Identifying TNC and TNCS

[Bazultra]

Does anyone else find it difficult identifying between the two. for example today I had an installation where the main incomer was not accessible without cutting off the metal ties that the electricity board had put on it. I just assumed it must be TNCS due to the continuity between the neutral conductors and CPCS. Any thoughts or advice on this would be much appreciated> Thanks fellow sparks.

Baz

 

[MDJ]

Never had a problem myself TBH no, maybe someone else may have had a similar problem to you.

 

[johnboy6083]

You cannot assume that. If the installation is fed from a dedicated transformer, then it is almost certainly a PNB, which is recorded as a TNS.
On all standard delta:star transformers, the star point must be earthed. This is called the neutral bond.
The neutral bond can be made at the transformer, or as is often the case in large industrial situations, at the incoming section of the panelboard.
This looks like a TNC-S system, however it is not. The bonding requirements are as a TNS system, as you don't have the added danger of other installations diverted neutral currents.

 

[Amp David]

Why would the continuity between the CPC and Neutral bar tell you its TNC-S? Why not TNS?

Never had a problem myself either, usually pretty easy to identify if a proper TNC-S.

 

[keniff]

Normally a loop test between l-e and l-n can help

 

[Bazultra]

Are there any other clear indications of what earthing arrangement it could be without access to the main incomer? In domestic installations I found it very easy to identify what earthing arrangement was being used as it was right in front of me or the earth stake was in the garden somewhere. Im not as experience in commercial or industrial installs

 

[kingeri]

As above, never really had an issue, seen a couple of instances where a former TN-S install has been PME'd and I had to double check I was thinking, but generally it's straightforward just by looking at the equipment.

 

[Sintra]

Ze testing followed by measuring impedance of L-N.

 

[MDJ]

Are there any other clear indications of what earthing arrangement it could be without access to the main incomer?

A loop test is a good start if you had to work it out without looking at the incommer, the ZE reading on a TNC-S will be completely different than that of a TT system for example and usually (not always) will be different than a TNS, why wouldn't the incommer be available to look at though?

 

[Silly Sausage]

A loop test is a good start if you had to work it out without looking at the incommer, the ZE reading on a TNC-S will be completely different than that of a TT system for example and usually (not always) will be different than a TNS, why wouldn't the incommer be available to look at though?

Not on one of E54's installs it wouldn't! :smilielol5: