Regards

OMS

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Failure is always an option

There are differences between PME and TN-S.

With PME, you have the neutral tied to earth at several locations, as a minimum these being on the main, at or before the junction with the first service cable and at or after the junction with the last service cable. In practice, there will be more than two connections to earth. At the customer's premises, the System Neutral is extended to the MET, for use as an earth connection.

With TNS, you can have the neutral tied to earth at several locations, as a minimum one connection is made, this is on the main, at or before the junction with the first service cable. At the customer's premises, an Earth Conductor (separate from the System Neutral) is extended to the MET. Just to confuse things, it is possible to have multiple connections between earth and the System Neutral, but this is still TNS as a separate Earth Conductor is run to the customer.

With PNB, this is a TNS supply in my mind, but with the neutral - earth connection made at the customer's end of the service cable. This neutral - earth connection is a Protective Neutral Bond. It is important to know this, as it is hazardous to the installation (and to life) to disconnect it with the transformer live. With a faulty transformer, an unearthed secondary winding could be as much as 6.35kV to earth.

Regards,

Alan.

From the new post by UKPN:

"this is for electricians who, faced with the very rare situation
of this special form of supply need to know the bonding
requirements.
PNB can be recognised by the fact it is found mainly in rural
areas.
it is ALWAYS an overhead line, pole transformer, single phase
or multiphase type of supply.
the neutral earth is NORMALLY crimped at the point at which
the insulator bracket is fixed to a consumers external wall, but
in rare situations can be in the neutral block of the DNO service.
because PNB is non-statutory, the consumers bonding requirements
are less onerous than PME regulations.
while the ELI is expected to be low, (less than 0.5) as with all DNO
ELI parameters external to the installation, this cannot be guaranteed.
an ideal situation, particularly as these are mainly rural installations
would be to use the PNB connection as an earth path with RCCD/S.

Regards"

Three points regarding PNB and domestic installations.

First I don't think this system is rare.

Second I think it still gets a PME sticker installed at the REC cut out.

Third the REC would expect to see the same earthing installed as a PME installation.

Regards Andy

I agree with UKPN PNB is only found on pole mounted transformers.
Some DNO's declare it as a PME supply so that the electrician bonds the installation to PME standard, However technicaly it is TN-S.
I belive the main reason for it use is to seperate the HV and LV earths. On a LV OHL the neutral is usually earthed at the first pole out from the transformer but if the line only feeds one customer it makes sense to earth the neutral at the customers cut out so that the neutral is tied 'closer' to local earth.
I dont know who or why the IET has started putting references to PNB in the guidance notes etc when it is not recognised in the regs themselfs.
The bottom line is that the earthing system is what the DNO tells you it is. Or if its a private transformer its TN-S

You guys need to get out more - PNB is offered by most DNO's for supplies to at least 1100A (ie a typical 800kVA DNO distribution transformer) - and for sure that will be ground mounted. I've just closed the file on a new development that had PNB offerd by the DNO in the Bournemouth area (sandy soil) - I've worked on many 10's if not hundreds of installations that have been offered PNB by the DNO's (and before them the Area Boards) - in many rural locations it was the only form of supply to anything other than domestic properties - schools, factory units, large agricultural installations to name a few .

Principally it will only be offered to a single consumer and in circumstances where the effective transformer resistance to earth exceeds 1 Ohm (but doesn't exceed 40 ohm). The metering CT's etc will be in a chamber pre agreed with the DNO (ie possibly in the client switchboard or in a seperate cabinet). HV and LV earthing will be seperated.

Typical arrangments are (for an example by SSE) here

As Alan points out - you need to know the animal you are dealing with - lift off that N-E link with a TX live and there could be a world of hurt going on for the unsuspecting (or uninformed electrician).

PNB most certainly is not reserved for pole mounted domestic supplies - and those of you with DNO backgrounds or similar will surely know that.

Regards

OMS

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Failure is always an option

Taken from central networks earthing guide

PNB Earthing
PNB is a special method of earthing, similar to PME, where the neutral earth is installed at
or adjacent to the customer's installation instead of at the transformer position. This
method can still be used provided the supply is from a dedicated pole mounted
transformer supplying a single customer..

As Alan Capon said one of the problems is that different DNO's use the term PNB for various configerations.

I worked for a DNO for over 20 years and to us PNB was pretty much as UKPN described, so it looks like SSE may be the odd one out.

