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Topic Title: PNB vs TN-C-S (PME)
Topic Summary: To get to the bottom of the debate in the other thread!
Created On: 25 July 2012 08:32 PM
Status: Post and Reply
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 01 August 2012 11:18 AM
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OMS

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Ok, I can agree that a PNB should be a TN-S system :-)

Just a final thought though... If PNB is to be defined as TN-S then you would have to add a clause in 7671 similar to 537.1.2 expressly prohibiting any switching of the "extended star point neutral" to prevent the transformer from floating. Maybe this is why it's currently defined as a PEN.

Regards, Tim


Well, the N-E bond typically is fitted prior to the consumer owned switchboard main switch or active protective device (ACB for example linked into the HV/LV and transformer protection (REF, UREF over temperature, OL and SC for example).

In several cases involving generators, CHP and UPS sets switching the neutral may be a requirement or may be prohibited (if you open the distribution system at 4 pole with a UPS you usually lose your bypass/fault clearance line neutral refernce so the UPS floats - you then need a neutral earthed transformer in the supply system) so even prohibiting switching the incoming N conductor may still result in alternative supply systems floating. You may also need to lift off a number of N-E bonds to prevent circulating neutral curents - neural earthing contactors being a common way of achieveing the outcome when running multiple generators or transformers.

It won't be an easy regulation to draft that's for sure - not least that the conductor is just a neutral not a PEN.

regards

OMS

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Failure is always an option
 01 August 2012 02:15 PM
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timothyboler

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Originally posted by: ArthurHall

The real world is not as clear cut as some of the text books.


I can believe it Arthur for sure but in fairness I'm not referencing from text books; I'm describing engineering standards including BS7430, BS7671, and IEC 60364-1. If we can't keep to the standard definitions of the TN systems -standard- then we shouldn't be using this 'TN-X' nomenclature at all.

I believe the British had to harmonize with IEC and CENELEC to the TN notation (originally from France) in the 16th edition?

Maybe we should scrap them and go with the old SNE, PME, CNE etc. definitions? And of course part of the issue is that DNO public distribution networks don't have to comply with 7671 and therefore they don't use or muddle the TN terms.

However in my mind, if a DNO connects the N to PE in more than one place then they cannot be electrically separated and therefore form a PEN. TN notation has to apply to the whole system including the supply and the installation and not individual zones.

I personally believe blanket bans on TN-C-S should be avoided also so we can get away from this "guaranteed TN-S" v "non-guaranteed TN-S" stuff put forward by the APEA.

Regards, Tim

-------------------------
Everyone loves a fireman - but hates the fire inspector.
 01 August 2012 03:02 PM
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Avatar for OMS.
OMS

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However in my mind, if a DNO connects the N to PE in more than one place then they cannot be electrically separated and therefore form a PEN. TN notation has to apply to the whole system including the supply and the installation and not individual zones.


Lets not muddle this with DNO and consumer installations - we already have TN-C-S to define systems with a PEN in part of the system and a seperate earth and neutral in another part of it. When we are talking about PNB there is generally only one connection to earth the neutral conductor - it just sits an a place unfamiliar to a lot of electricians.

Equally, I mentioned larger UPS systems earlier - which may also have a neutral earthed transformer in order to allow ADS when the main switch is open (4 pole) - describe that system - TN-S ?

Regards

OMS

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Failure is always an option
 01 August 2012 05:12 PM
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timothyboler

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UPS transformers, earthing :-/ will need a new thread for this one :-)

It most certainly is a separately derived TN-S earthing system if your UPS bypass input neutral is isolated from the UPS transformer star point neutral. Or are you saying that your neutral runs straight through from bypass input to output? BS7671 prohibits connecting any downstream earth to a neutral once the N and PE have been separated upstream.

On our UPSs we tie the main and bypass inputs together with a 3ph link and put the isolation transformer before this on the input side. Therefore the whole UPS becomes part of the new earthing system with a separately derived neutral. Problem is that you don't get a redundant bypass supply.

Regards, Tim

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 01 August 2012 05:21 PM
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OMS

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Problem is that you don't get a redundant bypass supply.


