Earthing and bonding sizing

[JulianC]

Righty, I may be suffering from brain fade on this so please bear with me....

In a domestic situation up to 100A it is generally accepted that for a TN-C-S supply, earthing and bonding must be 16mm & 10mm respectively and if this is not present, we must upgrade it.

The 10mm bonding is plain to see from reg 544.1.1 where it is categorically stated in table 54.8.
But where does it say that if you have a PME supply the main earth must be 16mm? From 543.1.1 we know that the earth conductor size can either be selected from table 54.7 (where 16mm is indeed the minimum) or it can be calculated from the adiabatic equation. Why then, providing the results of the adiabatic equation say so, can we not have a PME supply with 10mm for both earthing and bonding? Or can we and I've just not been paying attention?

General consensus seems to be that for T-T or TN-S we can calculate and perhaps get away with 10mm earth and 6mm bonding but every time TN-C-S is mentioned we for some reason need a 16mm earth & 10mm bonding. Why not 10mm for both if the adiabatic says it's OK?

Ta.

 

[ace design]

pay more attention and there is no getting away with anything when it comes to electricity

 

[JulianC]

You mean you don't know the answer either?

 

[Tester Steve]

The adiabatic equation is only used for a cpc, not main earthing.

Steve.

 

[JulianC]

The adiabatic equation is only used for a cpc, not main earthing.

Steve.

I disagree. Main earth is still a protective conductor and the adiabatic equation is equally valid.

 

[Deleted member 9648]

Table 4.1 of the OSG gives minimum earthing conductor sizes in relation to the line/neutral conductor for TNS and TNCS supplies.

 

[SirKit Breaker]

I dont do equations. But what i do do is, belt and braces just to make sure. I bond main earth 16mm, main equipotential at 10mm, and owt else gets 6mm, irrespective of TT, TNS, TNCS

The reasons behind this are several. The wholesaler stocks it, i keep less reels of cable in the van, it is adequate for all the current earthing arrangements i come across, and last but not least i dont have to do sums. Oh nearly forgot, the BRB says I can.

Cheers..........Howard

 

[chris gray]

I dont do equations. But what i do do is, belt and braces just to make sure. I bond main earth 16mm, main equipotential at 10mm, and owt else gets 6mm, irrespective of TT, TNS, TNCS

The reasons behind this are several. The wholesaler stocks it, i keep less reels of cable in the van, it is adequate for all the current earthing arrangements i come across, and last but not least i dont have to do sums. Oh nearly forgot, the BRB says I can.

Cheers..........Howard

my type of logic

 

[ezzzekiel]

The adiabatic equation is only used for a cpc, not main earthing.

Steve.

it can be used for sizing cpc/ earth, but not for bonding

 

[brucelee]

it can be used for sizing cpc/ earth, but not for bonding

hi mate for a tn-s system 100amp, 25mm tails, Ze 0.26ohm
to do the adibiatic equation for earthing conductor is it the voltage divided by the Ze to get the fault current then I squared x time then square root and finaly divide by K could you please explain if this is right cheers

 

[Sintra]

That's correct

 

[brucelee]

Cheers Sintra,
thought it was just for CPCs until reading this post its to check earthing conductor adequete at the mother in laws as it looks like 6mm

 

[sparkylad]

b l . if it was at the mother in-laws I would remove it altogether

 

[brucelee]

b l . if it was at the mother in-laws I would remove it altogether

lol mrs wouldnt feed me if i did that but it might be worth it haha