I was watching a lecture on earth grounding in which the professor assumed what would happen if we do not earth our appliances. He said that if the hot wire some how touched the outer body of the appliance and we came into contact with it, while it's ON, we will fell a terrible shock if we are bare footed. Then he did the calculations in which he considered the whole 120V across the body of the person.

I wonder why he did it, because during electrocution the current goes through the body to earth and then all the way back to the grounded wire of the transformer, I assume that the the section of the earth between our feet and the grounded transformer wire should be in series with our body? And because earth should have some resistance, the current through our body would not be as large as was calculated by the professor where he neglected the resistance of the earth. Why did he neglect the resistance of earth?


1 Answer 1


Since we can't read the professor's mind, it is impossible to definitively answer the question "Why did he neglect the resistance of earth?".

The most likely answer is that whilst the resistance of the earth is definitely a factor in the current going through a body in this instance, it is highly variable, ranging in it's effect on the electric shock from insignificant in the case of an earthed wet metal floor to highly significant in the case of a dry rubber mat.

Whether it was appropriate for the professor to ignore the earth resistance would depend on the context of the lecture. If, as seems to be the case, the context was one of warning of the risk of electric shock, it would seem to be appropriate to neglect the earth resistance, as doing so effectively matches the worst case scenario.

It is common and indeed necessary from a practical sense to ignore some aspects of electric circuits, especially when teaching. For example, it would likely overwhelm many students if in that lecture the professor had included taking into account the reactance of every wire, switch, fuse, circuit breaker, solder joint, and other connection in the circuit, and would have detracted from the message.


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