Does BS7671 allow fuse rating to exceed cable rating

[haptism]

Does BS7671 allow fuse rating to exceed cable rating where it is not required to offer overload protection ??

As in the case of a 100A fuse/16mm tails: supplying a CU with just a few final rcbo circuits ?

 

[Leesparkykent]

Does BS7671 allow fuse rating to exceed cable rating where it is not required to offer overload protection ??

As in the case of a 100A fuse/16mm tails: supplying a CU with just a few final rcbo circuits ?

Yes providing a few conditions are met.

 

[haptism]

Yes providing a few conditions are met.

Could you give me any pointers ? Reg numbers ?

 

[Wilko]

The tails shouldn't have to be protected against load by the suppliers fuse

 

[Leesparkykent]

Could you give me any pointers ? Reg numbers ?

Not specifics as the BYB is in the office but probably around the 433 mark in the BYB.

 

[haptism]

Not specifics as the BYB is in the office but probably around the 433 mark in the BYB.

Okay thanks great, !

 

[Wilko]

Good memory LSK !
Looking at BYB I think it's 433.3.1

 

[Deleted member 26818]

The supplier’s fuse is not there to protect any part of a consumer’s installation.
It’s there to protect the supplier’s equipment.

A consumer’s installation begins at the consumer connection point at the meter.
BS7671 applies to the consumer’s installation not the supplier’s network.
As far as BS7671 is concerned, there is no 100A fuse.
That said, if the supplier agrees, their fuse can be used to protect part or all of a consumer’s installation.

 

[HappyHippyDad]

Could you give me any pointers ? Reg numbers ?

The best practise guide (number 6 I think -consumer unit replacement) states in section 6.1 that 16mm meter tails with a 100A cutout could be deemed adequate if the maximum demand of the installation does not exceed the ccc of the tails AND also that the requirements of Regulation 434.5.2 for protection of the tails against fault current are met.

To meet the requirements of the above regulation with 16mm tails on a 100A fuse Ze needs to be 0.48 or less. Therefore 'in theory' a TNCS should always be ok with 16mm tails. A TNS would be ok if Ze less than 0.48. Not sure about a TT but K sqared x S squared is never going to be > I squared x t so I guess its not acceptable for a TT.

 

[HappyHippyDad]

Ps... can someone please tell me how to write a sqaured sign on an android device? Also an ohm! I used to be able to do it but the hash thing doesnt work any more!

 

[Zdb]

Ps... can someone please tell me how to write a sqaured sign on an android device? Also an ohm! I used to be able to do it but the hash thing doesnt work any more!

Symbols Shortcuts 2 with custom Keyboard – Android апликације на Google Play-у - https://play.google.com/store/apps/details?id=symbolsshortcuts2.gr8ly.co.za.symbolshortcuts2

 

[Ian1981]

Does anyone actually contact the supplier to confirm anything with them like ever??
Will their fuse protect our tails, can we extend our tails greater than 3meters, do we have any special considerations to think about when applying protective bonding in a pme installation other than using bs7671 etc?

 

[Wilko]

... if the maximum demand of the installation does not exceed the ccc of the tails AND also that the requirements of Regulation 434.5.2 ... Not sure about a TT but K sqared x S squared is never going to be > I squared x t so I guess its not acceptable for a TT.

Hi - the earth fault current for TT is very low. With typical Ze ranging 10 to 100 Ohms the earth fault current won't bother 16mm tails

 

[Midwest]

Does anyone actually contact the supplier to confirm anything with them like ever??
Will their fuse protect our tails, can we extend our tails greater than 3meters, do we have any special considerations to think about when applying protective bonding in a pme installation other than using bs7671 etc?

I have a few times. Once I rang 'em and was passed on to an engineer. I was asking if their supply was TN-C-S, 'what's that in English, do you mean PME?', and I've pulled your main fuse, sealed it (no Pinnipeds jokes please) with one of your sticky temporary seals; I've got the seal number etc for you, 'errgh what, seal number, ??'.

On the first instance, the engineer came out to inspect the head, said it can be converted to PME, just do it yourself.

 

[HappyHippyDad]

Ω
Testing to see if this works..

K²S²>I²t

S = (√(I²t)) / K