Extending Meter Tails

[Heisenberg]

I understand that when extending meter tails beyond a certain length, usually 3 mtrs, DNOs normally want you to add additional overload protection, typically in the form of a switchfuse.

I guess I'm struggling to understand that why, in a properly designed and specified installation, this is necessary.

What protection does a switchfuse add to a 5mtr cable that is different from that provided to a 3mtr cable as provided by the main cutout. I've also never seen anything about where abouts the switchfuse should be placed? Can it be placed as soon as is practicable or can it be placed at the end of the 3mtrs allowed by the DNO?

TIA

 

[telectrix]

you fit the switch fuse to protect the tails. as they are over 3m in length, they are more likely to get damaged than if they were just inside a meter box. the fuse fitted should have discrimination from the DNO fuse, so that your fuse blows first in the event of a fault on the long tails i.e. if the DNO fuse is 100A, you should fit a switch fuse with a 80A fuse.

 

[Des 56]

you fit the switch fuse to protect the tails. as they are over 3m in length, they are more likely to get damaged than if they were just inside a meter box. the fuse fitted should have discrimination from the DNO fuse, so that your fuse blows first in the event of a fault on the long tails i.e. if the DNO fuse is 100A, you should fit a switch fuse with a 80A fuse.

I don't tend to agree with that statement tel

The supplier assess the first 3 meters,so their fuse is assured to operate,then rely on the rest of the length being the installers decision

If they put a limit of 60 amp and you have 25 mm tails you can protect your tails with a 80 amp if you want
There is no discrimination with the supplier,but why should there be ?,its all been assessed by each party

It would hopefully be better if ever the fuse popped for the customers to go first,but that is conveniance only

 

[Guest55]

agree with Des , there is no requirement to have discrimination with the dno fuse , although its preferable on bigger commercial installs.
the requirement to have tails over 3 mtr fused down is due to the dno only wanting to take responsibility of the incoming supply & cabling within the immeadiate vicinity of their cut-out and not a 15 mtr run snaking through the house

 

[Heisenberg]

the requirement to have tails over 3 mtr fused down is due to the dno only wanting to take responsibility of the incoming supply & cabling within the immeadiate vicinity of their cut-out and not a 15 mtr run snaking through the house

This is more like the sort of response I was expecting because, aside from the convenience of having a fuse that could be replaced without DNO intervention, I don't see how the fuse in the switchfuse provides any more protection than the one in the main cut out.

I understand the comment about the longer they are the more easily damaged they can be, but that can apply to pretty much anything.

The difference of opinion regarding discrimination is interesting, because without that, the convenience of having the switchfuse blow first would probably be negated. I also assume that if the tails are not going to be surface mounted, then they would need extra protection, RCD, Mechanical etc.?

Cheers

 

[telectrix]

des. i stand corrected over the discrimination bit, but would prefer a lower rated fuse, just so that , hoefully, the customers fuse would blow first, thus saving the need to call out DNO>

 

[Des 56]

des. i stand corrected over the discrimination bit, but would prefer a lower rated fuse, just so that , hoefully, the customers fuse would blow first, thus saving the need to call out DNO>

I don't know about stand corrected tel,because in the end discrimination is a feature that we build into a system
Deciding to do so,,without losing capacity that may be needed, is good planning and good practice

It was just an emphasis on the word used "should" rather than "could"

 

[Deleted member 26818]

It is a common misconception that the DNO's fuse provides protection to an installation.
The DNO's fuse provides protection to their equipment, nothing else.
BS7671 requires that overcurrent protection be provided at a point in reduction in a conductor's CCC.
It is allowed for the protection to be placed along the run of the conductor no more than 3m from where the point in reduction in CCC occurs.
As such if your tails are 3m or less, the overcurrent protection for the tails is being provided by the protective devices in your CU, not as is often misconcieved by the DNO's fuse.
If you decide to install tails that are longer than 3m, you would be using the DNO's fuse to provide protection for those tails.

What right do you have to use the DNO's equipment to provide protection for your tails?
Have you asked permission?

