Advice needed for Earthing of PLC Panel

Dear All I am attaching a word file where as I have described the Earthing system followed by our co. I am presently commissioning these panels & when I checked resistence between electric earth pit & electronic earth pit I found it is 300K Ohm.
Kindly advice is it sufficient or it can create any problem in future for PLC Components.
Regards,
Neeraj

The distance is around 12 mtrs between electronic earth pit & Electric earth pit, still when I measure resistance between Earth Pit 2 & 3 it is only 300 Ohms.

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First, please correct the resistance. Is it "only 300 Ohms" as written in your document, or is it 300K Ohms (300,000 Ohms) as you stated in your question?

The U.S. National Electrical Code is strongly against separate grounding systems.

Quote from N.E.C. Article 250.6B(4):

"A solution often recommended by inexperienced individuals is to isolate the electronic equipment from all other power equipment by disconnecting it from the power equipment ground. In this ill-conceived corrective action, the equipment grounding means is removed or nonmetallic spacers are installed in the metallic raceway system. The electronic equipment is then grounded to an earth ground isolated from the common power system ground. Isolating equipment in this manner creates a potential difference that is a shock hazard. The error is compounded because such isolation DOES NOT establish a low-impedance ground-fault return path to the power source, which is necessary to actuate the over-current protection device."

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My answer then is that both 300 Ohms (OR 300K Ohms), are much too high. To meet the US National Electrical Code, the resistance between Earth Pits 1, 2, and 3 would have to be 25 Ohms or less. Essentially, connect all three ground points together into one grounding electrode conductor.

Your earth pit resistances are dependant on soil type - conectivity - and moistness.
The connectivety can change greatly - this is why they should be solidly bonded together.

The only seperate earthing we refer to is 'Shield Ground'
it still bonds to the earth but only at one point - the other end is open.
this is for signal noise not earthing of devices.

do not use the ending of grounding rails as connector to your pits.
Only use one end. Make a connection between all pits, it will lower the resistance.
Then connect from this net one connection to each rail. (wire must be 10 mm^2 )
All equipment must be grounded proper.
The shields from the cables should be connected to a separate rails on one side. Then you can test fro ground loops.

10mm^2 is one possible cable size.
the actual size needs to be calculated on earth fault so that cable may be good it may also be wrong.
It may need for example 150mm^2 in certain situations.

Ian, and Shooter,

My understanding of Neraj's question was: he wants to know if 300K ohms is TOO LOW for the resistance between the power ground and the signal ground.

In other words, he thinks that there should be a higher resistance between these grounding points, so that "is it sufficient [large enough],.." and will "it [] create any problem in the future for PLC Components?"

Certainly it will create problems for electrical workers or anyone else who happens to touch a signal ground and a power ground at the same time!

you could be right - hard to understand his english i think.
But if it is earth resistance, then it is dependant on soil resistivity and moisture content.
we may need to wait on the OP responce

But if it is earth resistance, then it is dependant on soil resistivity and moisture content.

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Take a look at the Ufer Grounding System. It is less dependent on soil resistivity and moisture content. I have used it almost exclusively for many years. It works even in desert sand.

If you have 3 different grounding systems in relatively close proximity, then it is important that the resistance between them NOT be dependent on the soil. You solve that problem by hard-connecting them together with large wires or rods.

sorry Lancie1 but that it what Shooter and I said

The Ufer Ground is usualy made inside a concrete slab. The weight of the concrete compresses the soil, making it more conductive. Also ions bleed from the concrete, doping the surface of the underlying soil. The normal reenforcing rods in the concrete make a very good large grouding grid.

i would agree with you. but
The electrical authority, over here, is no longer accepting that.
They, now insist on the Concrete reinforcement being bonded to the earth stake/s at an independant point.
There has been a lot of changes here due to the 10 year drought.
The soil is just too dry in some areas.
there are frequent changes to our reg.s - earthing has usually been one of them.

They, now insist on the Concrete reinforcement being bonded to the earth stake/s at an independant point.

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Ha-ha, that is a step back into the dark ages. Common sense would say to anyone who thinks about this for 3 minutes that the surface area of an earth stake is hundreds of times smaller than a concrete foundation, provides no soil compression, no soil doping, no sun shading, or no moisture retention.

Political decisions can be wonderful at maintaining a flawed status quo.

Properly done, a Ufer Grounding System in dry sandy soil has about 2.5 to 5 ohms resistance from the Ufer to a driven wet test rod 50' away. I have tested these myself, and watched while others run the test, with the same results. But no one has to take my word because Ufer systems were first designed 60 years ago and have been extensively tested many times.

yep your right again
the building code allows for what they call a class 'M' slab.
this is a floating concrete slab than is not dug into the ground as deeply as other slabs - because the ground is reconstituted land.
so they are worried about electrolisys.
- Not sure, but i think that is the full reason.
- We don't have a choice - just gotta do it.

It is important to mentally separate electrical earthing from the instrument grounding required for precise analogue control. Others have talked about the electrical grounding and this of course must be done properly. I strongly support using the concrete and its reinforcing for this purpose in a new installation.

However, regarding the instrument ground I insist on two independant instrument grounds installed at least 10Metres apart and cabled separately in 10sqmm cable to the control panel where they are terminated on an insulated earth bar. Before final connection to the earth bar I insist that the resistance between the separate cables be less than 1 Ohm.

I have installed many systems in India and know that particularly in the South it is very difficult to establish good grounding. But with the aid of many sacks of salt and a lot of water it is possible to get close to the 1 Ohm specification.

I reserve the instrument ground for connection to the screens of analogue wiring. In particular thermocouple wiring.

Thanks John,
I Don't do house wiring
never have - except my own home