NICEIC Certification Scheme Niceic

[MDJ]

TNC-S needs 10mm and always has done

Aye I have never liked that reg

 

[Buzz-light-bulb]

Electrical safety council.
When carrying out electrical work on an installation forming part of a TN-C-S system, is it necessary to upgrade existing 6 mm² protective equipotential bonding to 10 mm²?
Not necessarily. If the existing 6 mm² bonding connects all the extraneous-conductive-parts to the main earthing terminal, has been in place for a significant time and shows no signs of thermal damage, then it may not require to be upgraded.

Regulation number(s) 132.16 544.1.1

 

[Engineer54]

That's typical BS7671 for you, it's full of contradictions!!

 

[D Skelton]

The very reason it will never become a statutory document until it can be assured that everyone involved in writing it actually knows what they're talking about!

50% of it is good, 40% of it is far too open to interpretation, 10% of it is just fundamentally flawed.

 

[SPARTYKUS]

...and of course it's nothing to do with the fact that physics laws changed and it became safe to use the neutral as the earth.

but just in case make sure theres a whopping great fat thick bond to the pipes that go in the ground. Just in case. Because it is safe really. Honest. Just kidding. Maybe.

 

[NMc]

TN-C-S (PME) supplies come under the EQSCR not BS7671

 

[Engineer54]

...and of course it's nothing to do with the fact that physics laws changed and it became safe to use the neutral as the earth.

but just in case make sure theres a whopping great fat thick bond to the pipes that go in the ground. Just in case. Because it is safe really. Honest. Just kidding. Maybe.

It is, and always has been safe to use the neutral as an earth, when PME conditions by the DNO's have been met. It is, however an artificial earth. It may not be as safe in all circumstances as a TN-S earth, but still more than adequately safe in the majority of situations!!

The main advantage for the DNO is not having to provide expensive metallic sheathing or separate earthing conductors within the make-up of LV Network Distribution and branch off cables.

Oh BTW, the minimum main bonding conductors for TN-S installations should also be 10mm, ...eh, before you get to the contradictions that is!! lol!!

 

[D Skelton]

Oh BTW, the minimum main bonding conductors for TN-S installations should also be 10mm, ...eh, before you get to the contradictions that is!! lol!!

Nope, minimum of 6mm mate

 

[Engineer54]

Nope, minimum of 6mm mate

Only if existing, using the contradiction, not on a new build

 

[D Skelton]

Must be not less than half the required size of the main earthing conductor and not less than 6mm?

What am I missing mate? Do I need to run and get my green book out of the van?

 

[Engineer54]

Must be me then,...lol!! Can't say that i've seen any main bonding cables of less than 10mm on any TN installation for a good many years now....

 

[D Skelton]

I don't either, but I don't mark it down if I do, I just do the calcs and even if it's PME I'd just check for signs of thermal damage, if none, note it and move on.

I'll admit a while back I was installing bonding out in the sticks, TT system, I got my roll of 10mm out the van and was immediately worried that I might not have enough so I did a quick calc (diameter of the coil x 3.14 x number of coils) and it came out at X meters. I then paced the run and remember thinking it'd be fine. I chanced it and failed big time, I was about a meter short! I then got my tape and measured the run to find my pacing was way out!

Solution?

20-30 mile round trip to pick up a roll of 10mm or do I just unwrap the brand new roll of 6mm I had bought the week before? Calc confirmed, second option won!

 

[SPARTYKUS]

It is, and always has been safe to use the neutral as an earth, when PME conditions by the DNO's have been met. It is, however an artificial earth. It may not be as safe in all circumstances as a TN-S earth, but still more than adequately safe in the majority of situations!!

The main advantage for the DNO is not having to provide expensive metallic sheathing or separate earthing conductors within the make-up of LV Network Distribution and branch off cables.

Oh BTW, the minimum main bonding conductors for TN-S installations should also be 10mm, ...eh, before you get to the contradictions that is!! lol!!

hi E54,

yes, agreed, was being a bit facetious, it just amuses me thats all. I suspect the decision was based more on cost savings rather than "which was best" as in ...safest!

which leads me to suspect thats why they specify enormous bonding conductor csas to services for TNCS, so they have an incidental (!) backup earthing system in the event of the loss of Neutral, etc etc

call me a cynic.