Adiabatic Equation

[2glasseyes]

Hi

Is it acceptable to use the adiabatic equation to work out the size Main Earthing conductor to install in a standard domestic property for a consumer unit change.

Ive always been under the impression that its 25mm tails and 16mm main earth for a tns or tncs system, but a colleague thinks its acceptable to install a much smaller main earth as long as its ok by the equation.

Any help very much appreciated.

 

[telectrix]

why bother with equations when it's so simple to bung in a couple of ft. of 16mm.

 

[2glasseyes]

telectrix

I couldn't agree more, but I just wandered.

 

[topquark]

I certainly wouldn't use 25mm as a wander lead.

 

[telectrix]

if you assume a TN-S with a Ze value of 0.8, then your pefc would be 300A assuming 240v. that's the minimum fault current flowing through that earth conductor. could be considerably more if Ze is ( as it usually is ) lower. that's why we all use 16mm.

 

[Sparky Ninja]

Hi

Is it acceptable to use the adiabatic equation to work out the size Main Earthing conductor to install in a standard domestic property for a consumer unit change.

Ive always been under the impression that its 25mm tails and 16mm main earth for a tns or tncs system, but a colleague thinks its acceptable to install a much smaller main earth as long as its ok by the equation.

Any help very much appreciated.

Your colleague is correct. It can be obtained by calculation or measurement.

 

[Sparky Ninja]

why bother with equations when it's so simple to bung in a couple of ft. of 16mm.

Oh I dunno, how about to maintain certain levels of competence.

 

[telectrix]

true, problem is, if say, by using the adiabatic equation, you quite correctly chose to use , say, 10mm, then along comes mr. clipboard ans says " that's no good, it's got to be 16mm." sometimes you just can't win.

 

[Sparky Ninja]

true, problem is, if say, by using the adiabatic equation, you quite correctly chose to use , say, 10mm, then along comes mr. clipboard ans says " that's no good, it's got to be 16mm." sometimes you just can't win.

I can't argue with that. Happened a few times to me before, I took a white board to a building site once, embarrassed the hell out of him.

 

[RWJ]

Don't forget Table 4.1 in the OSG

Page 28

A PME install requires 16mm Earthing conductor even with 16mm Tails

 

[Deleted member 26818]

Don't forget Table 4.1 in the OSG

Page 28

A PME install requires 16mm Earthing conductor even with 16mm Tails

Unfortunately the OSG is incorrect again.
BS7671 requires only 10mm².

 

[telectrix]

probably be 25mm in the amendment, just to confuse us more.

 

[Andrewered]

I couldn't agree more, but I just wandered.

 

[Rob Smith 643]

Whole estates of 100's of houses built in the 90's onwards by reputable national buiilders under a NHBC buildmark have 10mm earthing conductors on 60A 1361 type 2's service heads marked with a pme label. Someone within their design teams must hold a firm belief that this is appropriate. I guess though that this will be discussed for ever more withopinion polarised and that I will be confused for evermore..

 

[steveberry11]

The Adiadatic equation should only be used for CPC calculations only. The BGB gives guidance on main earthing conductors and bonding conductor sizes that should always be followed.

Beware that this equation is only a small part of a cable calculation. If used incorrectly you may find that R1+R2 and Zs readings may be excessive.