Indeed - so if i wanted a UPS input supply for a tarnsformerless UPS, a fault clearance bypass and a totally independant maintenance bypass fed from 3 seperate transformers with 3 matched standby generators I'm going to need transformers with independant neutral earthing (as my 4 pole upstream devices will remove the neutral and hence any fault path)

Yes, it's still TN-S

regards

OMS

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 01 August 2012 06:39 PM
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UKPN

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--"yes, its still TN-S"

well I dont know where the "still" comes from, it never has been TN-S,
everyone in the industry knows that if this method of supply is near
any "wiring regulations" terminology it is TNC-S. but thats only near.
in fact, even the guidance note 8 which posters frequently refer to
shows a PNB source using the wording "alternative TNC-S system
(PNB) and, correctly showing alternative points of connection for source earth.
I highlighted this fact in my advice to electricians post. I have many PNB
systems on my area, in the rural parts, pole transformers feeding a
single consumer.
posters have mentioned they have seen ground mounted transformers
with this type of configuration. they are mistaken, this is not PNB.
there are many types of earthing/bonding configurations in use to
suit the vast number of building types, high rise for example.

Regards.
 02 August 2012 06:53 AM
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alancapon

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Originally posted by: timothyboler
. . . I can't agree with this Alan. With TN-S as defined in 7671 and 7430 you can only ever have one connection between neutral and earth. Yes there may be several earth rods connected to the PE conductor at several places but only 1 N-PE connection. Anything other than one N-PE connection as described above is a TN-C-S. . .

I know that Arthur has already answered this one, but I am just agreeing with his response. It is acceptable to introduce neutral - earth bonding within the supply network on a TT or TNS supply. This does not necessarily make the main suitable for providing PME supplies. At the risk of adding extra abbreviations, this is known as a "Multiple Earthed Neutral" and gives rise to the extra abbreviations TNS+MEN and TT+MEN. These are generally only used as abbreviations within the DNO network. The additional connections may be added as a step towards providing PME, to lower the N - E resistance, or to lower the N - E voltage on the distribution network.

Incidentally, on overhead pole transformers, we will often cable from the transformer pole to the first pole out (which may only be a few metres) and place the neutral - earth link at that point. This makes it easier to separate the HV and LV earthing with poor soil conditions - typically all steelwork on a transformer pole will be tied to the HV earthing system (apart from the LV neutral terminal). We still call this "TT", although it may be TT+MEN in some cases. We would not call it PME, TNC-S or PNB.

We only refer to PNB where a single customer is fed off a transformer with no LV network interconnection, and where the single N - E bond is located at the customer's LV switchboard, usually on the incoming circuit breaker. It is also the only occasion where the customer's earthing system is used to earth the neutral of the transformer.

Regards,

Alan.
 05 August 2012 11:07 PM
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paulskyrme

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OK alancapon,
NOT being argumentative, but just looking at a possible "option".

How would your company describe an 11kV:400V ground mounted transformer supplying a single consumer with the HV & LV earthing done at the transformer, typically say at a supply current of Ca 1600A/ph?
Sorry not sure what they quote the kVA at, but take this as the size of the clients incoming ACB which is their supply OCPD.
There IS a N-E bond at the client incoming ACB, but, it is reported by the DNO that their HV & LV sides are earthed at their substation.
So, if not PNB, what would this be, as the end user is effectively TN-C-S, but can never be PME as they are the only client from the HV:LV transformer?
 06 August 2012 12:29 AM
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alancapon

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Originally posted by: paulskyrme
. . . There IS a N-E bond at the client incoming ACB, but, it is reported by the DNO that their HV & LV sides are earthed at their substation. . .

My belief is that this could be TNS with a Multiply Earthed Neutral if the neutral and earth are two conductors back to the transformer, or TNC-S if there are combined.

. . . but can never be PME as they are the only client from the HV:LV transformer?

Why not? The ESQCR requires a minimum of two earth connections to provide PME. These are on the distribution main before the service joint with the first customer, and also at or beyond the service joint with the last customer. It doesn't tell us how many customers are required.

Regards,

Alan.
 06 August 2012 08:53 AM
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John Peckham

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"My belief is that this could be TNS with a Multiply Earthed Neutral if the neutral and earth are two conductors back to the transformer, or TNC-S if there are combined. "

That is my belief also.

-------------------------
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http://www.astutetechnicalservices.co.uk/
 06 August 2012 10:35 AM
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OMS

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Originally posted by: paulskyrme

OK alancapon,

NOT being argumentative, but just looking at a possible "option".

How would your company describe an 11kV:400V ground mounted transformer supplying a single consumer with the HV & LV earthing done at the transformer, typically say at a supply current of Ca 1600A/ph?

When you say earthing done at the transformer, Paul - what exactly do you mean - is it just the metalwork earthed - or is there a N-E bond at the transformer spill box down to the electrode.


Sorry not sure what they quote the kVA at, but take this as the size of the clients incoming ACB which is their supply OCPD.

There IS a N-E bond at the client incoming ACB, but, it is reported by the DNO that their HV & LV sides are earthed at their substation.

