Meter tails longer than 3m and mechanical protection

[Markymark981]

Hi I wonder if anyone to clear something up for me.

i have a customer who wants a consumer unit located in a more convenient position for them and the house users...main fuse and CU would be slightly longer than 3m (say 4m) and they wanted the meter tails run above a plasterboard ceiling (in between first ground and first floors)

i know anout the 3m rule and this hasn't been an issue on the CU I have changed before as they have been located max about a 1m from the meter.

from research 4m should be ok but 'm not happy about running them across the ceiling as it would be hard to mechanically protect them, but i wondered if I placed a 100a rcd main switch between the meter and the consumer unit would that then allow me to run across the ceiling as it would be mechanicaly proceeded?

surface mounting them on the main wall isn't an option as the main fuse is on the outside wall of a would be bathroom once completed so can't have them clipped visible in ththe bathroom. so only other way would be bury into brick wall (which would be the bathroom wall) and use metal conduit or buried in a stud wall but I have issues of mechanicaly protecting both sides if I used the stud wall option.

i am trying to get them to locate the CU on the same wall as the main fuse but the position of the main fuse (on the outside wall and Cu would be inside the property) would still mean part of the tails would need to be buried in a wall with some metal conduit and then it would be plasterboarded and tiled as I would locate the CU outside of the bathroom but on the same main wall that the bathroom and lounge share.

apologies for waffling

cheers

 

[Darkwood]

The 3m for tails is not relative to the BS7671 but it is generally accepted as such... the DNO has the final word on the tail length and they can even specify shorter than 3m.The tails should be as short as possible, easy for visual inspection and if passing external to internal have suitable mechanical protection throught the cavity.

If your tails are running like you express then front end SW/F and rcd protection required unless the full length is mechanically protected where required ...this is then a sub main and you might as well use SWA to negate the rcd need thus nuisance tripping of full power not such an issue.

I would deffo not be running standard tails through floorboards, joists etc.

 

[Leesparkykent]

Would a switch fuse at the meter position and SWA run to the new CCU be an option?

 

[telectrix]

Would a switch fuse at the meter position and SWA run to the new CCU be an option?

i agree.

 

[Darkwood]

If you can?... and dont want the SWA exposed then send under floor and run up in corner of the room ... even though you have mech' protection it still won't stop it been drilled to hang a picture etc so sending up in a corner as per safe zoning runs you reduce the risk of the buried swa been nailed or drilled.

 

[amlu]

25mm metal conduit buried in the wall, stick some armoured in there to make it safe on the ceiling run... easier to safely terminate with proper glands
Use a switch fuse at the beginning and rcd fuse board at the end of circuit.

 

[Deleted member 26818]

The 3m for tails is not relative to the BS7671 but it is generally accepted as such...

Should read:
The 3m for tails is as per the requirements of BS7671, but is not generally accepted as such...

 

[davesparks]

Should read:
The 3m for tails is as per the requirements of BS7671, but is not generally accepted as such...

With a regulation number to back it up of course!

 

[Deleted member 26818]

I'm in an Ice Cream parlour and don't have a copy of the Regs with me.
However, I belive it is to be found in Section 4, and deals with the positioning of protection devices, where there is a reduction in the CCC of a conductor.
It allows for the protective device to be within 3m of where the reduction occurs.
In the case of the tails, as BS7671 only relates to the installation (not the network), the point of reduction is taken as being the origin of the installation.
In other words, where the tails connect to the meter.

 

[davesparks]

That regulation is specifically only applicable to overcurrent protection. The next regulation down the page allows overcurrent protection to be omitted when the DNO allows their fuse to be used, so the length of the tails is set by the DNO not 7671.
For example UKPN specify a maximum length of tails of 4metres between a domestic meter and the consumers main switch. Other DNOs have their own rules

 

[Deleted member 26818]

It is extremely rare that DNO's allow their fuse to be used to protect a customer's installation.
I really don't understand where people get the idea that DNO's are willing to provide protection for installations over which they have no control?
This link provides the No. Of the relevant BS7671 Regulation: http://www.meteroperators.org.uk/stakeholder-info

Please note.
UKPN and other DNO's do not not work to the requirements of BS7671.

 

[davesparks]

Please note.
UKPN and other DNO's do not not work to the requirements of BS7671.

They do when their equipment is on the consumers property, this is detailed in the ukpn design specifications.

 

[Deleted member 26818]

Then perhaps you should point out to them, that providing a TT supply without providing RCD protection for the supply cable, does not satisfy the requirements of BS7671.
You can also point out, that denying consumers access to protective devices does not satisfy the requirements of BS7671.
Also providing 3 single phase supplies from a 3 phase supply without using a DB to distribute the supply circuits does not satisfy the requirements of BS7671.

I imagine, that I can probably think of a few more non-compliances whilst I drink my coffee.

Yep.
The use of a combined Earth and Neutral is prohibited by BS7671.

 

[Darkwood]

I do a lot of industrial liason with DNO and they can over-rule any BS7671 regulations ... they can demand tail length although this is rare as 3m is good in their books and more if mechanically protected -but more so they can increase the desired earthing CCC on a system due to network influences...hence my comment, the 3m rule is on the border when applied to tails as its a crossover of 2 regulation systems and at the end of the day if you want your power you have to go with the requirements of the DNO.

 

[Deleted member 26818]

The DNO's are legally obliged to provide a connection for any installation which complies with BS7671.
Whether it's worth the time and effort forcing a DNO to honour it's legal obligations, is another matter.