 

[Heisenberg]

It is a common misconception that the DNO's fuse provides protection to an installation.
The DNO's fuse provides protection to their equipment, nothing else.
BS7671 requires that overcurrent protection be provided at a point in reduction in a conductor's CCC.
It is allowed for the protection to be placed along the run of the conductor no more than 3m from where the point in reduction in CCC occurs.
As such if your tails are 3m or less, the overcurrent protection for the tails is being provided by the protective devices in your CU, not as is often misconcieved by the DNO's fuse.
If you decide to install tails that are longer than 3m, you would be using the DNO's fuse to provide protection for those tails.

What right do you have to use the DNO's equipment to provide protection for your tails?
Have you asked permission?

Spin, I'm not actually doing anything, just trying to get a better understanding.

I don't understand, given what you have said, how 3mtr tails are protected by CPDs up stream (CU), but swap those for 4mtr ones and the DNO fuse is providing the protection?
I understand however, the 3mtr rule when changing CCC but what about occasions where the CCC remains the same but the length of the tails are merely extended? I had always assumed that extending tails implied that they would stay the same but simply be longer.

I'm trying to understand the technical reasons behind the requirement, but aside from Tel's comment about added length, added danger of damage and Biff's comment about the DNO not wanting responsibility, I'm still none the wiser.

Cheers

 

[Guest55]

............I'm still none the wiser.

really ?
i thought me , spin and des wrapped up the issue quite clearly lol.

 

[Deleted member 26818]

Spin, I'm not actually doing anything, just trying to get a better understanding.

I don't understand, given what you have said, how 3mtr tails are protected by CPDs up stream (CU), but swap those for 4mtr ones and the DNO fuse is providing the protection?
I understand however, the 3mtr rule when changing CCC but what about occasions where the CCC remains the same but the length of the tails are merely extended? I had always assumed that extending tails implied that they would stay the same but simply be longer.

I'm trying to understand the technical reasons behind the requirement, but aside from Tel's comment about added length, added danger of damage and Biff's comment about the DNO not wanting responsibility, I'm still none the wiser.

Cheers

Basically, you have to ignore the DNO's fuse and in some cases their equipment. Pretend it's not there.
Your installation starts at the consumer terminals. By rights the CPD to protect your tails should be placed there.
Obviously that is not practical.
BS7671 allows for the CPD to be placed along the run of the conductor within 3m of where a reduction in the CCC occurs.
The reduction in CCC being taken as the consumer terminal in the meter.
Some DNO's only used to allow 2m tail lengths, because they would take the reduction in CCC as being at the fuse rather than the meter.
This would allow them up to 1m for their tails from the cut out to the meter, and leave you with 2m from the meter to your CU.
If the length of your tails is greater than 3m and you do not install a switch fuse, the CPDs in the CU would not provide protection for the tails.
You would be relying on the DNO's fuse to provide protection.
The DNO's may or may not allow this.
In any event, as far as the DNO's are concerned, if want to rely on their fuse to provide protection to your equipment, they expect you to ask permission first.

 

[HandySparks]

As such if your tails are 3m or less, the overcurrent protection for the tails is being provided by the protective devices in your CU, not as is often misconcieved by the DNO's fuse.

I don't see how this can possibly be the case. Yes, I understand about diversity, but ultimately, if the tails are overloaded, it will be the supplier's fuse that blows, not one of the individual final circuit over current protective devices. If what you say was correct, we would not be allowed to install a CU where the sum of the ratings of the OCPDs was more than the rating of the suppliers fuse. In my house, the sum of the circuit breaker ratings amounts to more than double that of the supplier's fuse.

 

[Deleted member 26818]

If you exceed the load provided by the supplier, you are effectivly misusing the supply.
If your misuse causes their protective device to blow and they have to replace it, they can charge you, or even refuse to replace the fuse. In extreme cases they can refuse to provide you with a supply.
However, if they have allowed their fuse to provide protection for your equipment, it's no longer so clear cut.
Why should you be charged for the replacement of the fuse when all that's happened is that it's done it's job.
Why should you have to wait for it to be replaced?
If you don't want to wait, why can't you replace it yourself?

