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# What is a safe earthing resistance?

Everybody would definitely prefer 0.0 Ohm resistance as well. However, discussing earth resistance (a.k.a. combined resistance of earthing system to the general mass of earth) involves answering the question: what is the purpose of making this resistance as low as practically possible? Or, better yet, what conditions the earthing arrangement needs to satisfy?

Does 1.0 Ohm resistance makes sense, if a \$0.5MM facility is installed on solid limestone and achieving 1.0 Ohm resistance costs another \$0.5MM? It's actually not unheard of here in Middle East. I specifically leave granite formation out of discussion, as achieving such values there could cost millions of dollars. Or do we get required level of safety for personnel working in a facility with 0.1 Ohm earth resistance and 63kA of available short circuit current from the equipment? 6300V of voltage drop on such the resistance – isn't it enough to kill anybody?

Thus, this resistance alone does not make much sense. It can basically be of any value, as soon as the earthing arrangement provides: i) step voltage & touch voltage levels below safe ones under conditions of s/c with maximum available current at the least favorable place; ii) enough current for relay protection under least favorable conditions to sense s/c and isolate faulty equipment; iii) ability to withstand the maximum available s/c current without getting damaged by electromagnetic forces and generated heat during longest possible time that takes the relay protection to operate. You can satisfy all of the above by placing copper mesh under your switchgear, for example, as soon as you stand on this mesh. But what would be combined resistance of earthing system to the general mass of earth then?

As long as step potential / touch potential are in safe limits, current doesn't flow through the body of the person, so it is safe for person to enter/walk in the switch-yard.

When person is on this earth grid/mesh, he is safe to walk/touch all ground connected equipment even during fault because there is no voltage difference between his legs, hand & leg etc which can cause current to flow through the body.

Further earth grid/mesh is normally provided at a depth of 600mm below ground level and earth electrodes of depth / length approx >= 2500mm are provided & are solidly connected to this earth mesh / grid, to ensure that fault current flows down the earth grid/mesh not to the equipment/person above ground. Resistance / potential may be there but what matters more is potential difference not just potential.

Size of earth mesh/grid & earth electrode with qty etc are calculated such that earth mesh/electrode can take max fault current for a specified period of time within which protection would operate and isolate the fault.

For some equipments (like TR neutral etc.) dedicated earth electrode is being provided without connection to earth grid/mesh (though many do not recommend this, but is being followed at many places), then earth resistance of this electrode becomes important because this single rod has to carry specified fault current for specified time & anyone touching the same during fault would obviously get shock due to potential between this rod & ground on which person is standing is not same. For this reason for all equipment earth electrode should be connected to earth grid/mesh or one more electrode should be provided which will be connected to earth mesh/grid.