CLOSED - Moving main fuse and meter board

[Entropy]

Hi all, I need a bit of a sense check as I am unsure if there was a miscommunication between myself and the guy from UK power networks. I was looking to move the house fuse and meter board from one wall to the adjoining wall. The main power supply comes up from underground at the front corner of my house runs along the side wall at about 3m up, just nips round the corner to the back of the house and comes in to the house right in the corner of my kitchen. So I wanted to move the meter and board 90 degrees to the other wall that the door hinge faces and the recess is located, see the attached pics.

UK Power networks said that they need to remove the main supply back to an underground location, add a new length of cable then install the main fuse into an external meter box. The approximate price given was £6K if they did the digging and £2.5K if I arranged the digging.

I am happy if these are the regulations but none of the electricians that quoted for connecting up the tails to the Consumer Unit mentioned anything about current regs and requiring an external meter box, they all seemed quite happy that the main fuse could just be relocated so that is why I am asking if there has been a misunderstanding in my chat with UKPN.

Thanks

 

[jj4091]

It doesn't really matter as they will still put a new cable in. Unless I am misunderstanding what you are asking.

 

[AndyPRK]

I think its now normal to have the fuse and meter in an outside meter box. (get on with it before they approve the idea of having a box sticking out your garden!)

Your electrican will have to run a cable between that box and the CU.

I guess you will have to consider where you want the meter box.

 

[Murdochcat]

You also need to involve the meter supplier too and get them to install an insulator switch too.

Then the spark will need to run new tails from the insulator to the consumer unit

 

[securespark]

Isolator.

And, if your CU is more than 3m as the tails fly from the isolator, then you will need a fuse switch to fuse down from 100A.

Best to place the external meter cabinet so that your 3m of tails is plenty to go direct into the Consumer Unit.

 

[JohnW2]

And, if your CU is more than 3m as the tails fly from the isolator, then you will need a fuse switch ...

It does seems that, for whatever reason, many/most DNOs do require a switch-fuse for tails >3m (even though tails downstream of the meter and/or 'isolator' are theoretically 'beyond their 'jurisdiction', aren't they?). However ....

to fuse down from 100A.

... even if/when a switch-fuse is required, it would not have to be "fused down from 100A" if the downstream tails were 25 mm², would it? (i.e. a 100A fuse in the switch-fuse surely ought then to be fine?)

 

[securespark]

Consumer unit feed / main tails more than 3m - Wiring and the Regulations BS 7671 - IET EngX - IET EngX https://share.google/o2IsvbBa5FFar2iOF

I think it appears not to be an electrical rule but more of an administrative one, which was my understanding.

As for fusing down, conditions in 536 should be met (selectivity, used to be discrimination).

Yes, electrically, 25mm tails are OK on a 100A fuse.

But, in the scenario where there is a 100A fuse switch 3m after the meter, there would be no discrimination between the 100A DNO fuse and the customer's 100A fuse.

Yet the whole point of discrimination is to make sure the device closest to the fault opens.

It (the 3m rule and fitting of fuse switches) can be very awkward, especially when you have to run the longer tails in SWA to protect against damage or penetration.

Then there's awkward cases where the DNO fuse is significantly less than 100A, the DNO will not upgrade it and the tails need to be significantly longer than 3m.

This was one of my jobs, the tails needed to be 9m the DNO fuse was 40A, and they would not upgrade the elderly TT supply or the fuse.

After lengthy discussions between us (me and the customer, the customer and the DNO and me and the DNO), I turned down the job.

Personally, I think if having tails longer than 3m can be avoided, do so!

 

[EFLImpudence]

The extra fuse is not an electrical requirement so presumably electrical conditions and rules need not apply.

 

[JohnW2]

Consumer unit feed / main tails more than 3m - Wiring and the Regulations BS 7671 - IET EngX - IET EngX https://share.google/o2IsvbBa5FFar2iOF

As that discussion underlines, we're talking about 'DNO requirements/rules', not a rational electrical requirement, nor even a requirement of BS 7671.

I think it appears not to be an electrical rule but more of an administrative one, which was my understanding. ... As for fusing down, conditions in 536 should be met (selectivity, used to be discrimination). .... Yes, electrically, 25mm tails are OK on a 100A fuse. ... Yet the whole point of discrimination is to make sure the device closest to the fault opens.

You need to tell the DNos that ....

Then there's awkward cases where the DNO fuse is significantly less than 100A, the DNO will not upgrade it and the tails need to be significantly longer than 3m.

Again, I think you would have to tell the DNOs that! My cutout has 3 x 60A fuses and, downstream of the meter, I have 3 x 80A switch-fuses (to protect lengthy distribution circuits). If there is any discrimination, it's 'the wrong way around'

I think that it is (electrically) a pretty stupid rule, given that, in general, it's ridiculous to suggest that a fuse can only protect the first 3m of a cable. If that were the case, there would have to be several fuses along the length of some of my final circuits!

 

[securespark]

As suggested in the chat, the DNOs want to demarcate between what they want to protect with their fuse and anything longer, require the customer to install their own fuse. The discrimination is to ensure that if the main fuse were to fail, it would be the customer's fuse, not the DNOs.

 

[securespark]

As that discussion underlines, we're talking about 'DNO requirements/rules', not a rational electrical requirement, nor even a requirement of BS 7671.

7671 does not cover DNO stuff.

My cutout has 3 x 60A fuses and, downstream of the meter, I have 3 x 80A switch-fuses (to protect lengthy distribution circuits). If there is any discrimination, it's 'the wrong way around'

Indeed. That doesn't seem right.

I think that it is (electrically) a pretty stupid rule, given that, in general, it's ridiculous to suggest that a fuse can only protect the first 3m of a cable. If that were the case, there would have to be several fuses along the length of some of my final circuits!

See my point above.

 

[EFLImpudence]

What will discriminate with a 60A DNO fuse?

 

[SUNRAY]

What will discriminate with a 60A DNO fuse?

63A MCB

 

[JohnW2]

As suggested in the chat, the DNOs want to demarcate between what they want to protect with their fuse and anything longer, require the customer to install their own fuse. The discrimination is to ensure that if the main fuse were to fail, it would be the customer's fuse, not the DNOs.

That presumably must be at least part of their thinking (all it doesn't really explain the "3 metres") - but in cases such as you described, in which the DNO's fuse is low in rating, if the customer installed a downstream fuse fairly likely to discriminate from it, they would be left with an insufficient supply - so the DNO then 'upgrade' the supply, shouldn't they?

 

[JohnW2]

7671 does not cover DNO stuff.

Exactly - that was (part of) my point. Nor do electrical considerations really support the idea that whether or not a cable needs additional over-current protection depend upon how long the cable is!

Indeed. That doesn't seem right.

Quite so - but I can't say I've ever heard of a DNO specifying the rating of the consumer's fuse that they say is 'required'. In any event, even if they did 'specify', and one did comply with that request, one could change (for a higher rating) one's own fuse the moment they 'walked out of the door'

See my point above.

If you're suggesting that the only reason (in their minds) for their 'rule' is that they want to reduce the risk of their fuse operating as a result of a fault (or faults) in the tails then I would, in turn, suggest that's probably an incredibly rare occurrence and only very slightly more likely with long tails than short ones.