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.

 

[Knobhead]

I'd prefer to see any device removed if it can no longer reliably perform the function for which it was designed, manufactured and installed. Under my local regs this would be a requirement.

Marvo, this argument has been rumbling on for years.

If you’ve never taken a voltage operated VOCB apart you won’t know what they are like.
Many of the old school have and trust them as an On Load switch. Unlike a RCD there is nothing in the current path other than the contacts, with the trip disabled it is a high quality switch. Even E54 has said this in the past when it’s cropped up.

 

[GLENNSPARK]

I'd prefer to see any device removed if it can no longer reliably perform the function for which it was designed, manufactured and installed. Under my local regs this would be a requirement.

yeah...well we work to BS 7671 around here..

and i still fail to see what the hell would be wrong with using it as a DP isolator.
As Tony (quite rightly) points out....this one has been doin the rounds for years....

nowt wrong with its new found use as a (very reliable) DP iso....

crack on i`d say...

 

[davesparks]

The only potential issue I can see is that most of the ones I have come across have been rated at 60Amps whereas our standard single phase supply is 100Amps these days.
Obviously there won't be much of a problem as the average house rarely goes over 20/30 amps consumption most of the time, but in theory something doesn't quite add up there

 

[Engineer54]

Marvo, this argument has been rumbling on for years.

If you’ve never taken a voltage operated VOCB apart you won’t know what they are like.
Many of the old school have and trust them as an On Load switch. Unlike a RCD there is nothing in the current path other than the contacts, with the trip disabled it is a high quality switch. Even E54 has said this in the past when it’s cropped up.

I have indeed, ...as a double pole on load switch, there is probably nothing to touch them. Why throw away a free perfectly good DP switch??

Have a good look at those moving and fixed contacts on these VOELB switches, then look at a modern 100A isolating switch and tell me what one you would trust handling 80A/100A?? Not that you'll ever see anywhere near 80A/100A on 99.99% of domestic installations.

Think how many old Wylex 3036 CU's (that were only ever made with 60A main switches) have been and still are working perfectly fine for literary donkey's years with 80A DNO service fuses, and probably in more than a few cases 100A services fuses.

 

[GMES]

Yes they should be disabled as E/L protection. Relabelled they make an excellent ON LOAD main switch.
Take one to pieces and see what the contacts are like. Fast make and break plus arc chutes.

Regarding condemning breakers with a “yellow” test button. 40 years ago RCCD test buttons were yellow.

Tony are they anything like Parachutes

 

[JD6400]

I can see the logic behind reusing them , but in my experience getting them to reengage is usually at best problematic .
As I have already mentioned though , the vast majority of the ones I see , have lived in normally very aggressive atmospheres for the past 40 years or more , so are probably not a very fair representation of them in general !?

 

[Knobhead]

Tony are they anything like Parachutes

OK it was late, I kept looking at and the brain didn't klick.

Just thought of another good reason to use the old switch, the incoming tails are there and connected.

 

[Marvo]

I guess we'll just agree to differ. I'm not stating that a VOELCB wouldn't function as a D/P isolator but I think the principal unnecessarily opens the door to other bad practice. If I had an old MCB that burned out or known to be faulty could I use it as a blank in a DB. It would probably be more effective than those blanks you just push into the cover. I wouldn't do this either but going by the same principals then why not?

 

[Engineer54]

I guess we'll just agree to differ. I'm not stating that a VOELCB wouldn't function as a D/P isolator but I think the principal unnecessarily opens the door to other bad practice. If I had an old MCB that burned out or known to be faulty could I use it as a blank in a DB. It would probably be more effective than those blanks you just push into the cover. I wouldn't do this either but going by the same principals then why not?

Don't think anyone is saying that they would go out of your way to use one of those old VOELCB units as an installations DP isolator switch. But if it's already there in place, i'd have no problem at all disconnecting any of it's protective cabling and continuing it's use solely as a DP isolator switch.

 

[Amp David]

Surely even after completing whatever a part p installer course is you would know where to be looking fir something like this. So basic it makes me cringe

 

[Richard3009]

Am I missing something here, can't you just look where the main incoming earth originates from either from an earth electrode TT, the sheath (armour) TN-S or linked N.TN-C-S or PME which to my mind is another bloody acronym that should never be introduced to students why add another.

 

[Knobhead]

Am I missing something here, can't you just look where the main incoming earth originates from either from an earth electrode TT, the sheath (armour) TN-S or linked N.TN-C-S or PME which to my mind is another bloody acronym that should never be introduced to students why add another.

I will say I prefer the DNO’s and the American terminology for PME. CNE Continuous Neutral Earth.
Its plain stupid that in the UK we can’t even agree amongst ourselves what it is.

As for TN-C-S who ever came up with that idea should have been drowned at birth.

BTW Richard, I’m retired from heavy industry where TN-S rules.
We did use IT systems for MV, but that’s a whole subject on its own.