Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Helpful Answer
  • Replies 59 replies
  • Subscribers 37 subscribers
  • Views 9904 views
  • Users 0 members are here
Options
You may also be interested in

Repost - Earth Rod not taken account in TN systems

fiftyhertz

Hi All,

I realise the function of the earth rod in a TN system is to provide a close reference to true earth for the neutral

The thing that has confused me slightly is the TNCS PNB, which has an earth rod located at the consumer end. When i looked at the old forums there was a debate between whether this was TNCS or TNS, as the neutral carries no current due to the earth rod, and therefore by definition cannot be a combined conductor. If the current is not dissipated into the ground via the rod, why would no current flow in the neutral of this system prior to the rod

Thanks in advance

EDIT: My question wasn’t phrased very well and I’ve tried to clean it up for future readers, but i think this is the correct summary.

Fault current CAN flow between the neutral/earth link and the neutral point of the transformer in a PNB earthing arrangement. The previous forum posters were essentially saying is that even though though the link is remote, fault current will still flow in the CNE cabling, but we can note that it also would in a pure TN-S system but more likely an internal section of busbar within the TX and the neutral bar, instead of external cabling and by that logic TN-S would be a form of TN-C-S if semantics were involved.

Link to thread

 What earthing arrangement is this? 

Parents
  • 0

    I think most of the points have been already covered - just to add:

    1. Any electrode has a very considerable resistance to the general mass of the Earth - generally several tens of Ohms (with a lot of effort you might get down to an Ohm or slightly lower). It's all down to the soil that surrounds the electrode rather than the resistance metallic electrode itself. So two (or more) electrodes on the same PEN conductor normally make a very small difference indeed  (tens of Ohms in parallel with probably less than 1/10th of an Ohm) - for all practical purposes the effect on normal loop impedance etc. is ignored. Forget any notion of there being no potential difference along a N or PEN conductor - there will be if there's any N or PE current flowing - Ohm's Law still applies even if the numbers are small.

    2. To be TN-C-S there must be a conductor in the system that carries both N current and provides a connection to Earth. To my mind that's down to the order between the electrode and the N-PE link, so TN-S looks like this:

    and TN-C-S:

    The PEN (or CNE) conductor only exists between points A and B to my mind (not between 0 and A).

    Note that there's nothing in those diagrams (or BS 7671's definitions) that say how far apart the transformer, N-PE link and electrode can be.

    Also we're talking about the path of the connection of the installation to Earth rather than the route taken by earth fault currents (whose complete path will of course be back to the transformer star point). It's the presence of N currents between the installation and Earth that causes some of the biggest problems with TN-C-S, (especially during broken PEN events, but some smaller problems under normal conditions too).

    Take the TN-C-S diagram and add a few more electrodes to the PEN conductor and you have PME.

    PNB can be either (in the limiting case, a single bolt provides the N-PE link and the connection point for the electrode's conductor) so the cases converge - it can be argued either way).

    "PME conditions apply" typically means the "DNO want you to treat it as if it were PME" - whether it actually is at the present time or not - i.e. they're keeping their options open if they want to convert to fully fledged PME at a later date (typically to allow additional consumers to be connected to the same transformer in the future). Where there isn't a DNO involved on the LV side (e.g. industrial site with a private transformer) it's then feasible to adopt a TN-S version of PNB and not worry about PME conditions.

       - Andy.

  • 0 in reply to AJJewsbury

    Hi Andy,

    Thanks for the post; some further useful diagram and helpful text. In the industrial park example, I’d presume wavecon or similar DNO cabling would be used for the CNE? I must admit I haven’t come across it before for Non-DNOs.
    Myself and I bet most people not involved in designing utilities would lean towards the use of SWA or AWA singles in the standard TNS (Non PNB) arrangement, what would be the deciding factors in you using this PNB TNS arrangement with the CNE? Probably length of run would be a factor 

  • 0 in reply to fiftyhertz
    fiftyhertz said:
    what would be the deciding factors in you using this PNB TNS arrangement with the CNE? Probably length of run would be a factor 

    Or just DNO policy

Reply Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines