extra fuse board CU

[Chris Dolby]

hi to all. a friend asked me a question the other day and i was unsure so thought i would see what you think. his customer wants an extra fuse board on its own supply (not a sub) with its own set of tails from the supply. do these tails have to be 25mm too as this would be difficult to pull another set through the wall (hard enough with one set sometimes) . the supply is on a 80 amp fuse and the new cu main switch is 32A. so can he get away with smaller tails??

 

[telectrix]

the tails are sized according to the DNO's main cut-out fuse. if it's 100A, you need 25mm tails.

 

[Chris Dolby]

well it says 80 on the cover but ive heard it isnt always what it says. is this true??

 

[telectrix]

if it's 80A you could get away with 16mm tails. but 80A is borderline.

 

[SirKit Breaker]

You dont need to pull another set of tails through the wall. Take the existing tails and put them into a henley block. The put new tails to new CU into henley block as well. Job done. But a word of warning, this is a very dangerous thing to do if you are not too sure about what you are doing.

Cheers.........Howard

 

[Blood]

HI All
So if you can run a 2nd CU then you can us a HENLEY block and run 10mm tails to a shower inclosure 63A RCD & 40A MCB if fitted after the METER?

 

[telectrix]

no, because those 10mm tails are then protected by the suppliers fuse ( 100A?) which would mean that the tails could be seriously overloaded without the fuse blowing.

 

[johnboy6083]

yes you can if im getting what you say.

its called backfusing, and relys on the fact that overcurrent protection can be up or downstream from the load.
SDo, from your meter, yourun 25mm tails to a henley block. From there you can run any size tails you like, as long as they are protected by an appropriately rated device in the CU or DB that the tails are feeding.

 

[gpz900jim]

thats a very interesting idea johnboy, ive never heard of that.

 

[malcolmsanford]

thats a very interesting idea johnboy, ive never heard of that.

Have a read of section 434 and in particular regulation 434.2.1 and it may help

 

[Blood]

So instead of 10mm tails to a shower (2nd CU) these would be better as 25mm meter tails this then puts the shower CU on par with the main CU, as far as these 10mm tails overloading and only being protected by the suppliers fuse but they have a 40A MCB and a 63A RCD in the cct before they get to the shower from the suppliers fuse, I am doing this as the fitted CU is not man enough to allow the current pull by the shower along with the rest of the household ccts

 

[malcolmsanford]

So instead of 10mm tails to a shower (2nd CU) these would be better as 25mm meter tails this then puts the shower CU on par with the main CU, as far as these 10mm tails overloading and only being protected by the suppliers fuse but they have a 40A MCB and a 63A RCD in the cct before they get to the shower from the suppliers fuse, I am doing this as the fitted CU is not man enough to allow the current pull by the shower along with the rest of the household ccts

If your 2nd CU is just going to be a small single circuit enclosure for a shower then there is no need to tail into that with 25mm tails, for a 63amp 30mA RCD with a 40 amp MCB then 10mm tails is more than sufficient.

As I posted read section 434 protection against fault current and in particular 434.2.1 and 434.2.2.

 

[nickblake]

dont forget you are required to provide one point of isolation for the installation if you havent got a nice switch supplied by the DNO what i would do is use a circuit on the existing CDU to power the new board if there is not enough ways then relocate a circuit to the new board then you can isolate the complete installation with one switch and the tails you are using will be protected too

 

[malcolmsanford]

Nick I know this as been batted about before, and the question always comes back how do you tie regulation 537.1.3 off on a large commercial/industrial site. You would not have one large single point of isolation after the incomer that would isolate the entire complex. Each new distribution point would be classed as an installation.

The definition of an installation in the BS 7671-2008 is "An assembly of associated electrical equipment having co-ordinated characteristics to fulfill specific purposes". Nowhere in that definition is the word origin. I think many believe that the installation refers to the entire system, but as the defination says, it is not.

If you include a point of isolation, and if conforms to 537.1.4 in a domestic situation on that small shower board then I think you find that the double pole 63amp 30mA RCD will give you this.

IMO that new shower circuit is an installation.

 

[JulianC]

Every now and than I read somewhere 'official' that "it is acceptable to have more than one installation in a domestic dwelling". Every time I read this, I forget where I've read it. Not much help I know. Anyhoo, the upshot of this as far as I'm concerned is that you could have a dozen CUs in your house and class each one as a separate installation if you wanted to. No need for a single isolator to switch the lot.