How good are you at telling the difference?
How good are you at telling the difference?
Wow since when did the simple act of earthing get so complicated! I don't think it was always so things really need simplifying.
As with may things there are different options at different stages, resulting in many overall permutations...
o The LV earth might be shared with the HV earth or it might be independent.
o The source earth may be connected at the source, or a bit further away (as near a reasonably practical) (ESQCR 8(3)b - allows that for any system) - or much further away (as a special case for PNB)
o Exposed-conductive-parts might be connected direct to the source earth without going via the N, or they can 'borrow' the N for that purpose
o There may or may not be additional deliberate connections to Earth along the line
o The consumer's installation may provide additional connections with Earth (or other potentials) - e.g. via bonded extraneous-conductive-parts to the supplier PE or even an entirely independent electrode (TT).
the trick is to distil out which differences are important and which are just arbitrary differences.
For me the interesting criteria is at how many different points the N is connected to the protective earthing system - if it's exactly one then there can't be any diverted N currents (as N current can't both leave an return from a single point) and a break in a N anywhere can't interfere with the Earthing of exposed-conductive-parts. (I'd call then TN-S.) If it's two or more then the possibilities of diverted N currents arise, as does the risks of shock from exposed-conductive-parts in thee case of a broken PEN (that would be TN-C-S in my book).
PME is one way of mitigating some of TN-C-S risks in large networks. The TN-C-S version of PNB follows the same general rules for electrode placement, except due to the small size of the network the 1st and last electrodes can effectively be at the same position (i.e.one electrode serves both purposes).
But there can be large scale TN-S networks that aren't PME, likewise I think there can be PNB systems that aren't TN-C-S but TN-S (in both cases it might be wise to treat them as if they could be TN-C-S, as the DNO might convert to TN-C-S at a later date, but that's a separate issue, and won't apply in all circumstances, e.g. private HV/LV transformers).
I think there is confusion about the phase "PME conditions" - not helped by the general English usage of "condition" as meaning either "the state of things" or "requirements that must be satisfied". As I understand it, it is meant to refer to the extra requirements imposed on a consumer (e.g. size of bonding conductors) that were originally over and above Wiring Regs requirements (but these days mostly incorporated into BS 7671), before the DNO will grant permission for use of its Earth terminal. - i.e. ensuring the installation is suitable for connection to a PME distribution system. That's subtly but significantly different to saying the system currently actually is PME.
BS 7671 doesn't help either - sometimes it says "where PME conditions apply" other times it says "a PME Earthing facility" - which can be interpreted rather differently.
- Andy.
Wow since when did the simple act of earthing get so complicated! I don't think it was always so things really need simplifying.
As with may things there are different options at different stages, resulting in many overall permutations...
o The LV earth might be shared with the HV earth or it might be independent.
o The source earth may be connected at the source, or a bit further away (as near a reasonably practical) (ESQCR 8(3)b - allows that for any system) - or much further away (as a special case for PNB)
o Exposed-conductive-parts might be connected direct to the source earth without going via the N, or they can 'borrow' the N for that purpose
o There may or may not be additional deliberate connections to Earth along the line
o The consumer's installation may provide additional connections with Earth (or other potentials) - e.g. via bonded extraneous-conductive-parts to the supplier PE or even an entirely independent electrode (TT).
the trick is to distil out which differences are important and which are just arbitrary differences.
For me the interesting criteria is at how many different points the N is connected to the protective earthing system - if it's exactly one then there can't be any diverted N currents (as N current can't both leave an return from a single point) and a break in a N anywhere can't interfere with the Earthing of exposed-conductive-parts. (I'd call then TN-S.) If it's two or more then the possibilities of diverted N currents arise, as does the risks of shock from exposed-conductive-parts in thee case of a broken PEN (that would be TN-C-S in my book).
PME is one way of mitigating some of TN-C-S risks in large networks. The TN-C-S version of PNB follows the same general rules for electrode placement, except due to the small size of the network the 1st and last electrodes can effectively be at the same position (i.e.one electrode serves both purposes).
But there can be large scale TN-S networks that aren't PME, likewise I think there can be PNB systems that aren't TN-C-S but TN-S (in both cases it might be wise to treat them as if they could be TN-C-S, as the DNO might convert to TN-C-S at a later date, but that's a separate issue, and won't apply in all circumstances, e.g. private HV/LV transformers).
I think there is confusion about the phase "PME conditions" - not helped by the general English usage of "condition" as meaning either "the state of things" or "requirements that must be satisfied". As I understand it, it is meant to refer to the extra requirements imposed on a consumer (e.g. size of bonding conductors) that were originally over and above Wiring Regs requirements (but these days mostly incorporated into BS 7671), before the DNO will grant permission for use of its Earth terminal. - i.e. ensuring the installation is suitable for connection to a PME distribution system. That's subtly but significantly different to saying the system currently actually is PME.
BS 7671 doesn't help either - sometimes it says "where PME conditions apply" other times it says "a PME Earthing facility" - which can be interpreted rather differently.
- Andy.
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