Cable sizing earthing conductor

[An Buachaillín Bán]

Hi all,

Hope you're keeping well

I'm currently wiring a small commercial premises. 3 phas incoming supply, 16sq cables

As it is in an urban area with a busy pedestrian street on both sides of the building I had to drive the earth rod just inside the door of the premises. However it is located about 30m from the distribution board. Could anyone tell me how I would calculate whether I would need to run a 16sq earth cable or if 10sq would be sufficient. I'm conscious if a large fault current were to flow and 10sq being adequate.

Building is also neutralised. Which brings me to my next question. If a premises is neutralised, why the need for an earth rod.

Appreciate any advice here. Thanks

 

[nicebutdim]

@Risteard

 

[Risteard]

What size is the supply (in kVA)?

 

[An Buachaillín Bán]

What size is the supply (in kVA)?

Hi Risteard, sorry for late reply. It is a 15kA supply.

 

[LastManOnline]

Hi all,

Hope you're keeping well

I'm currently wiring a small commercial premises. 3 phas incoming supply, 16sq cables

As it is in an urban area with a busy pedestrian street on both sides of the building I had to drive the earth rod just inside the door of the premises. However it is located about 30m from the distribution board.

Just spotted this thread.Sinking earth rods is probably the average sparks least favourite pastime.

Could anyone tell me how I would calculate whether I would need to run a 16sq earth cable or if 10sq would be sufficient.

There is no calculation involved .If you install a 16mm supply (in ROI) you are obliged to run a 16mm earth to the rod.10mm is more than adequate (4mm is actually more than adequate).But the regs require 16mm.

. I'm conscious if a large fault current were to flow and 10sq being adequate.

This is a popular misunderstanding.A "large" fault current cannot flow through an earth rod for the same reasons a "small" fault current cannot flow through it.Current flow through an earth rod (it's just another resistor) is entirely dependent on the rods resistance to earth.We can determine what current can flow through a rod by applying ohm,s law.

Building is also neutralised. Which brings me to my next question. If a premises is neutralised, why the need for an earth rod.

Well ,in the UK they don,t sink any earth rods.But this will likely result in higher "touch votages" under fault conditions.In the UK ,this is currently considered to be an acceptable risk.
In ROI ,our supply system is nearing 100% T-NCS.For that reason ,we are probably a little more conscious of the to mitigate the dangers of an open PEN fault.

 

[pc1966]

Well ,in the UK they don,t sink any earth rods.But this will likely result in higher "touch votages" under fault conditions.In the UK ,this is currently considered to be an acceptable risk.
In ROI ,our supply system is nearing 100% T-NCS.For that reason ,we are probably a little more conscious of the to mitigate the dangers of an open PEN fault.

I can see why the UK decided against mandating earth rods for existing properties: just imagine the amount of service cable/pipe damage from ~10 million new earth rods being driven down in urban areas! However I thought they were very foolish not to mandate either a good rod or proper foundation earths as part of every new building, as they would have little installation risk and greatly improve the distributed earth performance.

I guess the UK also differed from the ROI in that we can have TT earthing for situations where open-PEN touch voltages are considered a problem (agricultural, swimming pool, etc) but AFIK that is no longer allowed?

 

[An Buachaillín Bán]

Just spotted this thread.Sinking earth rods is probably the average sparks least favourite pastime.

There is no calculation involved .If you install a 16mm supply (in ROI) you are obliged to run a 16mm earth to the rod.10mm is more than adequate (4mm is actually more than adequate).But the regs require 16mm.

This is a popular misunderstanding.A "large" fault current cannot flow through an earth rod for the same reasons a "small" fault current cannot flow through it.Current flow through an earth rod (it's just another resistor) is entirely dependent on the rods resistance to earth.We can determine what current can flow through a rod by applying ohm,s law.


Well ,in the UK they don,t sink any earth rods.But this will likely result in higher "touch votages" under fault conditions.In the UK ,this is currently considered to be an acceptable risk.
In ROI ,our supply system is nearing 100% T-NCS.For that reason ,we are probably a little more conscious of the to mitigate the dangers of an open PEN fault.

Thanks for the advice! Much appreciated