Earth Electrode Size?

rob w

Hi,

Just flicking thorugh the ETCI regs on the section for earth electrodes 542.2.3, and it states the requirements of electrodes for installations not exceeding 250A.

Does anyone know what the situation is for installations which exceed 250A?? Cant find it anywhere!! :mad:

Thanks

2011

rob w wrote: »

Hi,

Just flicking thorugh the ETCI regs on the section for earth electrodes 542.2.3, and it states the requirements of electrodes for installations not exceeding 250A.

Does anyone know what the situation is for installations which exceed 250A?? Cant find it anywhere!! :mad:

Thanks

You won't find it because it is not there.

In larger electrical installations the electrical systems designer will not specify a size of the earth electrodes or earth pit. Instead a maximum resistance to earth will be given. The electrical contractor has to keep adding earth electrodes (connected in parallel) until the target resistance is achieved. Larger earth pits such as this normally use a combination of horizontal conductors (bare copper cable or bare copper tape) as well as vertical earth electrodes.

The target resistance will be influenced by a number of factors at the design stage, including the resistivity of the soil, the size of the installation, maximum prospective earth fault current etc.

rob w

Thats great, thanks!

Is there a particular way to calculate a value for the resistance to earth? For a tncs system i presume it doesnt affect the max earth fault impedance loop value, as its not on the metallic return path? Am i right in saying that?

2011

rob w wrote: »

Thats great, thanks!

Is there a particular way to calculate a value for the resistance to earth?

Yes, but it is a bit of an estimate.
The best is to measure and record the value.
This value can change quite a bit depending on the weather. When the soil is damp it will have very different resistivity than when it is dry. Also if the electrical contractor is unlucky and drives earth electrodes into a rocky area readings will not be as good.

On a rescent project I had a lot of involvement with the earthing (large installation well in excess of 10MVA). Earth pits were added in areas where there were large amounts of 804 hardcore present. Our target resistances were very low due to the size of the installation. Each earth electrode had a measured value of > 100 Ohms to the general mass of earth (Wenner method used). I know this as I had to witness and sign off on the tests.

For a tncs system i presume it doesnt affect the max earth fault impedance loop value, as its not on the metallic return path? Am i right in saying that?

It will effect the EFLI as it is a parallel path.
However if cabled path is low enough a resistance it may not be noticed on the test instrument.

Removable test links are installed at earth pits so that their resistance to earth can be measured and tracked over time.

It is possible to use and EFLI test instrument to measure the resistance of the connection of an earth electorde to the general mass of earth. However this method is not an approved method in Ireland.

BrianDug

As 2011 has said you will need to increase the number of earth electrodes.
This is a good study on the placement and spacing of earth electrodes that I came across last year while carrying out some research.

http://www.lightningprotection.com/pdfs/resources/knowledge-transfer/technical-papers/principles-of-grounding-design.pdf