One rule for DNO and another for us.

[rocker]

Did a job yesterday where the DNO changed an overhead TT for a PME.

As it was a long tails run, turned up with me switchfuse with a standard 80a fuse in it. Got told I'll have to put a 60 in because there's only an 80 in the main service fuse (even though it states 100 on the casing). Apparently the DNO will 'never go full whack' on the fuses. Told the guy it was going to be an 80 due to the size of the house and scope for development, and if his fuse goes first then tough. Then questioned the size of the incoming cable, which he said was 35mm concentric. not a chance. I'd say it was 16mm, possibly 25 if it had very thin sheathing. I then pondered using 16mm tails into my switchfuse (16mm armoured running to the cu) as mr dno had used 16mm tails from the meter to his cutout. Was told I'd have to use 25s.

So basically the guy who owns the house has been charged serious money to have this done, he's had to dig his own trenches, hire his own sparks to actually get the power to his house, and has been supplied with a substandard incoming service, a fuse which is more or less on the limit of what he's likely to consume, and tails that aren't to regulation.

But if we cut one of their seals we'll get told off. they're a law unto themselves and, not happy with screwing us with energy prices, they are ripping us off with their installations as well.

 

[Engineer54]

What are you saying here, that your customer has had an underground supply put in, from a PME'd overhead supply source?? Or are you talking about the connection at the house overhead position, coming down as a concentric cable into a trench and away to the service head position??


Once you get past the DNO's meter, they have no say in what size tails you install, you are the responsible person that decides the design on the consumers side of the installation. As long as your installation meets or exceeds BS7671 that's all you need to worry about....


I wouldn't be too bothered about the 80A service fuse either, i doubt if this house will ever see anywhere near a fraction of that load. All this crap about 100A service fuses and 25mm tails, 90%+ of domestic installations just don't warrant either. 60/80A fuses are more than enough for single dwelling units!!

 

[chrish_1977]

I ha the same with the dno where I am, they said that the tails had to be 25 or they wouldn't even connect the supply up, not a problem, but could see how this would be a pig for some Installations, I think they were just being stupid though

 

[driverman]

80 amp fuses have been fitted for years now by DNO's . I believe it was about the time voltage was reduced to 230v and it's all about Ze requirements at the cut-out (header) position. re above DNO cable size, it's usually 35mm Hybrid cable (solid core) installed as standard. As it's PME 25mm tails 16mm earthing conductor to carry worst possible fault current. Suppliers meters are only rated @ 80 amp anyhow.

 

[Engineer54]

Concentric single phase cables are quite deceptive when trying to determine conductor size. There is only a 3mm difference between 16mm and 25mm cables, and a 1mm difference in overall dia size between a 25mm and 35mm cable. and i'd agree that 35mm seems to be the DNO's standard size concentric cable.

Can't see a reason for 25mm tails though, just because it's a PME supply. Think you'll find that most DNO meters now days, are rated up to 100A....


I've never seen a solid core phase conductor in a concentric cable, i'm not saying they aren't available, but i've never seen such a cable.

 

[Knobhead]

I’ve got a section of DNO 35mm² solid Au tucked away. The neutral is copper though.

It’s tiny to look at. Will measure the OD later.

 

[Rockingit]

I've never seen a solid core phase conductor in a concentric cable, i'm not saying they aren't available, but i've never seen such a cable.

I've an off-cut kicking around on my bench somewhere - 16mm solid alu core as the line conductor and then copper spiral (ala armoured style) as the neutral. It's what WPD install as standard around here on an 80A fuse.

 

[GLENNSPARK]

80 amp fuses have been fitted for years now by DNO's . I believe it was about the time voltage was reduced to 230v and it's all about Ze requirements at the cut-out (header) position. re above DNO cable size, it's usually 35mm Hybrid cable (solid core) installed as standard. As it's PME 25mm tails 16mm earthing conductor to carry worst possible fault current. Suppliers meters are only rated @ 80 amp anyhow.

and after just reading this twaddle i believe you havn`t picked up a loop tester in some time either....