Extending Meter Tails

[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