Meter Tail installation that exceed 3 meters length passing through the building fabric?

[NozSpark]

Erm...yes. It has nothing whatsoever to do with running cables in cavities.---------- Post Auto-Merged at 07:56 ---------- Previous post was made at 07:44 ----------

Noz. I understood that the supply fuse (cut-out) is sized according to the size of the tails to the meter. The cut-out limits the total load on the supply, of course. Hence the diffferent rating of cut-out fuses encountered. The meter tails are protected by the cut-out. They do of course indirectly ensure excessive load is not placed on the street supply. What is the purpose of a rule that imposes a requirement to introduce a switched fuse at 3m? What justification would the DNO have for this?

Regards

No, sorry but I can't word that any differently.

The cutout fuse is sized to protect the supply cable (that is it primary role) - remember that your 16mm supply cable is connected to much bigger cables in the street which are protected by much bigger fuses at the sub station,,,,,, we then size our tails to the cut out fuse.

And as to why the rule,,, it's their kit, they can make whatever stipulations that they like

 

[Sharpend]

If upto the cut out is the responsibility of the DNO and they work to a different set of regs, then why would the Bs7671 be relevant to them?

Surely they can stipulate the 3m rule if that's what they feel is necessary to protect their equipment. It's up to you as the qualified spark to design according to our regs beyond the cutout, with respect for the DNO requirements. I'm pretty sure you get the hump if someone adds to you work without giving consideration to your design criteria?

 

[springcrocus]

No, sorry but I can't word that any differently.The cutout fuse is sized to protect the supply cable (that is it primary role) - remember that your 16mm supply cable is connected to much bigger cables in the street which are protected by much bigger fuses at the sub station,,,,,, we then size our tails to the cut out fuse.

And as to why the rule,,, it's their kit, they can make whatever stipulations that they like

Although I agree with all you say, Noz, I think the cutout fuse is often sized based on the prospective fault current. How many times have you seen 16mm tails on a 100A BS1361? On a TN-C-S this will clear comfortably under fault conditions but it's pretty rare to see a 100A fuse on a TNS because the fault current probably wouldn't clear the fuse in 5 secs due to the higher Ze.

I believe this is the main reason behind their 3-metre rule. Any increase in tail length increases the Ze (e.g. at the main switch on the CU) and extends the clearance time for the fuse so they insist on a switch-fuse to cover their a*ses.

 

[park1]

1, the incoming cable and service head belong to the don.

2, the outgoing tails and meter equipment belong to the supplier.

3, esqcr require that dno and supplier equipment meet BS 7671

4, your installation in general needs to meet BS7671

5, you actually use the dno's fuse to protect the outgoing tails, consumer units use the fuse for protection too, all of which is the consumers equipment.

6, The 3 meter requirement was the dno's way of ensuring protection was maintained without further calculation.

7, I have spoken to the design engineer for my dno. He had no issues utilizing the fuse on extended tails providing it met BS 7671.

 

[batty]

Erm...yes. It has nothing whatsoever to do with running cables in cavities.---------- Post Auto-Merged at 07:56 ---------- Previous post was made at 07:44 ----------

Noz. I understood that the supply fuse (cut-out) is sized according to the size of the tails to the meter. The cut-out limits the total load on the supply, of course. Hence the diffferent rating of cut-out fuses encountered. The meter tails are protected by the cut-out. They do of course indirectly ensure excessive load is not placed on the street supply. What is the purpose of a rule that imposes a requirement to introduce a switched fuse at 3m? What justification would the DNO have for this?

Regards

3 mtr rule is a bit like the speed limits you do 35mph in a 30mph because that's what everybody does not that that is right. If you didn't have a rule people would take the tails from one side of the house to the other with no fuse protection apart from DNO's. On some houses this could be 20 or 30 mtrs.

 

[sellers]

Is this really that difficult?

Under 3M no need to switched fuse

Over 3M you need a switched fuse

Cable run regulations apply the same as it would be wiring a 1mm radial.

In the wall less than 50mm deep, either RCD or mechanical protection against nails etc

Anything else? Also if you want a cheap switch fuse, TLC do them for around

 

[MSH1601]

What's wrong with running them under or in the floor? Then up to the board clipped? Nought wrong with a bit of grinding or chasing to cheer up the builder!!

http://www.cavitytrays.co.uk/associated-items/view/5/111/1/type-i-in-screed-services-duct