adiabatic equation

[Guest55]

Replying to ratttlehead -

But if the earthing conductor is oversized, i.e. larger than necessary (50 instead of 25 and fitted because that is all someone had) then the bonding conductor will not have to be half of this.

i never thought of that lol.
d'oh.

 

[telectrix]

what i've justr said with all the typos.

 

[i=p/u]

Least you got their guys.

 

[rattlehead85]

The regulation states the following.
"Except where PME conditions apply, a main protective bonding conductor shall have a c.s.a. not less than half the c.s.a required for the earthing conductor of the installation and not less than 6mm2"
Where does the word MINUM appear from in that sentence in the regs?
In your situation the earth needs to be 50mm2 therefore the bonding needs to be 25mm2. If you have an earth less than 50mm present and you have checked that it complies by using the adiabatic then your bonding has to be half the size of the earth you have!

so if he changes his main earth from 50mm to 25mm his 16mm bonds will comply lol
;-)

Yep but then his earthing conductor sizing according to the table would be wrong and he would then have to justify the 25mm by using the adiabatic. If he can prove by calculation that the 25 is ok then the 16 bonds would pass as they are half of the installed earth. They wouldn't be if the 50 was there this is the departure we are talking about.

 

[Guest55]

Least you got their guys.

i dont know how we'd have done it without you.
;-)

 

[telectrix]

Yep but then his earthing conductor sizing according to the table would be wrong and he would then have to justify the 25mm by using the adiabatic. If he can prove by calculation that the 25 is ok then the 16 bonds would pass as they are half of the installed earth. They wouldn't be if the 50 was there this is the departure we are talking about.

no. the earthing conductor is oversized, that doesn't make it wrong. it's like using 10mm cable on a cooker when you only need 6mm.

 

[i=p/u]

1700x1700 square foot loop the loop dipita doop = 16mm bonding conductors.

 

[i=p/u]

no. the earthing conductor is oversized, that doesn't make it wrong. it's like using 10mm cable on a cooker when you only need 6mm, /QUOTE]



Hmmmmmmmm

 

[telectrix]

please edit my quote back to what i put without the addition.

 

[rattlehead85]

Yep but then his earthing conductor sizing according to the table would be wrong and he would then have to justify the 25mm by using the adiabatic. If he can prove by calculation that the 25 is ok then the 16 bonds would pass as they are half of the installed earth. They wouldn't be if the 50 was there this is the departure we are talking about.

no. the earthing conductor is oversized, that doesn't make it wrong. it's like using 10mm cable on a cooker when you only need 6mm.

I was under the impression that if a bigger sized earth was used than what was required by the regs it became irrelevant. The bonding was sized in accordance with the earthing conductor stated by the book which for TN-S was half the line conductor size. So what i am trying to say is that if for example a 90mm earth was used the bonding would still only be required to be half
of the stated 50mm as set out by the book. The issue only arises when the bonding is not sized in accordance with what is stated in the book.

 

[rattlehead85]

If ya install more than what the regs require then you have automatically complied. The problem arises when you are dealing with less than what the regs require.

 

[telectrix]

i agree. but the 50% rule for bonding is 50% of the required size of the main earth, not the actual size installed.

 

[Geoffsd]

A good example is a small domestic tns.

When you use the adiabatic it says you need an 11mm earthing conductor.

This will mean the bonding has to be 6mm - at least half.

If you have an 11mm conductor for the earthing you can use it but you won't so you fit 16mm.

Then the bonding is less than half but compliant.

 

[rattlehead85]

i agree. but the 50% rule for bonding is 50% of the required size of the main earth, not the actual size installed.

I understand. So in that case why the hell are they bothering with the table in the regs book to confuse everything. Why dont they just state to use the adiabatic to gain a suitable earth size and size the bonding from that only.
One reg contradicts the other!

 

[Welchyboy]

Mate word of advice and please don't take this the wrong way but you need to be clear in your head about earthing and bonding and how to interpret sizing if you are carrying out EICR's. It is essential you confirm if the bonding is continuos by measurement not just by visually checking a bonding clamp! Bonding irregularities are one of the most important things to highlight during condition reporting!

Mate I can assure you I know about earthing and bonding maybe you need to re assess your own understanding

Telectrix ur bang on thank you