Calculate bonding sizes TT

[Bengaz]

Hi, searched but still niggling problem need help on...

Installation: -
4mm earth to earthing electrode (rod) Ra 42ohm
Zdb 33ohm
1361 60A main fuse
16mm tails
cant get acceptable result for using adiabatic as disconnection time too high (higher than 5 secs with that Zs reading)
no frontline RCD protecting tails.
What would the minimum bonding size be? (2mm therefore 4mm wiring?)

 

[davesparks]

exactly then halved = obv not 2mm as I stated.
Guess I'm trying to understand how we can calculate the main earthing conductor size then half that for bonding.
Was trying to understand the way it could be calculated, I know the regs say 6mm FYI.
Maybe I worded things not too great...

Earthing conductor size can be calculated using the adiabatic equation.

Main bonding normally can be half that size, but not less than 6mm and not normally more than 25mm.

So if your earthing conductor needs to be 25mm you need a bonding conductor of 12.5mm minimum so would install 16mm as it's the next nearest size above that.

If your earthing conductor needs to be 6mm then your bonding conductor would also need to be 6mm due to the minimum size requirement.

 

[mainline]

No they aren't, the thread title is about bonding size and the only question in their post is:

Right at the start of his post he said "4mm earth to earthing electrode (rod) Ra 42ohm"

4mm earth to earthing electrode (rod) Ra 42ohm
Zdb 33ohm
1361 60A main fuse
16mm tails
cant get acceptable result for using adiabatic, as disconnection time too high (higher than 5 secs with that Zs reading)
no frontline RCD protecting tails.

I thought he got confused with Bonding and \Main earth, otherwise he wouldn't be quoting adiabatic calculations.

 

[Bengaz]

No, i ai'nt got confused, what I am asking is because you cant use the adiabatic equation for the main earthing conductor then what formula result makes the bonding size acceptable. Instead of just using the regs as 6mm minimum how can we measure the acceptability of the bonding size? ie if the ze was less than 1 then the 4mm main earth wouldnt be large enough then that would increase the bonding size respectively. Guess maybe the 6mm bond would always suffice in a TT system with the expected fault currents?

 

[mainline]

No, i ai'nt got confused, what I am asking is because you cant use the adiabatic equation for the main earthing conductor

Why ?

then what formula result makes the bonding size acceptable. Instead of just using the regs as 6mm minimum how can we measure the acceptability of the bonding size? ie if the ze was less than 1 then the 4mm main earth wouldnt be large enough then that would increase the bonding size respectively.

Half the size of the Main earth

Guess maybe the 6mm bond would always suffice in a TT system with the expected fault currents?

Yes

The bonding minimum is 6 mm

The main earth can be less.

 

[davesparks]

No, i ai'nt got confused, what I am asking is because you cant use the adiabatic equation for the main earthing conductor then what formula result makes the bonding size acceptable.

You can use the adiabatic equation, but the result will usually be less than the minimum permitted size for a protective conductor. So you will end up using the minimum permitted size.

Again with the bonding, you will often end up using the minimum size.