Ground Resistance in Mining Industry

[sobeys81]

Hi...
You have been all of great help me in the past while answering my questions.

I am working on a grounding grid study and some how I found this document that talks about client requirements. I am just having a hard time understanding this. I would appreciate, if you could elaborate.
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" All grounds shall have a resistance to earth not exceeding 5 ohms throughout the year considering seasonal variations in soil conditions.
Grounds for low resistance grounded systems shall not exceeded 2 ohms.
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I have done some grounding grids in the past where based on CSA I have to only satisfy GPR, touch and step potential requirements but there was never a limit on ground resistance value. I did some research and found this link where it talks about 5 ohms but did not find no information about 2 ohms.

http://www.msha.gov/s&hinfo/techrpt/ground/groundre.htm

Thanks

 

[dpc]

The 2 ohms maybe referring to the grounding resistor. What voltage system is this?

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[sobeys81]

it is 4160 actually. I wonder, if it was a practice in the past to have 5 ohms and 2 ohms. I am just wondering, how this resistance ohm value can be a limitation.

I would appreciate a word.

Thanks

 

[dpc]

Without the actual document you are referring to, I can't say for sure what you are referring to. The quote in your original post doesn't make a lot of sense.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[NAZ55]

I think what you are referring to is more of an MSHA and typical industrial standard.

http://www.msha.gov/s&hinfo/techrpt/ground/groundre.htm

Please see extracts below from MSHA's site

"As previously mentioned, it has been established that the resistance of mine ground beds should be designed as low as practical with a 5 ohm value a design goal."




"In the real world, however, the ground system does have resistance. All ground beds, even the largest, have some measurable amount of resistance. "Earth resistance" is defined as the resistance of the earth to the passage of electric current. In comparison with metal conductors, soil is not a good conductor of electricity. Resistances in the two to five ohm range are generally found suitable for industrial plant substations, buildings and large commercial installations.

The National Electrical Code requires that "man made" electrodes shall have resistance to ground not to exceed 25 ohms and that where the resistance is not as low as 25 ohms, two or more electrodes connected in parallel shall be used. They should not be less than six feet apart.

"The 25 ohms value noted in the National Electrical Code applies to the maximum resistance for a single electrode. There is no implication that 25 ohms per se' is a satisfactory level for a grounding system."[2]

The Institute of Electrical and Electronics Engineers Standard 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems states: "The most elaborate grounding system that can be designed may prove to be inadequate unless the connection of the system to the earth is adequate and has a low resistance. It follows, therefore, that the earth connection is one of the most important parts of the whole grounding system. It is also the most difficult part to design and to obtain... For small substations and industrial plants in general, a resistance of less than 5 ohms should be obtained if practicable."[2] "

 

[NAZ55]

I should also add that a typical Utility standard practice it to obtain resistance of 1 ohm or below.

I believe the goal is to minimize GPR since higher grid impedance and higher ground fault current relate to higher GPR.

Since GPR = Ig x Zg

A typical utility grounding system can be designed for grid fault currents even as high as 80kA or more, in which case even 1 ohm impedance can contribute upto 80kV. Since we don't have much control our fault currents, impedance is the only comparatively easy number we can play with.