 

[Heisenberg]

really ?
i thought me , spin and des wrapped up the issue quite clearly lol.

Well Spin and Des did... :lips:

But seriously, none the wiser was probably the wrong phrase. What was eluding me was not thinking of the consumer side of the meter as being a point where the CCC changed.

I'm not trying to be obtuse, or wind anyone up but, ....why is the maincut fuse not considered as being overcurrent protection for the change in CCC from the supply cable to whatever comes out of the service head on the load size?

Maybe its just me being trying to be too literal with the regs, taking Spins advice and ignoring the DNO kit certainly seems like the best idea.

Cheers

 

[HandySparks]

If you exceed the load provided by the supplier, you are effectivly misusing the supply.
If your misuse causes their protective device to blow and they have to replace it, they can charge you, or even refuse to replace the fuse. In extreme cases they can refuse to provide you with a supply.
However, if they have allowed their fuse to provide protection for your equipment, it's no longer so clear cut.
Why should you be charged for the replacement of the fuse when all that's happened is that it's done it's job.
Why should you have to wait for it to be replaced?
If you don't want to wait, why can't you replace it yourself?

Are you going to provide some technical justification for your assertion that the DNOs fuse is not the overcurrent protective device for the tails between meter and CU (in a "normal" domestic situation) or do you like lobbing this stuff in just to cause controversy?

 

[Engineer54]

What Spin has stated, is to a greater extent correct, the DNO fuse is primarily there to protect the DNO equipment and installation. Can it also protect the householders installation, Yes, but is a coincidental bonus that is typically made use of in domestic installations. It has been argued for years now, that all installations (including domestic) should be provided with it's own main protective device. Possibilities are suitable DP MCB/MCCB within the CU, or a DP Switch fuse. The problem comes when you try to get a degree of discrimination between the DNO fuse(s) and the downstream protective device...

 

[Deleted member 26818]

Are you going to provide some technical justification for your assertion that the DNOs fuse is not the overcurrent protective device for the tails between meter and CU (in a "normal" domestic situation) or do you like lobbing this stuff in just to cause controversy?

If you disagree with what I've said, why don't you offer up your opinion?

 

[HandySparks]

If you disagree with what I've said, why don't you offer up your opinion?

Have you not read post 12?
If the sum of the ratings of the fuses or circuit breakers in a consumer unit significantly exceeds the current carrying capacity of the tails (which is not an uncommon situation), they cannot possibly provide adequate overload protection for the tails. Therefore the device providing the overload protection for the tails is the supplier's fuse, whether you recognise the fact or not.

 

[Deleted member 26818]

Please get a grip.
Do you look at car speedos and think that the high number is the top speed of the car?
The combined rating of the CPDs in most CUs, exceeds the rating of the main switch.
So what?

 

[HandySparks]

The combined rating of the CPDs in most CUs, exceeds the rating of the main switch.
So what?

So what?
Well, what's actually providing the overload protection to the tails is therefore the suppliers fuse, whether permission has been granted or not.

If not, what is?

 

[Deleted member 26818]

The CPDs in the CU.
Are you seriously stating that you work out your maximum demand by adding all the ratings of the CPDs?
For instance a typical 3 bed semi, could have upstairs RFC (32A), downstairs RFC (32A), kitchen RFC (32A), separate radial for fridge freezer (20A), radial for immersion heater or boiler (16A), upstairs lighting (6A) downstairs lighting (6A), cooker (45A) and shower (50A).
Total = 239A.
Typical maximum demand about 40A.
Hang on a minute, I forgot about the storage heaters.
Two in the front room at (16A), one in the kitchen/diner (20A) three in the bed rooms (16A), that's another 100A.
So a total of 339A, but still with a typical maximum demand of only 40A.

