Earthing systems

[milliamp]

Just to add a comment to nozsparks note. I recently did some work on a TN-S service and the Ze was 0.11 Ohms. The property is located close to a distribution transformer so that may account for the low loop reading. The loop value is as good as TN-C-S in other locations

 

[ban-all-sheds]

For TNC-S your EFLI should be below 0.35 ohms
For TN-S your EFLI should be below 0.8 ohms

Aren't they just the figures you get from an enquiry, nothing to do with reality?

 

[ebee]

For TNC-S your EFLI should be below 0.35 ohms
For TN-S your EFLI should be below 0.8 ohms

Aren't they just the figures you get from an enquiry, nothing to do with reality?

True.

at the end of the day (or any other time of day for that matter) only the supplier can tell you the earthing type you`ve got.
Measured values of 0.10 to 0.25 for both TNS and TNC-S are quite common around here

 

[Lectrician]

Going back to the original question (how do you tell between TN-S and TNC-S)

Isolate the installation, disconnect main earth from bonds & circuit earths, test EFLI (P-E) & note reading, repeat but this time (P-N)

For TNC-S your EFLI should be below 0.35 ohms
For TN-S your EFLI should be below 0.8 ohms

If your (P-E) & (P-N) readings are about or below 0.35 ohms and are the same then you have a TNC-S installation

No - That is not a method of determination. The only sure fire way to tell a system is using your eyes or asking the DNO.

The figures you quote are MAXIMUM values expected, but in reality they will be much lower. TN-S systems can (and do) have a lower Ze than a TN-C-S. These are the figures you are meant to use for design and cable calcs before a service is installed if you do not know what the actual Ze is to be, however, 0.8 can lead to problems with achieving disconnection times.