Main Earthing Conductor and Earthing System question.

[bambubar]

Hello all,

Just a quick question regarding I+T that I wanted to get some clarification on:
1. I recently went to a job where the incoming cables from the street came up through a metal conduit from the ground and then ran through some metal trunking before terminating in three sealed fuses (which was essentially the cutout). A 6mm earthing cable was clamped to this incoming piece of metal conduit and ran to an MET before terminating.
My questions are: a) is this a TN-S system? b) is the length of 6mm what I would class as the main earthing conductor for the installation.

Any constructive response to this would be appreciated. Thank you.

 

[Deleted member 26818]

Sounds like TN-S from the way you describe it.
Have you measured Ze?
What are the 3 conductors?
Where is the neutral?

 

[bambubar]

Hi Spinlondon,

Yes I have. I measured 0.21 ohms so got a normal reading. Would the 6mm earth be what I class as the main earthing conductor? I'm guessing so, in which case it would be undersized for the installation.

 

[bambubar]

The three conductors are the three line conductors as it is a 3 phase installation. There is also a neutral conductor coming up through the conduit aswell.

 

[bigspark17]

Yes & yes i would say from your post.
Can you post any pics?

 

[Deleted member 26818]

0.21 suggests PME conditions apply.
As such the earth conductor should be 10mm2.
It is possible, that at the time of design/construction only 6mm2 was required.

 

[bambubar]

I can't unfortunately bigspark17 as I didn't take any. It's moreorless exactly as I described however. I assumed the 6mm cable would be what I would have to use as there was no other earth cables present and there was nothing to suggest that the earthing system was anything other than a TN-S. I was just after clarification really.

 

[Ian1981]

0.21 suggests PME conditions apply.
As such the earth conductor should be 10mm2.
It is possible, that at the time of design/construction only 6mm2 was required.

Why does a reading of 0.21 ohms suggest PME?

 

[Deleted member 26818]

Standard Ze readings for the different types of earthing systems.

 

[westward10]

Where do you find this?

 

[Deleted member 26818]

Where do you find this?

I find it in a file I have in my head called memory.

 

[westward10]

What would 0.16 suggest.

 

[Ian1981]

Standard Ze readings for the different types of earthing systems.

What an odd thing to say

 

[Deleted member 26818]

Are you guys seriously saying you don’t know the typical Ze values for TN-S and TN-C-S?

 

[Des 56]

Are you guys seriously saying you don’t know the typical Ze values for TN-S and TN-C-S?

Ditto

 

[Risteard]

Are you guys seriously saying you don’t know the typical Ze values for TN-S and TN-C-S?

A typical MAXIMUM value is not the same thing as a typical value.

A TN-S reading can be extremely low if near the transformer.

 

[Deleted member 26818]

A typical MAXIMUM value is not the same thing as a typical value.

A TN-S reading can be extremely low if near the transformer.

And?

 

[Risteard]

Are you guys seriously saying you don’t know the typical Ze values for TN-S and TN-C-S?

A typical MAXIMUM value is not the same thing as a typical value.

And?

And it disproves your earlier remarks?

 

[Deleted member 26818]

A typical MAXIMUM value is not the same thing as a typical value.

And it disproves your earlier remarks?

Disproves nothing.
It’s a statement which has no value.
There are no typical maximum values.
There is either a maximum or a typical value.

 

[Risteard]

Disproves nothing.
It’s a statement which has no value.
There are no typical maximum values.
There is either a maximum or a typical value.

They are typical maximum values as the DNOs are not compelled to remain within those values.