 

[HandySparks]

The CPDs in the CU.
Are you seriously stating that you work out your maximum demand by adding all the ratings of the CPDs?
For instance a typical 3 bed semi, could have upstairs RFC (32A), downstairs RFC (32A), kitchen RFC (32A), separate radial for fridge freezer (20A), radial for immersion heater or boiler (16A), upstairs lighting (6A) downstairs lighting (6A), cooker (45A) and shower (50A).
Total = 239A.
Typical maximum demand about 40A.
Hang on a minute, I forgot about the storage heaters.
Two in the front room at (16A), one in the kitchen/diner (20A) three in the bed rooms (16A), that's another 100A.
So a total of 339A, but still with a typical maximum demand of only 40A.

No, I wouldn't estimate maximum demand that way.

But, if it's theoretically possible to load up the installation to 339A without any of the CPDs opening, what's providing overload protection to the tails? It's certainly not the CPDs, so what's the only other device in the circuit? The suppliers fuse.

Which will have popped well before 339A is reached.

 

[Robert Turnbull]

Do you not fit a main switch to your boards? That would go before 339A aswell.

 

[HandySparks]

Do you not fit a main switch to your boards? That would go before 339A aswell.

On a typical CU, the main switch is not an overcurrent device, it's a switch! :banghead:

The clue's in the name.

 

[Engineer54]

Do you not fit a main switch to your boards? That would go before 339A aswell.

Main switches in CU's and even many DB's are normally only isolators with no incorporated protective elements. So it might well smoke a bit or even melt, but it won't operate the switch!! lol!!

 

[sjm]

I recently contacted the NIC helpline about this particular issue. My question was if I extend the meter tails over 3 mtrs do I need to provide additional fusing? The answer was No you can rely on the DNO fuse for protection, however you will need to provide a means of isolation.
Not convinced I then contacted Western Power engineering and asked the same question and was given pretty much the same answer. Isolation YES, additional fusing NO, why would you need to?
If you check through the ESC website you'll find a very similar answer to the same question.

So make of this what you will.
Tin hat on, duck and cover.

 

[Engineer54]

I recently contacted the NIC helpline about this particular issue. My question was if I extend the meter tails over 3 mtrs do I need to provide additional fusing? The answer was No you can rely on the DNO fuse for protection, however you will need to provide a means of isolation.
Not convinced I then contacted Western Power engineering and asked the same question and was given pretty much the same answer. Isolation YES, additional fusing NO, why would you need to?
If you check through the ESC website you'll find a very similar answer to the same question.

So make of this what you will.
Tin hat on, duck and cover.

Working on that basis, ....Why would you need an isolator then?? lol!!

I don't believe for a moment that a DNO Engineering dept would come out with that load of old tosh, so i haven't a clue who you were talking too!! I wouldn't take much credence on anything the NICEIC advises, and this only reinforces that opinion!! lol!!

 

[sjm]

Working on that basis, ....Why would you need an isolator then?? lol!!

I don't believe for a moment that a DNO Engineering dept would come out with that load of old tosh, so i haven't a clue who you were talking too!! I wouldn't take much credence on anything the NICEIC advises, and this only reinforces that opinion!! lol!!

So who do you suggest I was talking too then? When you phone the engineering dept and ask to speak to an engineer I would expect to be speaking to an engineer, I'm not sure which part of this you don't understand!!
Or are you suggesting that I don't know one part of my anatomy from another?
If the ESC website gives the same advice as the NIC helpline then perhaps, with the greatest of respect, it might be you swimming against the tide.

 

[Engineer54]

So who do you suggest I was talking too then? When you phone the engineering dept and ask to speak to an engineer I would expect to be speaking to an engineer, I'm not sure which part of this you don't understand!!
Or are you suggesting that I don't know one part of my anatomy from another?
If the ESC website gives the same advice as the NIC helpline then perhaps, with the greatest of respect, it might be you swimming against the tide.

No idea, ...you tell me?? lol!!

Then i'll swim against the tide, rather than lower my own time proven standards!!

 

[leexgx]

I hate the DNO fuse to blow, most places I been to have an DP MCB between the meter and the CU this is all under 1 meter (easier the isolate the CU with out having to pull the DNO fuse)
as you may be extending the tails anyway best to just install an DP MCB and run the extended tails off that so any fault that happens from the CU to the meter/DNO fuse is protected by the DP MCB.

