all sorts of problems, especially in hi-tech environments. The only system worth anything is TNS (i.e. Neutral and Earth only join at one place - the transformer). So I commend anyone wanting to rid a system of TNC-S and return this to TNS.


Before you start, you must confirm the integrity of the N-E bond at the source. Should this bond be in any way weak, then voltages can develop throughout the rest of the system. Also, you will need to investigate the Earth itself ensuring that the Rise Of Earth Potential (ROEP) will not exceed local codes during a fault condition (they usually don't because the Earth is normally made up of many paths, not just a yellow/green wire from source to load!).

You will also need to confirm the integrity of the Neutral will be maintained while removing the N-E links at the downstream distribution panels. Failure to ensure these are solid could cause serious imbalance across the phases that can damage primarily (but not exclusively) single phase equipment.

If the above is satisfied, then,..

You should be able to remove the N-E links on the downstream distribution panels. You are likely to see sparking, especially if the circulating currents are in the order of 50A, but this should be about the only effect.

And with regards the specific question of an ROEP of 50V; You are also likely to have transients develop while you are doing the removal, but these are extremely short-term and should not cause any damage or pose any threat to life. Depending on the Earth system, you are effectively breaking a huge loop, and when you break a loop (coil) you get back-EMF. I wouldn't like to have my fingers across the bond while you're doing that because one could jump, but that's about all.

Once the N-E links are removed, please conduct a proper PQ survey to establish that no lurking problems exist e.g. a bad Neutral you were not aware of. You should do this at the end of each of the distribution systems you have modified (you only need to do this at the end, because any problem between the end and the source will be seen).


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© 21.06.05