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Topic Title: TN-C-S / TN-S
Topic Summary: Confusion
Created On: 28 September 2017 04:28 PM
Status: Read Only
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 28 September 2017 04:28 PM
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MikeLuker

Posts: 21
Joined: 01 February 2017

Hi All,

Today we have a previously certified TN-S Main Switch Panel with 300mm single line conductors 3 phase and a seperate bare earth conductor. Test readings show as TN-C-S as tested line conductors to earth and neutral, very much the same reading. Opened up the bottom of the panel and the DNO have a 35mm black single cable link to the earth bar, so effectivly changing this to a TN-C-S arrangement via a small link cable lowering the incoming impedance. Would TN-S bonding conditions apply as this is not PME. Is the link cable a DNO neutral reference to earth, is this correct with such a small cable on a 300mm supply or would this not effect any neutral currents, would this still be certified as TN-S with a note that a reference link has been installed thus creating TN-C-S low impedance readings, very confused by this one at the moment and any good advice would be appreciated.

Regards Mike
 28 September 2017 05:25 PM
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weirdbeard

Posts: 3116
Joined: 26 September 2011

Hi Mike, if PME conditions do not apply then the main earthing conductor and bonding can be calculated/selected the same whether TN-S or TN-C-S. The only definitive method of defining the earthing arrangement is ask the DNO.

-------------------------
:beer)
 28 September 2017 05:26 PM
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leckie

Posts: 4323
Joined: 21 November 2008

Well I presume you mean they have linked the incoming neutral to the earth bar, so that makes it a TN-C-S in my book.

Anyway, unless you can guarantee it is TN-S (own transformer for example) you should treat it as a TN-C-S in terms of bonding. That what it says in the DNO bumf as it will normally be connected as such somewhere external to the building anyway, service cable joints, etc.
 28 September 2017 05:45 PM
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MikeLuker

Posts: 21
Joined: 01 February 2017

Hi yes own transformer adjacent so is not connected via multiple earthing, no chance of any electrodes downstream of breaking down, so would TN-S conditions apply
 28 September 2017 06:17 PM
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MikeLuker

Posts: 21
Joined: 01 February 2017

The book only states where PME conditions apply and not a neutral to earth reference link which makes it TN-C-S with a transformer is not PME in my mind, but need to clarify this really as currently all the main bonds are sized for TN-S the NIC do verify this but I need to ensure this is correct
 28 September 2017 06:29 PM
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AJJewsbury

Posts: 15838
Joined: 13 August 2003

Hi yes own transformer adjacent

In which case it might be PNB (protective neutral bonding) rather than PME. While there's a long standing debate about whether to classify PNB as TN-S or TN-C-S, but I think there's agreement that it's not PME in any event. (Remember that BS 7671 requires different sized bonds for PME, rather than TN-C-S). The usual way of thinking about PNB is that it's like TN-S but with the one and only N-PE link at the consumer's end of the supply cable rather than at the transformer.

- Andy.
 28 September 2017 06:43 PM
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MikeLuker

Posts: 21
Joined: 01 February 2017

Hi Andy

Thank you for that response, yes this was my thoughts as well, as there would not be a risk of a broken PEN conductor in these instances would TN-S bonding be appropriate in your view, I seem to come across these instances a lot more lately.
 28 September 2017 08:00 PM
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AJJewsbury

Posts: 15838
Joined: 13 August 2003

as there would not be a risk of a broken PEN conductor in these instances would TN-S bonding be appropriate

As an aside, PME bonding sizes aren't based on a broken PEN condition (as they're typically significantly smaller than the N they can't be expected to carry the installation's full N current for any significant length of time) - just diverted N currents in normal service (all those metallic piped services in parallel with the supplier's PEN running down the road from one property to the next). It's because there's only one N-PE link in TN-S (and TT) systems that there can't be any diverted N currents.
- Andy.
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