I really would not think about Not installing an DP MCB before extending the tails (its as easy to install a DP MCB then to just use an Block to join the tails to extend them with the added risk of blowing the DNO Fuse)

I seen some CU boxs that have an 100A DP MCB or MCCB in the CU (it is real one not an switch just not sure about Type,long time ago) but it only has 40A DNO Fuse (incoming from Pole so Low amp) you can have an guess what did blow first (shower + kettle + toaster, maybe something els as well), they upped it to an 60A now and i think its all been put underground now

If your DNO has fitted at 40A/60A/100A fuse then your DP MCB should be rated at it or you be with out power until the DNO comes around to fit an new fuse and part with your money or customers money

 

[HandySparks]

I hate the DNO fuse to blow, most places I been to have an DP MCB between the meter and the CU this is all under 1 meter (easier the isolate the CU with out having to pull the DNO fuse)
as you may be extending the tails anyway best to just install an DP MCB and run the extended tails off that so any fault that happens from the CU to the meter/DNO fuse is protected by the DP MCB.

I really would not think about Not installing an DP MCB before extending the tails (its as easy to install a DP MCB then to just use an Block to join the tails to extend them with the added risk of blowing the DNO Fuse)

If your DNO has fitted at 40A/60A/100A fuse then your DP MCB should be rated at it or you be with out power until the DNO comes around to fit an new fuse and part with your money or customers money

Hi. I've not seen an MCB used as a stand-alone device installed in the tails before. Can you provide a link or photo of one of these DP MCBs that you are advocating? (Preferably installed as you describe.)

And what rating would you suggest using with a 100A DNO fuse?

By the way, are you an electrician and are you in the UK?

 

[leexgx]

the currant house here has that type installed here (DNO Fuse 100A > meter > DP MCB 100A > CU) there is to much junk in the way (wife) to get an picture of it

its quite standard in Warrington UK to see this type of set-up when an CU is replaced

most seem to install an 100A DP MCB even if the DNO fuse is lower then it not sure why

 

[HandySparks]

Why do you think it's an MCB, not an isolator?

A photo would be good, even if cluttered.

 

[sjm]

No idea, ...you tell me?? lol!!

Just putting "lol" doesn't stop your implication from being insulting.
I was merely passing on information gained from 3 sources.

The information supplied is from the Electrical Safety Council Website.
Q1.50
Is it permissible to install a consumer unit more than 3 m from the distributor’s service head?
Yes, in certain circumstances. Distributors generally accept a maximum of 3 m from the service head. Where the tails need to be longer, the distributor may require a means of isolation to be provided within 3 m of the service head. Any particular requirements should be ascertained in advance from the distributor.

 

[leexgx]

the CU has an 100A isolator but very sure its an MCB between the CU and the meter really there is a lot of junk where it is i be back in 2-3 hours got some work to do see if i can move it out

just from looking at it from the top it looks like an DP MCB, the DP MCB looks like size wise as what's in the Wynex CU MCBs, as the isolator in the CU is just an big red switch isolator, i look bit closer when i get back (you probably right its most likely an isolator thought)

 

[Engineer54]

No idea, ...you tell me?? lol!!

Just putting "lol" doesn't stop your implication from being insulting.
I was merely passing on information gained from 3 sources.

The information supplied is from the Electrical Safety Council Website.
Q1.50
Is it permissible to install a consumer unit more than 3 m from the distributor’s service head?
Yes, in certain circumstances. Distributors generally accept a maximum of 3 m from the service head. Where the tails need to be longer, the distributor may require a means of isolation to be provided within 3 m of the service head. Any particular requirements should be ascertained in advance from the distributor.

Listen, if i wanted to insult you, there would be no implication about it, you would have had no doubt you were being insulted!!

As for the other junk, i'll stick to how i have always done things in this situation, and how it was when i was trained and well beyond!! Likewise how every other experienced electrician here, would advise installing a fuse switch on extended tails!!