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Topic Title: Isolator for long meter tails
Topic Summary: Plastic or metal?
Created On: 21 September 2017 12:22 PM
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 21 September 2017 12:22 PM
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BrucieBonus

Posts: 1035
Joined: 20 February 2007

Hi all

Can I ask your advice please?

I've been asked to move a fuse board to a more sensible location - this will involve installing new meter tails about 5 m long. I was planning to fit a double pole isolator after the meter.

This popped up - but it's plastic - should it not be metal??

https://www.tlc-direct.co.uk/Technical/DataSheets/Wylex/WYREC2.pdf

It's TNS 60/80 main fuse for a 1 bed flat

I wasn't thinking I need to use a switched fuse, making it a submain - the run is from the front of the cellar to the back, will be in trunking. Am I right?

thanks in advance
BB
 21 September 2017 12:36 PM
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AJJewsbury

Posts: 15823
Joined: 13 August 2003

Just a simple switch/isolator is neither a consumer unit nor switchgear according to BS 7671 definitions, so the non-combustible requirement doesn't apply (and probably preferable for a TN-S system where Zs could be higher than permitted for 5s disconnection).

However, 5m is a bit long for meter tails - most suppliers stipulate a maximum of around 3m if you're relying on their fuse, or even don't promise anything at all and tell you to fall back on BS 7671 434.2.1 which puts you back to a 3m limit (and adds further requirements for reducing the risk of faults, fire and danger to persons to a minimum).

So I'd check with your local DNO to see if 5m is acceptable to them, if not you'd be looking at a (non-combustible) swicthfuse.

- Andy.
 21 September 2017 01:05 PM
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geoffsd

Posts: 1702
Joined: 15 June 2010

Does the fact that the DNO might say whatever mean that their fuse will not protect longer tails against fault current causing the implementation of 434.2.2(i)?

Also, if the house has an external meter box, does this count as "within" domestic (household) premises as stated in 421.1.201?
 21 September 2017 01:24 PM
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AJJewsbury

Posts: 15823
Joined: 13 August 2003

Does the fact that the DNO might say whatever mean that their fuse will not protect longer tails against fault current causing the implementation of 434.2.2(i)?

I suspect it's more of a 'organisational' rather than physical constraint - i.e. the DNO retain the ability to uprate the fuse in future without having to either risk inadvertently degrading the existing level of protection or having to ferret out many metres of hidden cabling to check it's c.s.a., length and so on.
- Andy.
 21 September 2017 02:08 PM
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tomgunn

Posts: 3916
Joined: 25 May 2005

Why can this not be classed as a 'sub-mains' supply? Go with PVC T&E, of the appropriate size, or a short run of SWA / MICC etc as I have run many sub mains like this.

regards...

-------------------------
Tom.... (The TERMINATOR).

handyTRADESMAN

Castle Builders
 21 September 2017 02:35 PM
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BrucieBonus

Posts: 1035
Joined: 20 February 2007

Thanks for the replies. I'm in UKPN area and just found this from one of their guides

"BS 7671 Regulation 433.2.2 allows a maximum distance of three metres between overload
protection devices and their source, therefore the customer shall provide and install an
intermediate switch fuse or isolating switch if the distance between the meter and consumer
unit is greater than three metres"

But I think their inclusion of isolating switch is not correct as then it wouldn't comply with the need to have a protective device.....

So switch fuse and SWA or 16mm T and E it is then

thanks again for pointing me in the right direction!
BB
 21 September 2017 02:57 PM
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AJJewsbury

Posts: 15823
Joined: 13 August 2003

So switch fuse and SWA or 16mm T and E it is then

Or insulated & sheathed singles and 16mm2 G/Y if you prefer - probably easier to work with and less worry about T&E's c.p.c. being too small for an earthing/bonding conductor (presuming the MET's at the flat's CU). Definitely SWA if it's going to be concealed in a wall though.
- Andy.
 21 September 2017 03:12 PM
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geoffsd

Posts: 1702
Joined: 15 June 2010

Originally posted by: BrucieBonus

"BS 7671 Regulation 433.2.2 allows a maximum distance of three metres between overload

protection devices and their source,

That is not what 433.2.2 says.