Again - earthing would mean something slightly different - you need to establish the location of the N-E bond(s)

So, if not PNB, what would this be, as the end user is effectively TN-C-S, but can never be PME as they are the only client from the HV:LV transformer?

Well, it could be TN-S, it could be TN-C-S and it could actually be PME depending on the number and location of N-E bonds - but we need to be clear where the "public supply" exists as a PEN conductor would be illegal in a UK installation fed from a public supply.





Regards

OMS

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 06 August 2012 05:32 PM
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UKPN

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---"there is a N/E stake at the consumers C/B pos but the
DNO say HV/LV are one and the same stake"

it will be interesting to see what "wiring regulations" label
OMS attaches to this one!

PNB- long before the "wiring regulations" invented their own
terminology.

Regard.
 06 August 2012 05:47 PM
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OMS

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Originally posted by: UKPN

---"there is a N/E stake at the consumers C/B pos but the

DNO say HV/LV are one and the same stake"

Well the electrode may be common to HV and LV systems - but if there is only one N-E bond at the consumers switchgear - so TN-S characteristics - effectively PNB. You don't know from the description what arrangements connect the transformer to the electrode and if that electrode also connects to the neutral at that point - if it does then multiple earthed neutral arrangements - and depending on the configuration of cabling it could be TN-S or TN-C-S

There are plenty of systems out there where the electrode sits in the vicinity of the transformer, but the N-E bond is at the switchboard - although, the DNO would need to be pretty clear about the impedance of that electrode if they wanted to keep voltage rise of consumers metalwork low in the event of a system HV fault on thier side


it will be interesting to see what "wiring regulations" label

OMS attaches to this one!

Well you have my view - what's yours ? - although I guess you'll have to clarify exactly what Paul was describing

PNB- long before the "wiring regulations" invented their own

terminology.

PNB was in much earlier versions of the wiring regs actually


Regard.


regards

OMS

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Failure is always an option

Edited: 06 August 2012 at 05:59 PM by OMS
 06 August 2012 07:33 PM
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psychicwarrior

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I have been following this thread with interest (always trying to learn something!).

I had cause a while ago to consider a 'CNE' supply. I cant contribute much more to this - you guys seem fairly up on it all.
Did come across this on the internet that seems to address some points made on this thread. I dont know the author btw. He could be on here for all I know.

http://www.marcspages.co.uk/pq/3315.htm
see the disclaimer here too just in case: http://www.marcspages.co.uk/

Thanks
Chris
 08 August 2012 05:16 PM
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John Peckham

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I see in the latest Edition of GN8 has an explanation and very nice drawing at Figure 4.9 showing PNB to be TN-C-S.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 08 August 2012 05:56 PM
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OMS

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OMS

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 08 August 2012 06:23 PM
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UKPN

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--"i can see that the latest ed of gn8 shows PNB to be TNC-S"

does it also say that 1 phase 3 wire is really 2 phase 3 wire?

Regards.
 08 August 2012 07:11 PM
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John Peckham

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-"i can see that the latest ed of gn8 shows PNB to be TNC-S"

does it also say that 1 phase 3 wire is really 2 phase 3 wire? "


Err No.

You need to look at BS7671 Chapter 31 to understand the difference between Single Phase 3 wire and 2 phase 3 wire arrangements.

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John Peckham

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 08 August 2012 09:00 PM
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kj scott

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Originally posted by: John Peckham

-"i can see that the latest ed of gn8 shows PNB to be TNC-S"

And why not, that is exactly what it is in BS 7671 terms.

does it also say that 1 phase 3 wire is really 2 phase 3 wire? "

All depends on the transformer arrangement, it can be either; the only one that can tell you is the supplier.
Err No.

You need to look at BS7671 Chapter 31 to understand the difference between Single Phase 3 wire and 2 phase 3 wire arrangements.


-------------------------
http://www.niceic.biz
 16 August 2012 12:29 PM
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AJJewsbury

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-"i can see that the latest ed of gn8 shows PNB to be TNC-S"

And why not, that is exactly what it is in BS 7671 terms

I'm not convinced.

page 43 of big green describes the 2nd letter as the "relationship of the exposed-conductive-parts of the installation to Earth". Note "earth" - not the 'star point' (or single phase equivalent).

To me that means we're considering at the path between the consumer's earthed metalwork and the earth electrode(s) only.

If N current also flows along any part of that route, it looks like TN-C-S to me. If it doesn't it's surely TN-S.

So PNB with a single electrode looks like TN-S. Add additional electrodes to N and it becomes TN-C-S.

- Andy.
IET » Wiring and the regulations » PNB vs TN-C-S (PME)

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