Main earth and bonding csa?

[D Skelton]

Hey guys, I'm getting a bit confused with regards to the regs for the sizing of bonding/main earth. Anyone able to shed some light on the regs or point me to the exact one?

My thoughts at the moment are that as a rule, the main earth has to be no smaller than half the size of the neutral and no smaller than 10mm, and the bonding has to be at least half the size or more of the main earth and no smaller than 6mm. For example, 25mm tails means a 16mm ME and 10mm bonding, and 16mm tails means a 10mm ME and 6mm bonding are acceptable, am I wrong? I'm guessing that the adiabtic equasion comes into this too?

 

[ackbarthestar]

Hey guys, I'm getting a bit confused with regards to the regs for the sizing of bonding/main earth. Anyone able to shed some light on the regs or point me to the exact one?

My thoughts at the moment are that as a rule, the main earth has to be no smaller than half the size of the neutral and no smaller than 10mm, and the bonding has to be at least half the size or more of the main earth and no smaller than 6mm. For example, 25mm tails means a 16mm ME and 10mm bonding, and 16mm tails means a 10mm ME and 6mm bonding are acceptable, am I wrong? I'm guessing that the adiabtic equasion comes into this too?

That appears fine for TN-S and TT earthing arrangements but for PME the minimum csa of MPB is 10mm2. (table 54.8) It should also be noted that for final circuits the earthing conductor must be the same size as the phase conductor <=16mm2 (table 54.7)
The requirements of earth fault protection and thermal constraints are somewhat different to providing a Faraday's Cage using MPB. However, by providing MPB you may also be providing a parallel path for fault currents, but this should not be relied upon.

 

[D Skelton]

Thank you, do you have any specific regs you could point me too. It appears this sort of info is scattered all over BS 7671 and I keep being told different things by different people. Your description of regs is one I commonly hear though.

 

[Octopus]

Regarding equipotential bonding I'd recommend reading BS7671 131.8

 

[D Skelton]

Lovely jubbly

 

[HandySparks]

It should also be noted that for final circuits the earthing conductor must be the same size as the phase conductor <=16mm2 (table 54.7)

Surely this is only true if you choose not to calculate the size using the adiabatic equation? (BRB 543.1.1)

 

[ezzzekiel]

Providing main earth meets adiabatic and main bonds have a resistance low enough to limit touch voltage to 50v then size is irrelevant

 

[Deleted member 26818]

There are two methods to determine the CSA of earth and bonding conductors.
One is to use the adiabatic equation (something that should always be done when conducting a PIR), and the other is to use the applicable tables in BS7671.
If using the adiabatic equation, you must first have knowledge of the fuse rating and type, and the PEFC.
Once you have this information, then using the time/current characteristics in BS7671 you apply the adiabatic equation, and the result will be the minimum CSA required for the earth conductor.
The CSA of the bonding conductors is half of that required for the earth conductor, with the minimum allowed being 6mm².
Where PME conditions apply, the minimum CSA for both the earth and bonding conductors is 10mm².
With TT installations, you could very well have an earthing conductor with a smaller CSA than that of the bonding conductors.

 

[ackbarthestar]

Thank you, do you have any specific regs you could point me too. It appears this sort of info is scattered all over BS 7671 and I keep being told different things by different people. Your description of regs is one I commonly hear though.

It may be referred to throughout the regs but it is specifically mentioned in chapter 54 sections 543 and 544. 434 deals with short circuit faults and refers to the adiabatic effect (ie keeping all even whilst hell abounds - albeit for less than 100ms :devilish: ), but not the necessary requirement of protection against faults exterior to the installation.
Oh by the way, sizing of the cpc will be found 543.1.3

 

[Chr!s]

Protective conductors other than bonding, Calculated using adiabatic or using table 54.7. there are other requirements that may need to be met within chapter 54 which may be relevant, e.g table 54.1.

Bonding conductors. Except PME, no less than half the REQUIRED csa for the main earthing condutor, minimum of 6mm and need not exceed 25mm if copper or equivalent.

