TN-S or TNC-S or both??

[DTM20]

Hi guys,

weve been having trouble identifying the supply supply type at a building were we are carrying out the installation of EV charge points and it's obviously important we get it right as it will derermine wether we have to install earth electrodes for the charge points.

it is a large newly built commercial building with a 3 phase 4 wire supply. On first glance it seems to be a PME system, the bolt on the side of the service heard is linked to the neutral bar inside, however there is also a separate 70mm Earth coming in with the main cable (I assume comes from the transformer) terminated to the same bolt.

Is it possibly both supply types are present and we don't need to install electrodes? Cheers in advance

 

[Engineer54]

How can it be TNC-S and TN-S if the separate earth conductors connection point is connected to the neutral at the service head?? The separate earthing conductor is far more likely to come from a local earth rod than a TN-S source earth!!

You can safely say that this installation is a 3 phase PME/TNC-S system....

 

[davesparks]

Could it be PNB with the 70mm earth being the earth electrode?

Or maybe an additional earth electrode on a PME supply.

Have you tried asking the DNO?

 

[Engineer54]

Could it be PNB with the 70mm earth being the earth electrode?

Or maybe an additional earth electrode on a PME supply.

Have you tried asking the DNO?

Not at a DNO consumers service head it won't be!! That's what you'll see at a TX or maybe at the TX's LV main switchboard.

 

[Farmelectrics]

phone call to the dno if in doubt

 

[Engineer54]

phone call to the dno if in doubt

What more confirmation would you want, than the earthing conductor being connected to the neutral connector at a DNO Service Cutout then??

 

[Farmelectrics]

none at all e54 but if the op was in doubt he could allways ask the dno

 

[gazj82]

What more confirmation would you want, than the earthing conductor being connected to the neutral connector at a DNO Service Cutout then??

That will do me, in my mind if they are connected together at source it's C-ombined

 

[DTM20]

Thanks for the replys, I'm going to contact electricity North west on Monday, never done this before so not sure how easy it will be to get in contact with someone who has a definitive answer. I was sure it was a PME system at first glance but then couldn't find an explanation for the incoming 70mm Earth, depending where that Earth is from it can make a big difference to our installation, it won't be easy to install the earth electrodes if they are required

 

[Engineer54]

Thanks for the replys, I'm going to contact electricity North west on Monday, never done this before so not sure how easy it will be to get in contact with someone who has a definitive answer. I was sure it was a PME system at first glance but then couldn't find an explanation for the incoming 70mm Earth, depending where that Earth is from it can make a big difference to our installation, it won't be easy to install the earth electrodes if they are required

Now i'm totally confused, why will this make a big difference to your installation, if you were already sure it was a PME supply?? So what's changed?? You would have known that earth electrodes would be required for this commercial multi EV charging point installation....

So where do you think this 70mm earth conductor is coming from, and what difference do you think it's going to make for your installation??

 

[DTM20]

Now i'm totally confused, why will this make a big difference to your installation, if you were already sure it was a PME supply?? So what's changed?? You would have known that earth electrodes would be required for this commercial multi EV charging point installation....

So where do you think this 70mm earth conductor is coming from, and what difference do you think it's going to make for your installation??

Hi E54, please correct me if I'm wrong, but as I understand it the separate Earth electrodes are required in case the neutral conductor fails then a person would be the path to Earth in the event of a fault, however on this installation if the neutral conductor did fail and there was a fault condition the 70mm Earth would still provide the path to Earth, if it's coming from the transformer, or even from another electrode.

 

[Engineer54]

Hi E54, please correct me if I'm wrong, but as I understand it the separate Earth electrodes are required in case the neutral conductor fails then a person would be the path to Earth in the event of a fault, however on this installation if the neutral conductor did fail and there was a fault condition the 70mm Earth would still provide the path to Earth, if it's coming from the transformer, or even from another electrode.

Correct... A local earth electrode system connected to the MET or to the Neutral connection itself will greatly reduce the adverse effects of an extremely rare scenario of a broken neutral. If the Ra value of that electrode system is very low and at the very least complies with DNO values which as i remember is below 5 or 10 ohms... It's basically just another N-E connection on a ''Protective Multiple Earth'' system, albeit being locally provided. Very common on larger DNO 3 phase installations of say 400A and up...


If this was a single domestic EV charging point you were installing, i would be telling you not to bother about the supply being PME or not. But as i understand it this is a commercially based installation so a TT system will need to be provided for the multiple EV charging points...

EDIT.... If as you say there is a Transformer in this building or on the property, then the 70mm could well be coming from the transformers LV's main distribution boards earthing system. but everything still remains the same, it's still a PME/TNC-S supply you have there.