Earthing question.. (arent they all?)

[dw3101]

I've normally only ever had to consider PME / TNC-S supplies, but what is the common practice for earthing in this scenario in a TNC-S scenario..

If you have a TP&N submain terminating in a barn with local earthrod, and the structural steelwork is bonded with 16mm² cable.

Say you were looking to run an additional submain to an adjacent barn 15m away. What are the options?

1) Rely on the earthing on building A assuming CPC on cable is sufficient, or run a CPC equal to largest phase conductor.

2) Earth The supply end of of the cable, and isolate at remote end and provide local earth here.

3) Add new earth rod in / at new barn bond steelwork and connect this in parallel with the first submain via cable CPC or additional conductor.

4) I've missed something obvious.

 

[RF Lighting]

what is the common practice for earthing in this scenario in a TNC-S scenario..

Did you mean to write TT?

 

[dw3101]

erm yeah..

Embarrasing mistake

 

[dunny123]

I have been researching similar scenarios recently and my understanding is as follows:

1) Rely on the earthing on building A assuming CPC on cable is sufficient,

You would need to connect the structural steelwork in building B (if its an E.C.P) with a main bonding conductor back to building A.

.. or run a CPC equal to largest phase conductor.

Not applicable if you do the above.

2) Earth The supply end of of the cable, and isolate at remote end and provide local earth here.

Yes, if you treat building B as its own TT system.

3) Add new earth rod in / at new barn bond steelwork and connect this in parallel with the first submain via cable CPC or additional conductor.

Thats the same as option 2 except the last bit, you shoulddn't connect the two earth systems together (unless the protective conductor is capable of the maximum likely fault current (calculated by the adiabatic equation)

If the structural steel work (in either building) is reliable and low enough inpedance then you could use it as the means of earthing (i.e. instead of a rod)

 

[dw3101]

Your saying that I can use the Earth in A so long as I bond the steelwork in B (Which goes without saying!)

and assuming the cable CPC is ok for the maximum fault current (likely to be a 4core 25mm² at the moment) then it should be ok.

David

 

[Spark123]

It is worth consulting guidance note 8 regarding this issue, TN-CS and livestock may not be a good mix without additional measures, I'm thinking bonded grid in the floor etc.

 

[dw3101]

Cheers, will check it out tomorrow when I have my red book to hand.

No livestock in one barn.. other has some stables in it, bonded all steelwork up at the weekend with 16mm² Earth cable..

David