tncs ot tns

[soulman]

hi can you chaps can you just explain something to me that confused me this week, after installing a new power circuit in a block of flats i was told to do a R1+R2 and then a ze i was unable to disconnect the earthing conductor as the consumer could not be isolated. firstly is this exceptable to work in a live consumer unit taken a ze reading. secondly the reading was 0.47ohms for ze yet there was a large sticker on the giant lucy enclosure stating p.m.e. the guy i was working for said that it must be tns. as we got a similar read in the other blocks of flats. the flats are about 3 years old and i thought tncs/pme was installed in most new properties. would the correct procedure be to contact the supplier to confirm.

cheers

 

[Larnacaman]

hi can you chaps can you just explain something to me that confused me this week, after installing a new power circuit in a block of flats i was told to do a R1+R2 and then a ze i was unable to disconnect the earthing conductor as the consumer could not be isolated. firstly is this exceptable to work in a live consumer unit taken a ze reading. secondly the reading was 0.47ohms for ze yet there was a large sticker on the giant lucy enclosure stating p.m.e. the guy i was working for said that it must be tns. as we got a similar read in the other blocks of flats. the flats are about 3 years old and i thought tncs/pme was installed in most new properties. would the correct procedure be to contact the supplier to confirm.cheers

If the Lucy's got a great big sticker on it saying that the supply is PME then it Ain't TNS is it!!! So no need to contact the DNO... TNS is usually around 0.8ohm mark, while PME is generally around the 0.35ohm, ....So there you are, your reading is a lot closer to the PME levels than the TNS. Your supply could well be taken from a previously designated TNS system that's now been converted to PME....

 

[soulman]

hi thanks for reply so it is exceptable on tncs to have a reading higher than 0.35 cheers

 

[Larnacaman]

soulman,

As i said, this may well be a converted PME from an older TNS system. It could therefore be, not fully converted at this time, ie ...not all of the joints have been roded. Also, as this is a block of flats your working on, the reading at the flat, may not be the same as at the MDB (if it has one)

Nothing much wrong with a Ze of 0.47ohm, plenty good enough to operate any form of protective device... As for statutory limits on DNO supplied earths, i'm a little rusty on UK stats these days .... So perhaps others here can give you a better answer on the PME 0.35ohm limits!!!

 

[sparkytim]

Report to DNO, if it is a 60 Amp cut-out fuse then they may well not do anything, and as our learned friend has said if it is a Flat then it may be 0.35 or less at the supply intake position and the rest may be internal.

Does this flat have a Red Head Service Fuse carrier? If so then the actual Service fuse is at the supply intake position and that is just a solid link isolator commonly used on blocks of flats.

 

[soulman]

hi this is the main distribution board and at college we were told that pme should never be above 0.35ohms?

 

[Sidewinder]

soulman,

That is NOT strictly true.

Do a disconnection time / impedance calculation on 60, 80 & 100A cut out fuses and solve for Z and see what you get?

 

[kme]

In Soul`s defence; that is EXACTLY "what they teach" now - if a PME supply, Ze must be no higher than 0.35 ohms.

Where I am confused is this:

If the earthing conductor could not be disconnected; how did you find the Ze? What you may have is Zs (DB), which is a different animal; and suggests that the Ze may well be excessive.

If I had those results; I would be STRONGLY recommending a block isolation, to test Ze; or informing DNO of possibility of fault on PME earth.

KME

 

[Larnacaman]

Any particular reasoning behind that statment about ''PME being no higher than 0.35ohm'' KME??

 

[sellers]

is there any chance of a picture? If your unsure of whether it is tns or tncs I wouldn't 100% trust a PME sticker.

 

[kme]

Any particular reasoning behind that statment about ''PME being no higher than 0.35ohm'' KME??

I`m simply repeating the current "standard" being taught at C+G level, mate.

is there any chance of a picture? If your unsure of whether it is tns or tncs I wouldn't 100% trust a PME sticker.

If the DNO have put a sticker on a HED saying it`s PME - then that is what they consider that supply to be. If your car has sticker on the fuel filler saying "diesel only", would you question that?

 

[sellers]

I`m simply repeating the current "standard" being taught at C+G level, mate.If the DNO have put a sticker on a HED saying it`s PME - then that is what they consider that supply to be. If your car has sticker on the fuel filler saying "diesel only", would you question that?

If you isolated a circuit marked up as downstairs lighting, would you work on it without testing for dead, of course not!

Never pressume.

0.35 is given in the OSG, can't remember if its a guideline or not. Don't know if its in the BRB

 

[aptsys]

It may well have a PME sticker since it has the provision for PME, but might still be a TN-S supply from earlier days. Best ask the DNO, or report a high Ze.

 

[Zeespark]

TNCS is the earthing system at the head, PME is supposed to be rodding at every joint - the two are not the same.

AndyGuinness

 

[S60TEM]

in my opinion and experience, report it to DNO as being above the allowable 0.35ohms and wait as they tell you 0.47 is fine! It's still well below anything to worry about with regards to disconnection times but at least you'll have informed them and covered your arse.

 

[soulman]

thanks for replies interesting