Earthing Resistance

Any resistance with a current flowing thru it, have an inherent (and proportional)voltage drop across itself. And you want to minimize the potential difference between ground and the grounded device.

Yahlasit

The limiting values of maximum earth resistance are clearly indicated in whatever standard (be it building or equipment) you are working to and depend on the equipment and application. As for the decision behind how these limits were chosen in the first place, it all has to do with minimising the risk to people and property while at the same time making it practical (and economical) for builders and product designers to meet these limits.

"As far as I know, no standard, I repeat, no standard - National or International - has anywhere specied a statutory maximum value to be maintained for Earthing System Resistance"

The Ontario Electrical Safety Code does not give a specific value, but does say that the value should be low enough to limit potential rise of any grounded equipment to 5000 volts, (rule 36-304), this section of the Code deals specifically with installations over 750 volts. Depending on available short circuit current, this could require the resistance to ground to be less than 1 ohm.

I have to respond to the comments listed and make sure the facts are correct. I am a master electrician, master inspector and MSHA certified for low medium and high voltage. In the 2008 NEC code book ART. 250.56- if the resistance of a single grounding rod is over 25 ohms is shall be augmentd by an additional ground rod. In mining for substation locations 2 ground fields are required. 1 for the grounded conductor and 1 for the grounding conductor. Both ground fields are

limited to only a few ohms of resistance

Your question caught my eye because I compiled data on electrical differences between strata and distant locations up to a kilometer. This was to study the geochemical activity in an area of Northern California. But accuracy was lacking due to the various sources of these differences was skewed seasonally, thermally, time of day, moon position (affecting water flows in mountain tributaries) cloud motion and traffic all affected the results. To help reign in these oddities I sought to find a site that was most solidly connected to the rest of the planet and choose a site where lightening had struck more often than the nearby countryside for my study.

I decided to quit this project after my gear was destroyed by lightening three times in a month.