Pfc / pscc

[uksparks]

Hi,

If you have a TNS supply, you would have a separate earth path by way of lead pipe or split concentric etc... true?

Ok, so if you have a PME / TMC-S supply, the earth connects to the neutral at the service head, yes... true?

Ok, so just out of interest, if you had an overhead TT thats now been converted into a PME, would you expect the PFC and PSCC readings to fluctuate if you keep taking the reading?

I just changed a board yesterday on this supply, and it was overhead TT, now PME, and im getting fluctuating readings anywhere from 0.22 to 0.28, ive checked my meter and checked again etc, and its all ok, but its fluctuating.

The only reason I say this is because, i also changed a board i dont know 6 months ago, i put it down as TNC-S / PME as thats exactly what I saw, the readings between PFC and PSCC were 0.1 ohms different. The NICEIC bloke said You are wrong, its not TNC-S and marked me down.

I said ok, well what is it then? He said its TNS on split concentric cable...ok fair point I said, he said that if its PME / TNC-S the readings will be exactly the same.

Now this one I have just done, its quite clean cut, theres two lines and one neutral, its NOT TNS, its not possible, but I am getting fluctuating readings.

Can anyone with a bit more knowledge enlighten me please.



I have connected the Earth into the left hand side as instructed by WPD as its been converted to PME and even has a sticker on it saying TNC-S / PME.

 

[uksparks]

I've got to pop back next week for a couple of other jobs so will take a better look.

the readings were good either way just makes me think!

 

[uksparks]

I did see only two weeks ago on a board I changed, it was TNS with split concentric originally, then had a new head and a label put on it saying pme TNCS, so again I get two different readings, I call WPD and ask them and they say it's pme, I say how comes I'm getting a different reading and they were bang on the same each time so was confident it was as thought, they then looked at it and sure enough it was separate with a sticker saying otherwise.

 

[guest119]

I can only say I didn't check if there was continuity but surely the fact that I've got 230v line to earth would that not suggest there's continuity?

No, not with the installation connected.

 

[Deleted member 26818]

Fluctuations could be due to the quality of the supply, could be due to high demand, a number of things.
However, differences in values of the two readings, is most likely due to poor terminations.

 

[uksparks]

No, not with the installation connected.

The installation was not connected, the main switch was open and the main earthing conductor disconnected.... It was just a ze reading which I was changing on the meter from PFC to PSCC, I know that it should be connected to take that reading which I do, but I was taking a Ze.

Everything I did was sound and tight and to the best of my knowledge it was correct, I can only put it down to a poo supply, it is in remote little hamlet in the middle of nowhere on a long run of overhead poles.

 

[davesparks]

What meter are you using?
Why do you need to change settings for PSCC and PFC, both tests can be done on the same setting which should be the high current 2 wire loop test.
If your meter has been using 3 wire low current/anti trip test for one of the tests then you can expect the two results to be different.

 

[Deleted member 26818]

I would expect the two tests to require the test leads to be connected in the same manner irrespective of being 2 or 3 wire?

 

[uksparks]

Sorry wasnt making my self clear, Fluke 1654B, all three wires connected, but I just press a button to switch between L-N-E and L-N, maybe I had it on the non trip setting by mistake... I doubt it though as that takes ages and this was fast.

 

[Deleted member 26818]

It's begining to sound like a problem with you meter rather than with the supply, unless it isn't TN-C-S.

 

[uksparks]

I do regularly check my meter, I think ill get it calibrated this week just in case, but I am confident that its ok, Im going back to this property on Tuesday so will check again then.

 

[uksparks]

I think I need some new test leads as I am looking now and I keep getting different readings, try a different lead and im ok... It must have been playing up..

 

[Deleted member 26818]

I'm glad that's sorted.

 

[uksparks]

I have seen people say about the Fluke test lead issue before and have to buy new ones to resolve the issue, admittedly I have plugged them in before and when I zero the leads I occasionally get a strange reading, but I think its time to replace them! I would imagine this is all the problem is.

 

[guest119]

Phew!!!!!