Where PME conditions apply use table 54.8

Regards Chris

 

[ezzzekiel]

all very well for new installs but diferent kettle of fish for existing installations, without getting into same old debate:

as advised from various sources (eca, esc, nieec etc)
providing existing earthing arrangements satisfy adiabatic equation and main bonding conductors are showing no signs of mechanical or thermal damage and have a low enough resistance then they may be left in situ.

So if you are willing to leave existing in place and sign the cert then that is fine - but remember if it all goes pete tong your names on the cert so if upgrade is necc or possible then do it, if not make sure all details are fully recorded on the condition and reason for leaving them in place

 

[edexlab]

This is exactly what I've been discussing with another electrician today re TN systems
He says he's definitely read somewhere that when working on an older installation it is ok to leave a 6mm main bond in place when the tails are 25mm main earth 16mm and install new 10mm to services previously left unbonded.

As far as I'm concerned I would be changing this existing 6mm to a 10mm and have always done so before ,as ezzzekiel says above you need to have a good reason to leave the smaller csa and I can't think of one in this case

can anyone point me to where I can find the info/article saying that you can do this ?

 

[Traineenoob]

There are two methods to determine the CSA of earth and bonding conductors.
One is to use the adiabatic equation (something that should always be done when conducting a PIR), and the other is to use the applicable tables in BS7671.
If using the adiabatic equation, you must first have knowledge of the fuse rating and type, and the PEFC.
Once you have this information, then using the time/current characteristics in BS7671 you apply the adiabatic equation, and the result will be the minimum CSA required for the earth conductor.
The CSA of the bonding conductors is half of that required for the earth conductor, with the minimum allowed being 6mm².
Where PME conditions apply, the minimum CSA for both the earth and bonding conductors is 10mm².
With TT installations, you could very well have an earthing conductor with a smaller CSA than that of the bonding conductors.

Can someone please explain the adiabatic equation ie when working out the size needed for bonding conductors?
So the bonding counductor needs to been half the size main earth conductor?
Fuse rating im assuming you use the main service head fuse.

lets say a 1361 type 2 60A
Pefc of 1.7kA

Can someone put this into a example please?

 

[Deleted member 26818]

First you look up the page in Appendix 3 of BS7671 for BS1361 fuses.
You then look at the values for your particular fuse i.e. 60A, and choose the largest value in Amps that is less than your PEFC. In this case 880A.
This gives a disconnection time of 0.1 sec. If your PEFC was only 0.7kA you would pick the value of 600A, which would give a disconnection time of 0.4 sec.
Once armed with the Amperage and the disconnection time, you can use the values for the Adiabatic equation.
i.e. 880A X 880A = 774400 X 0.1 sec = 77440. The square root of this is 278.280430948174.
You then turn to the page in BS7671 in Chapter 54 which has the k values.
As you are determining the size of a Main earth, and it is most likely copper and not incorporated in a cable or bunched with cables, you would choose the value of 143.
You would then divide the square root value by 143, which will produce the result of 1.94601702164208mm².
So in this instance the minimum CSA required for a bonding conductor would be half of 1.94601702164208mm², which would be 0.9730085108210398mm².
Obviously you would not use a conductor of such a size; for one, they probably don't make one, and for another, the minimum CSA allowed by BS7671 for a bonding conductor is 6mm².

Some people will tell you that you should use the actual value of PEFC when carrying out the Adiabatic equation.
This can really only be done, if you also know the disconnection time that would occur with that value.
Using the disconnection time associated with a lower value of fault current, would not be accurate and would produce a larger value for the CSA required.
The lower the fault current, the slower the disconection time, the greater the CSA.
The higher the fault current, the faster the disconnection time, the smaller the CSA.

 

[Octopus]

this is exactly what i've been discussing with another electrician today re tn systems
he says he's definitely read somewhere that when working on an older installation it is ok to leave a 6mm main bond in place when the tails are 25mm main earth 16mm and install new 10mm to services previously left unbonded.

As far as i'm concerned i would be changing this existing 6mm to a 10mm and have always done so before ,as ezzzekiel says above you need to have a good reason to leave the smaller csa and i can't think of one in this case

can anyone point me to where i can find the info/article saying that you can do this ?

bs 7671 131.8