Well, perhaps no so odd - I've just looked in a WPD document and clearly they offer PNB to single consumers from pad mount transformers where they need to segregate HV and LV earthing (typically by a stipulated 9m in WPD's case - they also wish to retain the HV earthing in the vicinity of the transformer, so PNB offers an easy way to get some seperation from HV and LV earthing.

I've also just checked an Eon document offering 400A TP&N supplies at PNB or PME (depending on number of consumers)

I've also been involved with DNO's and area boards before that, along with a stint down at Millbank with EC - PNB was, and still is, widely used by many suppliers across the UK and for sure ther are many thousands of legacy installations still out there.

Equally we have exactly the same set up where clients take supplies at HV - the NE bond is invariably at the main switchboard by default

Regards

OMS

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Failure is always an option

As a onlooker to the supply industry with no experience of working with large scale distribution transformers. It comes down to:
1/ terminology and design used by different organizations
2/ Local soil resistance
3/ Position of the neutral earth bond

1/ Terminology
A PME system can be described as a TN-C-S or TN-S which can also be regarded in certain situations as PNB
2/ Local Soil conditions
Dependent up the above conditions and whether or not an earth resistance is less than 1 ohm will be the deciding factor whether or not a neutral earth link is made at the transformer.

3/ Position of the Neutral/earth bond
I can see that a very low resistance between HV and LV will mean a fault on the HV side will cause the voltage to rise on the LV side, the consequences of such a fault will be limited by having a low touch voltage below 430V but would encourage a very high fault current. To reduce the possibility of fire, this would be detected by CTs connected around the incoming supply conductors and then disconnected at the Tx.
Alternatively, a ground resistance between 1 ohm and 40 ohms will require the need for isolation between HV and LV. The neutral to earth bond is placed as far as possible from the Tx in order to reduce effects of touch voltages being imported from the faulty Tx and to provide an earthing point to maintain those touch voltages.

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."

SSE
Power
Distribution
Applies to
Third party installers of new connections


SPECIFICATION FOR BONDING AND EARTHING DURING THE INSTALLATION OF NEW CONNECTIONS


Page 4 of 5



SP-PS-312









Prepared by: JB, Network Services
Rev: 2.01













4.3 Separate Neutral and Earth Distributors (SNE, TN-S)

Where properties are unsuitable for the application of PME and/or electrical installations
do not comply with the Regulations, the distributor services shall be of SNE type and separate
earth terminal provided at e service termination. The neutral shall be earthed at the source
substati only (SEN).

The separate earth conductor shall provide a continuous path back to t source substation and have a
current carrying capacity not less than one that of the phase conductor; and must in any case be
not less than 16 mm

4.4 Protective Neutral Bonding (PNB, TN-C-S (PNB))

PNB is not a current standard or use on SSEPD systems.

Scottish Power is the same saying that PNB should be applied to pole mounted only see here (page 7); I don't know why they would say that.

Agreed that PNB (per strict BS7430 definition) is used all over on ground mounted substations. Every one on our site is done this way. Also for EMC reasons all double ended switchboards are PNB in accordance with fig. 9A and 9B of BS7671 - also described as... TN-C-S multiple source set-ups.

I still don't like the idea of PNB being TN-S If your neutral is going to carry earth fault current (from out of the spill box to a switchboard) then it's a PEN at least in my mind.

...and, as has been pointed out, by calling it TN-S on the paper you increase the risk of the unsuspecting electrician disconnecting the link whilst live.

Regards, Tim [Edit link]

-------------------------
Everyone loves a fireman - but hates the fire inspector.

OK Arthur, without starting a war of "cut and paste" then from :

PROCEDURE FOR INSTALLATION OF LARGE LOW VOLTAGE SUPPLIES USING 3 CORE BUNCHED WAVECON CABLES
(GREATER THAN 250KVA)

Published by SSE and Applies to Power Systems Staff and New Connections

we have in section 7.0 - Provision of Earthing for Customer's Equipment

this:

7.2 LV Cable Boxes where earth resistance is greater than 1 ohm.

Where the earth resistance at our transformer is greater than 1 ohm the BWC copper screen wires will be cut flush with the outer covering of the cable and taped to form an insulated end. The armour clamp rings shall be discarded and the covering of the cable at the armour clamp position built up using hessian tape until it can be securely clamped by the armour clamps. On no account must a connection be installed between the transformer neutral bar and the cable box earth stud.
An HV steelwork earth electrode will be connected to the steelwork earth of the transformer. (A workshop copper bar will have been installed to connect the cable box earth to the transformer earth and this shall remain).
On the customer's equipment a link must be installed between the incoming neutral busbar and the earth connection. An earth electrode will be connected to the neutral busbar. This electrode must be segregated from the transformer HV steelwork earth by at least 3 metres. The resistance of this electrode shall not exceed 40 ohms.

