Buried Earth Network. 70mm.sq?

[davva]

I have an earthing layout drawing for a chemical plant with a recommendation to use a 70mm.sq bare stranded copper conductor to be used to form a main loop buried earth network (BEN) around the plant at a depth of 500mm. The 70mm.sq cable will be connected periodically to earth rods.

I struggling to find where the requirement for 70mm.sq cable originates from (which standard?). Unfortunately the engineer who wrote these words of wisdom has long since left the company and is probably drinking a pina colada on a sunbed somewhere!

This drawing has been left to degenerate into one which is churned out again and again. Having recently moved from the marine industry I have not had to be so worried about buried earth networks so excuse my lack of knowledge!

I have collected quite a number of standards and documents mainly through a paper chase, but I am pushed for time to read through all of them extensively in time to get this drawing out. The standards are:-

BS 7430
BS 6651
IEC 61024
ANSI 80
ANSI 142
Earthing practice CDA publication 119
IEC 61000-5-2
Power Quality Application Guide (Leonardo da Vinci) Earthing & EMC 6.1

The latter one has a statement that quotes “The buried earth electrode ring around the building should be located at a distance of least 1 metre from the exterior wall. It should be deep enough to ensure that it will not be affected by freezing in winter and will not dry out in summer. Where there is no applicable local guidance the depth should be at least 0.5 metres. The earth ring should be made of copper with a cross-sectional area of at least 50 mm.sq.” There is no reference to any standard.

Thread 238-83781 was of some use, but I was hoping someone in the UK/Europe might shed some light from a BS/IEC standpoint.

Many thanks.

 

[jghrist]

The conductor must be large enough to withstand the maximum fault current that can flow through it. IEEE Std 80 has formulas for sizing based on fault current. The conductor also must be mechanically reliable. I would not specify anything smaller than #2/0 copper (67 mm²) for mechanical reasons, but this is not a code requirement in the USA. Normally we use #4/0 copper (107 mm²) for substations.

 

[cuky2000]

Earthing conductors should be sized to carry a significant amount of current for short time without fusing during short circuit or lightning strike. The conductor should be large enough to avoid degradation by annealing the metal, compensate for corrosion, and preclude hot spot that may initiate fire or expose the facility to hazardous conditions.

Usually earthing conductors are sized for initial conductor temperature of 30°C and a maximum temperature of 250°C.

The enclose link may contain some information and engineering practice applicable in UK and other part of the world.

http://www.cda.org.uk/megab2/elecap...MoM Electromagnetics touch voltage earthing"