PME or TT ?

[sprigsteen]

Hi Everyone, New to the site, newly qualified Part P installer. I have encountered two properties which appear to be TT systems with ZE readings of 65 and 100 ohms, on live testing ring main ZS results 0.5ohms and 0.56 ohms. There is no PME earthing connections at DNO cutout fuse. Does this mean the DNO have connected neutral and earth at the pole? How do I know if it is PME? Cheers guys.

 

[Rockingit]

The OP must have lot the use of his legs, as anyone else would have traced the earthing conductor from the MET looking for the rod............

 

[trev]

The OP must have lot the use of his legs, as anyone else would have traced the earthing conductor from the MET looking for the rod............

Too obvious mate.

 

[Meggerman]

Hi Everyone, New to the site, newly qualified Part P installer. I have encountered two properties which appear to be TT systems with ZE readings of 65 and 100 ohms, on live testing ring main ZS results 0.5ohms and 0.56 ohms. There is no PME earthing connections at DNO cutout fuse. Does this mean the DNO have connected neutral and earth at the pole? How do I know if it is PME? Cheers guys.

Hi Sprigsteen. As you state there is no earth terminals at supply cut out then these properties supplies are not TN-CS they are TT and you should record them as such. The low readings you are getting are parallel earth paths through main bonding and other PME supplies in area. As Zs must be taken with main bonds connected just record what it is and carry on.

 

[7029 dave]

The OP must have lot the use of his legs, as anyone else would have traced the earthing conductor from the MET looking for the rod............

Well yes, but let me tell you I have seen TT arrangements with NO rod, just elcb that's it. (old installs)

 

[Meggerman]

Well yes, but let me tell you I have seen TT arrangements with NO rod, just elcb that's it. (old installs)

I have had one in the last year that still had ELCB and 2.5mm to water pipe as earth.

 

[GLENNSPARK]

I have had one in the last year that still had ELCB and 2.5mm to water pipe as earth.

wow...

guess you dont get out much then...


i`v come across loads of em...

and before anyone starts:
those VOELCBs can be used as purely as a DP isolator....they can break fault....due to them having arc-shutes...

 

[GLENNSPARK]

:rofl: you think you'll get an answer

hmm..

we can all live in hope....cant we?

 

[Meggerman]

wow...

guess you dont get out much then...


i`v come across loads of em...

and before anyone starts:
those VOELCBs can be used as purely as a DP isolator....they can break fault....due to them having arc-shutes...

I didn't state how many I have seen, I have seen this one in the last year.

 

[Badged01]

those VOELCBs can be used as purely as a DP isolator....they can break fault....due to them having arc-shutes...

Good point, hadn't thought of using them like that!

 

[sprigsteen]

well well well

and here we go again...

OK:

its all about the PFC here....

do a Zpn...then a Zpe...check results on PFC...

if the same then its more than likely to be a TN-C-S

can you tell us why?

answer please:

One test is live to
earth, the other live to neutral. If neutral and earth are connected by DNO results will be the same. Is this common practice for the DNO? On the installations in question they have the old DNO voltage isolator with the yellow button. Would this have any bearing on the results? Thanks again

 

[sprigsteen]

When testing......PFC.....PSCC.....If same reading obtained it's PME

Thank you driverman. Is it common practice for the DNO to do this? I am assuming they are connecting the neutral and earth at the pole. The installation is protected by old DNO voltage trip with yellow button. Could this have any bearing on test results? Thanks again.

 

[sprigsteen]

Can't you phone the DNO for the information?

Phoned DNO, they didn't know and said they'd have to send out an engineer!

 

[JD6400]

We tend to come across them ( all be it far less regularly now days ) mainly whilst testing on old dairy farm supply's and alike .
And to be honest I am very wary of them , as I find once operated with the test button they can be an absolute pig to reengage again if at all .
I agree with you Glenn but for what a main switch and enclosure costs , is it worth the saving ?

 

[GLENNSPARK]

One test is live to
earth, the other live to neutral. If neutral and earth are connected by DNO results will be the same. Is this common practice for the DNO? On the installations in question they have the old DNO voltage isolator with the yellow button. Would this have any bearing on the results? Thanks again

depends on where they are connected.

do you think the starpoint is common for both neutral & earth?

can you think where there may be another reference point....nearer the installation in question perhaps?

 

[Rockingit]

I often find installations which have been mixed up when E7 has been installed - the original installation has been TT, but the meter fitter has come to fit an e7 meter/clock/whatever onto a separate board and then connected that board direct onto a PME head and ignored the rest. So within the same house you can find TT and TN-C-S systems running from the same supply that don't cross-over anywhere because the e7 circuits have always stayed separate and no-one has needed to check the bonding.

Until, that is, that house is on the Somerset levels and is sat in 5' of water for a few weeks and all the back boxes start talking to each other through soaking wet walls... it's been fun picking my way through some of my most recent inspections and reports!!