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?