What the max difference PFC for TNC-S for PEFC / PSSC

[JONOXNAM]

Hi I was wondering if anyone could let me know what the max difference in PEFC and PSSC in a TNC-S system:

I ask as I measured PSSC 3.8kA & PEFC 3.0kA and have never seen it that far apart on a TNC-S before.

This was repeatedly the case, the tails are 25mm, 1m in length and the EC is 16mm.

0.08Ze, The cutout also feeds the next door neighbor.

Thanks.

 

[malcolmsanford]

I would say because of the very low range of values you are working to if your using a normal MFT, then your going to get differences in results, as these meters are just not designed to for such precise testing.

 

[Silly Sausage]

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[tony mc]

If its TNC-S both your readings should be the same have I missed something ?

 

[JONOXNAM]

Hi, I use a Megger MFT 1553 and it was cal'd the first week of december (it was bang on and nothing was needed to recal according to Relay calibration).

As I said I never saw any where near this difference before as TNC-S should be basically the same +/- meter tolerance... hence my question...

I did not know if the loop in and out to the next door neighbor could in anyway vary these results? that was my only thought....

I did another job the following day TNC-S and the PSSC PEFC results were almost identical so why the difference, I don't know?

 

[DNO]

PSSC and PEFC are the same on PME/TNCS

Kind regards, DNO

 

[Engineer54]

Hi, I use a Megger MFT 1553 and it was cal'd the first week of december (it was bang on and nothing was needed to recal according to Relay calibration).

As I said I never saw any where near this difference before as TNC-S should be basically the same +/- meter tolerance... hence my question...

I did not know if the loop in and out to the next door neighbor could in anyway vary these results? that was my only thought....

I did another job the following day TNC-S and the PSSC PEFC results were almost identical so why the difference, I don't know?

To stand a better chance of getting a accurate figure when undertaking these tests try IQ's little tip here.

Sticky: Higher-accuracy low Ze test procedure

 

[JONOXNAM]

PSSC and PEFC are the same on PME/TNCS

Kind regards, DNO

Hi DNO, yep that should be the case and I was asked that very Q on my 2391 practical... that's the reason for the post they are not returning the same value +- measurement variance at this property....

 

[JONOXNAM]

To stand a better chance of getting a accurate figure when undertaking these tests try IQ's little tip here.

Sticky: Higher-accuracy low Ze test procedure

Many thanks for that, but I have no problem with the Ze value its the PSSC+PEFC values that are not as should be....

 

[telectrix]

with a Ze that low, you only need a tiny bit of resistance on a connection to get a pefc lower than pscc.

 

[topquark]

If its TNC-S both your readings should be the same have I missed something ?

Should be roughly the same Tony. But it would only take a difference of around 0.01Ohms in taking the readings to account for that difference with such a low Ze that's quite likely to show a large difference in fault current (meters will measure the Ze and calculate the fault/short circuit current using the measured Ze and nominal voltage). The 0.01Ohms could easily be a poor connection when moving the leads for example.

 

[JONOXNAM]

Should be roughly the same Tony. But it would only take a difference of around 0.01Ohms in taking the readings to account for that difference with such a low Ze that's quite likely to show a large difference in fault current (meters will measure the Ze and calculate the fault/short circuit current using the measured Ze and nominal voltage). The 0.01Ohms could easily be a poor connection when moving the leads for example.

Many thanks, in this instance the difference does not really matter since its the higher value that's what is of concern and recorded but I had never seen such a big difference. Could it be if there is contact resistance in the EC connection at the head could cause this as some have stated such a low Ze will cause this difference... I might pop around and do the Ze right on the head block vs the CCU end of the EC...

Thanks all Jon.

 

[Deleted member 26818]

It could be that there is a high resistance on one of the connections.
Are you using three probes when conducting the tests, or two?
What I would suggest is conducting a PEFC test on the neutral rather than a PSSC test, and comparing the results with a PEFC test on the earth conductor.