Dear Mr. Emadshaaban1987 (Electrical)(OP)23 Nov 22 12:59
"....#1. The ground system of our (industrical plant) is baised on many ground rods in diffents areas ,that are connected to the same ground network."
This is the usual practice. In numerous cases one or two ground rods may not achieve the resistance required. Therefore with many ground rods planted at different areas which are connected to to the same ground network is a good solution, to achieve the required resistance.
Note: (a) The distance between ground rods shall be as far apart as possible (but NOT closer that say 8m). They may be in any location and any direction.
(b) It is usually better to have numerous ground rods connected together than driving a single or two rods only deeper or increasing the diameter/material of the ground rods.
"... #2. The measurmetns of some rods show relativluuty high grounding resistance (40 to 110 Ohm) .what is the minium reading and if there any stantrdes for grounding rods"
There is NO standard that stipulates the resistance value of the ground rod. It varies as what your measurement shows (40-110 Ohm). This is normal/expected. It shows that some are no good and some are bad!.
Note: To improve/lowering the resistance by
(a)change the location to a (usually) wet area,
(b) drive it deeper but usually need not exceeding say 8m. Any deeper may have limited effect. (c) plant more rods at different directions see #1. would be a better choice.
(d) the total combined resistance that is within your requirement is more important than the individual value.
Che Kuan Yau (Singapore)

Hi che1234, thank you for such details information, this information is really helpful.

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