ZE on tnc-s system

[Km1777]

Evening everyone .
I was changing a fuse board today . Everything tested out fine and good . Until I measured the Ze which is on a tnc-s system . The result was 0.36 ohms now I know the max value for tncs is o.35 ohms . What would you guys do in this event call the dno or accept the result . Would they do anything about it as it’s only just over the limit . No bomdong was connected whilst doing this test

 

[mainline]

Evening everyone .
I was changing a fuse board today . Everything tested out fine and good . Until I measured the Ze which is on a tnc-s system . The result was 0.36 ohms now I know the max value for tncs is o.35 ohms . What would you guys do in this event call the dno or accept the result . Would they do anything about it as it’s only just over the limit . No bomdong was connected whilst doing this test

Accept the result, it could be that the supply has long lengths of low voltage cable.

Worth carrying out a test L-N and see if the reading is the same.

 

[pc1966]

As @mainline says if this is a rural 60A (or less supply) it is not too out of order, and the 0.35 ohm is a typical guidance limit only (the DNO is not bound by it). Also if you PSCC and PFC are much the same then nothing untoward in the DNO's earth connection to the CU either.

Can you give us any more context on the system?

 

[westward10]

Deduct the resistance of you test leads it will be fine or keep testing it until you get a lower reading.

 

[Lou]

Welcome.to the forum!

Could you post any pics?

 

[Km1777]

As @mainline says if this is a rural 60A (or less supply) it is not too out of order, and the 0.35 ohm is a typical guidance limit only (the DNO is not bound by it). Also if you PSCC and PFC are much the same then nothing untoward in the DNO's earth connection to the CU either.

Can you give us any more context on the system?

Thanks
It’s a 100amp
Main fuse not a rural area it’s on a min street . The pfc is 0.6ka so happy with that . All Zs readings are below max values . I’m just use d to seeing lower readings on a tnc-s system

 

[pc1966]

Thanks
It’s a 100amp
Main fuse not a rural area it’s on a min street . The pfc is 0.6ka so happy with that .

If your PSCC is more or less the same (which it ought to be if TN-C-S) then you are looking at a 10% voltage drop at 60A (as that is 600A at 100% drop).

At 100A you are looking at 36V drop hence 36/230 = 15.7% and that is way too close to the ESCQR limits of +10/-6 % to be acceptable.

It need not be the PEN that is high impedance here, it could be the line conductor has a poor joint.

All Zs readings are below max values . I’m just use d to seeing lower readings on a tnc-s system

If you have checked your MFT & probes on another system after then and it got sensible numbers (so your test is correct) then for an urban area it is suspiciously high, so a call to the DNO to explain this concern might be worth doing.

 

[Km1777]

If your PSCC is more or less the same (which it ought to be if TN-C-S) then you are looking at a 10% voltage drop at 60A (as that is 600A at 100% drop).

At 100A you are looking at 36V drop hence 36/230 = 15.7% and that is way too close to the ESCQR limits of +10/-6 % to be acceptable.

It need not be the PEN that is high impedance here, it could be the line conductor has a poor joint.



If you have checked your MFT & probes on another system after then and it got sensible numbers (so your test is correct) then for an urban area it is suspiciously high, so a call to the DNO to explain this concern might be worth doing.

Thanks

I re tested this morning as I had other bits to finish off . I tested three times and it was 0.30 every time . It may have been my leads not being zeroed that caused the higher reading yesterday .