The copper screen wires of the BWC will be connected to the earth electrode at the customer's end by connecting them to the earth stud connection in the customer's switchgear.
This type of installation will be PNB (Protective Neutral Bond).

I guess it depends on which manual you look at !!

Although this is semantics - my comment was simply aimed at UKPN's statement that PNB was only ever applicable to small scale single consumers (ie domestics) fed from pole pig transformers - clearly not the case

Regards

OMS

-------------------------
Failure is always an option

No war OMS
The only thing that it seems we have established is that there is no standard. Or if there is a standard then it is applyed in a different manners across the country.

I guess so Arthur - I suppose we are all clear in our own minds as to what we mean when talking about PNB - but clearly that doesn't translate across the DNO's or the wider engineering community

Regards

OMS

-------------------------
Failure is always an option

One thing that I think has been cleared up is that if it is a private transformer with the Neutral earth at the LV board then it is definatly TN-S as a private installation dosent have permission to use TN-C-S.

I can't reconcile that with Figure 9A (see note C) from BS7671 titled as a TN-C-S?

-------------------------
Everyone loves a fireman - but hates the fire inspector.

Why not Tim - the diagram shows the consumers installation as a solid line - there is no PEN conductor within a consumers installation (when fed from a public supply. The N-E link is effectively the cut out for a domestic supply.

For a larger non domestic supply note C is also true. what is not shown is the multiple earthing of the PEN conductor so calls into doubt the TN-C-S nature of the title (and couldn't be PME if note a is true)

Note a clarifies the earthing of the neutral - ie it occurs only at point c so actually reflects PNB (or TN-S)

Note b just cocks the whole thing up by reference to "similar to a PEN" - but isn't one. The insulated requirement is correct though

As I said initially, BS 7671 committees really do have thier knickers in a twist over this - principally because it's a long time since the IEE wiring regs contained any reference to PNB. The BS 7430 committee haven't helped either.

Regards

OMS

-------------------------
Failure is always an option

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.

However what you said does reflect what's written in the 2012 Spring Wiring Matters regarding earthing of Petrol Stations. They mention that the APEA Blue Book requires an on site transformer with "guaranteed TN-S" with a fully dedicated PE from the site back to the supply transformer because you can't guarantee this with a public TN-S supply? Hope I'm not getting confused about TN-S now :-/

Regards, Tim

-------------------------
Everyone loves a fireman - but hates the fire inspector.

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

-------------------------
Everyone loves a fireman - but hates the fire inspector.

Tim
In a DNO TN-S system the neutral is earthed at multiple points. Normaly every joint, every service joint and every pot end. In addition the neutrals of adjoining transformers are solidly connected. This system is known as MEN multiple earthed neutral and was brought in about 1960 I think.
The crutial point is that there are seperate neutral and earth conductors, even if they are linked along the way. The natural progression was to combine the neutral and earth into one conductor and PME was born.
A SNE main (with MEN) can have a TN-C-S service taken off it or a TN-S service or a TT if the earth terminal is with held.
A CNE main can only supply a TN-C-S or TT service.
The real world is not as clear cut as some of the text books.

Take supply authorities transformer/transformers 1000KVA or larger located in a transformer compound 30,40, 60 meters from an large industrial main-switchboard room, a three strand bare copper conductor leaves the LV neutral, goes into the ground and arises and connected to the earthing bar at the LV switchboard. What would this supply be, it could be PNB, (because that what it is). Is it TN-C system (earthed concentric wiring) with the combined earth and neutral conductor (PEN)., technically not correct, because it is not concentric wiring. Is it TN-C-S not really unless linked across MET and neutral at main switchboard. Is it TT, no, unless you loose the neutral and earth acts as an earth electrode?
Note: the above is still in use, but superseded by PVC sheathed earthing conductors as appropriate.