Main Earthing conductor max resistance?

[dokkan1080]

I know main bonding to water and gas need to be 0.05 ohms

But what about the main earthing conductor?

Does it have a maximum resistance value from MET to the CU?

 

[Dustydazzler]

Only one off the top of my head is 0.35 ohms for a tncs (pme)

 

[loz2754]

I know main bonding to water and gas need to be 0.05 ohms

According to what regulation?

 

[loz2754]

The maximum impedance of the earthing conductor is determined by the adiabatic equation.

 

[timhoward]

According to what regulation?

No regulation that I’m aware of, I think it’s in GN3 from memory.

 

[timhoward]

In practical terms assuming you have a CU very near the MET the resistance should be almost negligible, so if Zs@DB is measured it would be well under the max Ze figures of 0.35 (TNCS) and 0.8 (TNS).
It sounds as though this question relates to a particular situation and maybe more details would help.

 

[loz2754]

No regulation that I’m aware of, I think it’s in GN3 from memory.

Nope.
GN3 refers to a measurement made between the cable connected to the bonding clamp, and the actual pipe. This reading to be <0.05 ohms. There is no regulation specifically mentioning a maximum resistance of any bonding conductor.

 

[telectrix]

Nope.
GN3 refers to a measurement made between the cable connected to the bonding clamp, and the actual pipe. This reading to be <0.05 ohms. There is no regulation specifically mentioning a maximum resistance of any bonding conductor.

lots of sparks get this wrong. the 0.05 is as you say. absofruitly nothing to do with the resistance of the bonding conductor.

 

[timhoward]

lots of sparks get this wrong. the 0.05 is as you say. absofruitly nothing to do with the resistance of the bonding conductor.

Guilty as charged m’lord. Better read GN3 again...!

 

[telectrix]

it's to ensure that :

1. the bonding clamp is securely connected to the metal of the pipe, not through paint.

2. wet-pants hasn't fitted clamp to a plastic pipe.seen that done a couple of times.

 

[dokkan1080]

In practical terms assuming you have a CU very near the MET the resistance should be almost negligible, so if Zs@DB is measured it would be well under the max Ze figures of 0.35 (TNCS) and 0.8 (TNS).
It sounds as though this question relates to a particular situation and maybe more details would help.

Not situational I have just seen a really long 16mm earthing conductor today on a tncs a few days ago and was just wondering if it had a maximum length

 

[timhoward]

Not situational I have just seen a really long 16mm earthing conductor today on a tncs a few days ago and was just wondering if it had a maximum length

Does that also mean tails were really long and over 3m?

 

[dokkan1080]

Does that also mean tails were really long and over 3m?

Yes they were fused at an isolator

 

[timhoward]

Yes they were fused at an isolator

So would it be fair to say that the main earthing conductor only runs as far as the fused switch and from there on it becomes a cpc which has to meet the max Zs requirements of the fuse in the isolator which in turn dictates its max resistance?

 

[Lucien Nunes]

Noting that one has to be careful to define where the MET is, according to what is connected where. E.g. if the MEB conductors terminate in the CU, the cable from the origin to the CU is not just a CPC.

 

[pc1966]

So would it be fair to say that the main earthing conductor only runs as far as the fused switch and from there on it becomes a cpc which has to meet the max Zs requirements of the fuse in the isolator which in turn dictates its max resistance?

Maybe, maybe not:

• It has to allow the fused-switch to meet its Zs for 5s disconnection at the end of the sub-main (as you just said).
• It has to allow all final circuits to meet the Zs requirement.
• It has to meet the adiabatic limit for all OCPD clearing (supply fuse & finals circuits, but obviously they will be smaller and can be ignored if supply is met)
• And also any possible extraneous bonding on TN-C-S (which ought to be back to the MET, but if sub-main to the remote board, such bonds might well be connected there) so you might have the (typically) 10mm minimum.

If you have no reason to doubt the design, then the resistance check is basically to verify it is working as designed, so based the GN3 figure would be:
R measured <= 0.05 + expected cable R2

 

[dokkan1080]

So would it be fair to say that the main earthing conductor only runs as far as the fused switch and from there on it becomes a cpc which has to meet the max Zs requirements of the fuse in the isolator which in turn dictates its max resistance?

Maybe, maybe not:

• It has to allow the fused-switch to meet its Zs for 5s disconnection at the end of the sub-main (as you just said).
• It has to allow all final circuits to meet the Zs requirement.
• It has to meet the adiabatic limit for all OCPD clearing (supply fuse & finals circuits, but obviously they will be smaller and can be ignored if supply is met)
• And also any possible extraneous bonding on TN-C-S (which ought to be back to the MET, but if sub-main to the remote board, such bonds might well be

16mm to fused isolator, then 16mm to CU
Or it might have had 16mm from the met and a separate 16mm to the isolator