The 3m. limit applies only where the fault current protection is not met - and then calls for fire protection measures.
This would therefore not normally be done in dwellings.
 21 September 2017 03:12 PM
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geoffsd

Posts: 1702
Joined: 15 June 2010

duplicate post
 21 September 2017 07:54 PM
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Fm

Posts: 1710
Joined: 24 August 2011

A Wyler kmf may come in handy
 21 September 2017 07:54 PM
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Fm

Posts: 1710
Joined: 24 August 2011

A Wyler kmf may come in handy
 21 September 2017 09:53 PM
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alancapon

Posts: 6880
Joined: 27 December 2005

The distance quoted by the DNO does not come from BS7671. It comes from the DNO's design guide for their LV network. To make their lives easier, they say that if you use a certain size and length of meter tails, they will design their network to ensure their cutout fuse will always offer overload protection. If the meter tails are longer / thinner than they request, it may still work, but if for some reason it doesn't operate in a fault scenario, the liability is not the DNO's.

Regards,

Alan.
 21 September 2017 11:05 PM
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mapj1

Posts: 9349
Joined: 22 July 2004

However a moments calculation, augmented by squinting at the fuse curves, suggests that for 100A supplies and 25mmsq tails, 3m is very, very conservative.

Even if at the cut out Zs is 0.8 ohms, as it may be for TNS, the max earth fault current is only 290A ,we are looking at about 100 seconds (a touch under 2 minutes) to open.
The TNC-S figure of 0.3 ohms gives 770A, and more like second.
Its not going to fail to open due to an extra 100milliohms in either case, TNC-s becomes 550A and 5 seconds, and TN-S becomes 0.9 ohms and 255 amps and more like 3 minutes to blow.
Not much more hazardous than it was before the extra 0.1 ohms.
So how long is an extra 100 milli-ohms of 25mmsq. well its 16 milliohms per 25m per core, so about 6*25metres = 150m round trip so 70m of meter tails pair. A factor of more than 20 is a huge margin of conservatism.

bigger pics here

-------------------------
regards Mike
 22 September 2017 02:18 PM
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tomgunn

Posts: 3916
Joined: 25 May 2005

You know, when I think about this... I have run miles of single insulated, (not even double insulated), , including SP & TP&N , 10mm / 16mm etc, in metal trunking all around factories for sub supplies... never had any issues.

Good luck...

regards

-------------------------
Tom.... (The TERMINATOR).

handyTRADESMAN

Castle Builders
 23 September 2017 02:03 PM
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weirdbeard

Posts: 3116
Joined: 26 September 2011

Originally posted by: BrucieBonus

Thanks for the replies. I'm in UKPN area and just found this from one of their guides

"BS 7671 Regulation 433.2.2 allows a maximum distance of three metres between overload

protection devices and their source, therefore the customer shall provide and install an

intermediate switch fuse or isolating switch if the distance between the meter and consumer

unit is greater than three metres"

But I think their inclusion of isolating switch is not correct as then it wouldn't comply with the need to have a protective device.....

So switch fuse and SWA or 16mm T and E it is then

thanks again for pointing me in the right direction!




Hi BB, sorry for the late reply, but can you get the DNO to fit an isolator when you request the disconnection? REC type isolators have sealing facilities, and once the DNO have sealed their side the device is not applicable to BS7671. Where the tails are a little bit longer than the 3meter allowance and fully visible in one room I wouldn't feel obliged to fit switch fuses and SWA.

-------------------------
:beer)
 23 September 2017 02:33 PM
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leckie

Posts: 4320
Joined: 21 November 2008

Well its a nice idea WB, I'm not sure the DNO would do that though, it would depend if they bother to measure the tails, or if it was obvious. In my experience if the tails are longer than 3m they wont want to carry out alteration work.

I have recently found out that if you ask the DNO, Western Power where I am, they charge much more than the electricity supplier. About £200 for Western Power but either free or about £40 depending who the supplier is - so worth asking.

I think BB said these tails where in trunking, do presumably surface, so no need to bother with SWA.
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