Minimum bonding size TNC-S

[robd]

Afternoon all,

Just trying to clear something up, going through regs book at present.

Basically have got a TNC-S/PME installation, 16mm tails, 60/80a main fuse (think it is 60A), 6mm bonding to water and 6mm coming from service head to block then into CU as main earth.

Am I right that being as it is PME I am obliged to upgrade bonding to 10mm as per table 54.8 and that the note about bonding must be not less than half size of CSA for main earth only applies to conditions other than PME.

Just wanted to be clear as that's how I'm reading it, the job is a mains change. Thanks in advance.

 

[andyb]

I agree about the adibiatic not applying to bonding that's how I read it

It does not apply to a PME supply, but it could be used for a TNS suppy.

 

[davesparks]

but it could be used for a TNS suppy.

Not really, the adiabatic is can be used to select the size of earthing conductor, then the bonding conductor is worked out based on the earthing conductor. The size of the bonding conductor isn’t a direct result of the adiabatic calculation. And if the earthing conductor size has been selected according to the table then the adiabatic has nothing to do with the bonding conductor size.
For example if the adiabatic shows that a 19.7mm earthing conductor is needed you would use 25mm, then the bonding would be 16mm (next size up from half of earthing conductor) if the adiabatic result was used for bonding it would be 10mm.

 

[Deleted member 26818]

The minimum CSA of an earthing conductor is 2.5mm2.
The minimum CSA of a bonding conductor is 6mm2.
Where PME conditions apply, the minimum CSA of both the earthing and bonding conductors is 10mm2.
There is no requirement to upgrade the Earthing conductor to 16mm2 unless the adiabatic equation dictates a CSA greater than 10mm2 is required.

 

[andyb]

Not really, the adiabatic is can be used to select the size of earthing conductor, then the bonding conductor is worked out based on the earthing conductor. The size of the bonding conductor isn’t a direct result of the adiabatic calculation. And if the earthing conductor size has been selected according to the table then the adiabatic has nothing to do with the bonding conductor size.
For example if the adiabatic shows that a 19.7mm earthing conductor is needed you would use 25mm, then the bonding would be 16mm (next size up from half of earthing conductor) if the adiabatic result was used for bonding it would be 10mm.

I think it can Dave, the reg's state 'not less than half the csa required for the earthing conductor'.
So the way see it is, if you are adding to an installation that has 25mm tails and a 16mm main earth but only say a 6mm main bonding conductor, then in that case you could use the adabatic equation to determine the required csa for the main earth then go from there. So if the equation gives you a requrement for say an 9mm main earth, despite the fact that someone has installed a 16mm earth you can say half 9mm = 4.5mm so the 6mm installed is adequate.

 

[Risteard]

No you are not obliged to do this unless you do the adiabatic equation and the 6 MM is not suitable.
If it's under 6mm then you are obliged to increase to 10mm for bonding and 16 mm for main earthing.

The adiabatic equation is not relevant to main protective bonding conductors, or any bonding conductors for that matter.

 

[Toneyz]

If you wait a couple of weeks and its got plastic incomer won't need bonding big tin hat on-off the cuff remark.

 

[Deleted member 26818]

If you wait a couple of weeks and its got plastic incomer won't need bonding big tin hat on-off the cuff remark.

No need to wait, there has never been a requirement to bond plastic incomers.

 

[robd]

Sadly it's copper not alkathene

 

[Midwest]

If you wait a couple of weeks and its got plastic incomer won't need bonding big tin hat on-off the cuff remark.

As I understand the draft, I might be wrong, the metallic pipe entering the building need not be bonded;

'Chapter 41 Protection against electric shock
Section 411 contains a number of significant changes. Some of the main ones are mentioned below.
· Metallic pipes entering the building having an insulating section at their point of entry need not be connected to the protective equipotential bonding (Regulation 411.3.1.2).' (Hager)

Whereas before, it was a slam dunken?