Pme

[Maff]

Laugh if you like, I am new to this, is a TN-S sytem classified as a PME system, also is it possible for domestic houses sharing the same supply (same street) to be a mix of TN-S and TN C-S

 

[telectrix]

TNC-S is PME. usually all houses in a street are served by the same cable so all would be PME. TN-S is using the melal sheath of the supply cable as earth.

 

[Maff]

Thanks, I thought so, I have TN C-S, but 5 houses along the row my neighbour is TN-S???

 

[Harry Worth]

Hi Maff,
They can laugh at us both. I'm possibly newer than you - I'm on my electrical course at present (just done my 17th and Part P) and still learning so the experienced guys will set me straight if I've understood incorrectly. PME (protective multiple earthing) is TNCS. The Neutral and Earth (protective conductor is the correct terminology I think) are combined until they enter the house where they seperate. TNS is a system where the two functions , Neutral and Earth are entirely separate conductors. I would have thought from the same supply they would all have to be the same. The only doubt in my mind is that I remember there being a gradual transition from TNS to TNCS and nobody has explained how this transition is achieved. A question for my tutor tomorrow.

Too slow again - Telectrix has already responded.

 

[IQ Electrical]

Thanks, I thought so, I have TN C-S, but 5 houses along the row my neighbour is TN-S???

It's possible that your neighbour could even be supplied from a different transformer, the DNO LV network is a strange creature!

 

[IQ Electrical]

Hi Maff,
They can laugh at us both. I'm possibly newer than you - I'm on my electrical course at present (just done my 17th and Part P) and still learning so the experienced guys will set me straight if I've understood incorrectly. PME (protective multiple earthing) is TNCS. The Neutral and Earth (protective conductor is the correct terminology I think) are combined until they enter the house where they seperate. TNS is a system where the two functions , Neutral and Earth are entirely separate conductors. I would have thought from the same supply they would all have to be the same. The only doubt in my mind is that I remember there being a gradual transition from TNS to TNCS and nobody has explained how this transition is achieved. A question for my tutor tomorrow.

Too slow again - Telectrix has already responded.

Hey guys, you won't catch anyone laughing at your questions here, the only time questions like this are derided is when guys are half way through an installation and then start wondering about the earthing system and consequent bonding requirements.

Harry, the combined conductor in PME is the PEN conductor and your TN-S description is spot on.

The transition from TN-S to TN-C-S PME on a system is achieved by adding multiple earthing (the 'ME' of PME) to the neutral conductor along the route.

Transition is sometimes neccessary due to poor TN-S earth connections at street joints etc.

 

[Harry Worth]

Thanks for that IQ - That's now got me thinking about Maff's original post and whether if transition was ongoing then would there 'appear' to be a mix of TNCS and TNS in a street.

 

[Deleted member 26818]

Yes it is quite possible to have a mix of TN-S and TN-C-S, and even TT in the same street.
The systems are generally only upgraded, at the request of homeowners, and often there is a charge for doing such.

 

[malcolmsanford]

Yes it is quite possible to have a mix of TN-S and TN-C-S, and even TT in the same street.
The systems are generally only upgraded, at the request of homeowners, and often there is a charge for doing such.

And this is another good reason why things like PIR's should be done. On many occasions you will find TN-S systems have deteriorated to such an extent that RCD upfront protection as been installed to counter it. You guys will find over your careers many places where there is no earth at all in premisses due to TN-S degradation, or earth clamps on sheaths loose or missing, it's the same old story well it was ok for years until Granny/Children got electocuted.

 

[richy3333]

Yep, I agree. I recently found a TNS system with a failed earth. Customer didn't care he had no earth and was put out that he had to call the DNO!!

 

[Maff]

if you come across a TN S system where the earth clamp has failed/come undone would you repair/replace it yourself or get the DNO to do it as it is their kit

 

[alli]

I wouldn't touch it. Put in a earth rod and make it a TT system. Tell the customer to notify their DNO and let them repair it.

 

[topquark]

if you come across a TN S system where the earth clamp has failed/come undone would you repair/replace it yourself or get the DNO to do it as it is their kit

I think you've answered your own question. It's not your kit to repair, so get the DNO in to sort it, they're being paid to provide the supply.

 

[roukel01]

Just going back a few posts...... an easy way to remember these systems...

TN-S (Terra Neutral - SEPERATE) Terra is latin for Earth

TN-C-S (Terra Neutral - COMBINED - SEPERATE) ie Earth and Neutral enter the property combined (i.e on same conductor) and are then Seperated

TT (Terra Terra) Basically Earth to earth, a physical connection of the earth in the system is made with Earth

Not quite sure what IT stands for, my guess would be insulated Terra (I.e high impedance connection to earth or no connection at all)

 

[Amp David]

Thanks, I thought so, I have TN C-S, but 5 houses along the row my neighbour is TN-S???

My house in TN-S, but when next door had to have their supply replaced, it was a TNC-S.