Neutral Screen Cable

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TPower
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Neutral Screen Cable

Post by TPower »

I was having a discussion with another electrician who was intending on installing two underground 1 core NS cables to create a three phase supply for a builders temp. One cable would be an active+screen as neutral; the other cable would be 2 actives (so the screen would be an active conductor).

It sounds completely bonkers and unsafe to me using the screen as an active and I don’t believe any electrician or inspector would think this is acceptable.

The fact that it’s called a ‘neutral screen’ cable infers that the screen is reserved for the ‘neutral conductor’. Most (if not all) electricians would recognise and expect the screen not to be an ‘active conductor’ so potentially very dangerous.

I’m struggling a bit to find concrete wording in 3000, that indicates this is prohibited? The best I’ve found is 5.5.3.2(b) ‘…conductive screening of cables shall be EARTHED where required at the end where cable originates..’ Is this clause satisfied when you use the screen as a neutral conductor, as the neutral and earth are at the same potential?

Any advice much appreciated.

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gregmcc
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Re: Neutral Screen Cable

Post by gregmcc »

effectively the N/S cable using the screen as a phase would no longer be a N/S cable, table 3.5, this cable would now be classed as
Insulated, unsheathed conductor so this cannot be buried direct and must be enclosed in conduit.

Your right, it is bonkers, just a problem waiting to happen.

If this is buried direct then it is not compliant, a signed coc saying it is, is a false declaration and this should not be passed by an inspector.
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TPower (Fri Dec 17, 2021 4:02 pm)
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Re: Neutral Screen Cable

Post by TPower »

Thanks. Appreciate the feedback.
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Re: Neutral Screen Cable

Post by AlecK »

First; 5.5.3.2 (b) is not relevant.
It's a clause about how (ie at which end) you earth a screen (or armour or sheath), IF it is required to be earthed.
Thar's why it's under 5.5 "earthing arrangements".

what's required to be earthed is in 5.4 "earthing of equipment".
Where the screen of a N/S cable is used as a live conductor (active or neutral); it's not required to be earthed at all.
Earthing an active would be silly.
Earthing a neutral would be non-compliant; essentially creating a parallel connection to the MEN link, and so violating 8.3.3.83.1(a).

As to using the screen of a N/S as an active, I don't like the idea at all - but like you I can't find anything that makes it non-compliant.

Agree the name implies that the screen will be N; but the term is defined in clause 1.4.26
- and the definition does not specify that the screen can only be used as N.

The fundamental principle of basic protection is that live parts shall not be accessible [1.5.3.1], and 1.5.4 lists the acceptable methods - the applicable one being insulation. Remember neutral is a live part [ESR 4]. Thus neutrals, same as actives, must be insulated unless protected by barriers, obstacles, or being out of reach.

True 3.11.12 (via Table 3.5) says insulated, unsheathed conductors can't be buried direct.
But nor can all N/S cables - only those with extra-thick sheath.
Also 3.10.1.1 says “insulated, unsheathed” cables must always be in a wiring enclosure (5 Exceptions).
and Table 3.8 specifies (some) different clearances for N/S than for "insulated, sheathed, live conductors"

But to suggest that this means the screen of a N/S cable can't be used as active doesn't hold up.
If it's true that the screen amounts to "insulated, unsheathed" conductor; then that's the case regardless of what function the screen is used for.
The nature of the sheath doesn't change; so if the screen is "insulated, unsheathed" when used as active; then it's equally "insulated, unsheathed" when used as N. In which case these rules would prevent use of N/S in the usual manner.

Therefore the only reasonable interpretation is that the sheath of N/S cable is doing double duty; being both insulation and sheath for the screen.
and that both the inner core(s) and the screen are classed as insulated & sheathed.

In the absence of anything definite to the contrary, I am - reluctantly - forced to conclude that using the screen as active may actually be compliant.
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TPower (Wed Dec 22, 2021 8:50 pm)
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Re: Neutral Screen Cable

Post by DougP »

Yes AlecK, NS insulation is classed as double insulated as per ASNZS4961.

Using the sheath as an active won't comply with 3.9.4.4 as a means of protection, so another means of protection would need to be used.

As for the other uses of NS cable such as Cat A underground, it seems that they have just assumed that the screen would only be used for neutral or earth as there's no details in any of the relevant clauses prohibiting the screen being used as an active.
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Re: Neutral Screen Cable

Post by pluto »

Prehaps I am showing my age in the electrical industry, but in the Electrical Wiring Regulations 1961 R82 (6) there was a mandated requirement that the screen of Neutral screen cable can only be used as a neutral conductor or Earth conductor.

Since that date (1961) i don't recall any changes made in Regulations or Standards made since that date to change the requirement.
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Re: Neutral Screen Cable

Post by JamieP »

That's really interesting Pluto, I've just had a look and the same requirement stands under the 1976 Regulations, 82 (6) and I can't see this changed in any of the amendments but under the 1997 Regs I can see no such requirement

Edit: Apologies I completely missed the 1993 Regs but same comment on the requirement disappearing from the earlier 1976 Regs, which lines up with Alecs comment below about the move from detailed rules to standards/ECPs etcetc
Last edited by JamieP on Wed Jan 05, 2022 11:32 am, edited 1 time in total.
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Re: Neutral Screen Cable

Post by pluto »

I have a feeling that the neutral or earth must be the only connections to the screen was transfered to the Standard AS/NZS 4961, I don't have a copy to check, but the following statement from a well konow cable manufacturer clearly indicates the requirment.
I quote
Service entrance or consumer mains for underground residential distribution and unenclosed situations where the neutral/earth screen provides protection against the hazards of electric shock. Suitable for direct burial. The screen offers the same resistance as a phase conductor with sufficient coverage to facilitate mechanical protection of the phase conductors.
Quote ends

I would doubt that the outside sheath would have sufficient voltage rating to be able to be used to insulate for at least 250 Volts.
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Re: Neutral Screen Cable

Post by AlecK »

WRT DougP's statement that using screen as active won't comply with 3.9.4.4 (for protection against mechanical damage, when required by either 3.9.4.2 or 3.9.4.3.2);
I agree; but we have no info as to whether any such mechanical protection is required in this case.
Being a "builder's temp"; seems unlikely.
Incidentally, using the screen as neutral doesn't comply as mechanical protection under this clause either;
as there's a difference between a screen being earthed and it being a neutral (ie a live conductor).

Appreciate the reference for sheath being classed as DI, as I suspected.
Which confirms that both the inner core(s) and the screen of a N/S cable are classified as 'insulated & sheathed".
And also means that the voltage rating of the sheath is not an issue.

As for what used to be in older Regulations; each new set of Regs includes a revocation of all previous;
along with a 'grandfather" Reg that allows systems installed under earlier rules to continue in service.
There are many instances of older requirements simply being discontinued.
Especially with the 1993 & 1997 Regs; when there was an intentional move from specifying installation detail in Regs,
to having the Regs simply set high-level outcomes, with detail rules being moved to CoPs & Standards.

Statements from manufacturers as to (their understanding of) intent are interesting, but can't be relied on .
Any rule about how cable is used is an installation rule.
To be enforceable, it needs to be in an installation Standard that's mandated by ESRs.
I can see no reason it would be in a product Standard such as "4961"; and even if it is, it's not enforceable under ESR 59.

Overall,I still don't like the idea. But if I was asked to inspect this set of (presumed) mains; I haven't got a good enough reason to issue a 'fail" RoI.
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