tns /tnc

[trickyb]

Hi i know most domestic installations are TNC but how do you tell? if theres an earth terminal on the side of the cutout does this automaticaly mean its TNC ? How do you test for this ?:confused5: thanks ian

 

[Des 56]

A earth terminal on the side of the cut out does not mean its tnc-s
I have known of many where that terminal is not linked to the neutral


In a Tnc-s system the earth loop test on the neutral and the test on earthing conductor are as near as damn it the same, they are connected at the cut out

In a tns system the earth path will usually have a higher reading than the neutral loop reading

 

[Engineer54]

A almost sure way of knowing it's TN-S is the main earthing is via the lead sheath of the incoming cable. Where concentric cable has been used, then sometimes they bring the earth out under the cut-out and sometimes they use that earth terminal at the side of the cut-out connected internally, but Not linked to the Neutral. So to be sure, when in doubt, as Des say's, the loop tests should tell you if it's TN-S or PME....

 

[trickyb]

when you say test if is 0.35ohms TNC or 0.80 TNC ? yes !

 

[trickyb]

Sorry i mean 0.80 TNS

 

[Engineer54]

What Des is saying, is that both loop tests L-N and L-E will be as near as dam it the same value, if the supply is PME/TNC-S. Whereas on a TN-S supply they will be a significant difference.

A TN-S earth can be below 0.35 ohm the 0.80 ohm is the maximum allowable. Same goes for PME, so that is no way to confirm an earthing supply type!!

 

[trickyb]

Could the supplier company tell me?

 

[SimpleSimon]

Could the supplier company tell me?

By the time they are able to tell you you'll wish you never called.

 

[telectrix]

Could the supplier company tell me?

they could, but can you afford the time let alone the phone charges?

 

[SimpleSimon]

With your loop tester, if you have a three lead tester you Will put your Leads on L-N-E ~(earth disconnected from MET).

If your system is PME (TNCS) than lets say you get a value of 0.35 ohms.
If you than link your E-N test leads together and test L-E your still going to get a value of almost exactly 0.35ohms because your Neutral is earthed.

If you do the same method on a TNS supply, you'll get a notable difference between the two readings.

 

[telectrix]

disagree simon. you need to measure PSCC and PEFC. if they are virtuallt identical then you have PME.

 

[MarkieSparkie]

I'm in agreement with Tel, the only certain way identify TN-C-S (PME) is to check that PSCC and PEFC are virtually identical.

 

[ezzzekiel]

me thinks tell is correct as usual

 

[Engineer54]

What are PSCC and PEFC measurements, if they are not loop tests?? I don't know the connection arrangements of particular MFT's so maybe that's what Tel is questioning?? Then again, i might be missing something... lol!!

 

[telectrix]

with the meggers, Ze and PSCC are 2 lead tests.