Suppose that a man is working on the roof of a mobile home with a hand drill. It is early in the day, the man is barefoot, and dew covers the mobile home. The ground prong on the electrical plug of the drill has been removed. Will the man be shocked if the “hot” electrical line shorts to the

Question

Suppose that a man is working on the roof of a mobile home with a hand drill. It is early in the day, the man is barefoot, and dew covers the mobile home. The ground prong on the electrical plug of the drill has been removed. Will the man be shocked if the “hot” electrical line shorts to the case of the drill?

Accepted Answer

To analyze this problem, we must construct a model that adequately represents the situation described. In his book Medical Instrumentation (Boston: Houghton Mifflin, 1978), John G. Webster suggests the following values for resistance of the human body: [latex]R_{skin}[/latex](dry) = 15 kΩ [latex]R_{skin}[/latex](wet) = 150 Ω, [latex]R_{limb}[/latex](arm or leg) = 100 Ω, and [latex]R_{trunk}[/latex] = 200 Ω.
The network model is shown in Fig. 9.28. Note that since the ground line is open-circuited, a closed path exists from the hot wire through the short, the human body, the mobile home, and the ground. For the conditions stated previously, we assume that the surface contact resistances [latex]R_{sc_{1}}[/latex] and [latex]R_{sc_{2}}[/latex] are 150 Ω each. The body resistance, [latex]R_{body}[/latex], consisting of arm, trunk, and leg, is 400 Ω. The mobile home resistance is assumed to be zero, and the ground resistance, [latex]R_{gnd}[/latex], from the mobile home ground to the actual source ground is assumed to be 1 Ω. Therefore, the magnitude of the current through the body from hand to foot would be

[latex]I_{body} = \frac{120}{R_{sc_{1}} + R_{body} + R_{sc_{2}} + R_{gnd}}[/latex]
= [latex]\frac{120}{701}[/latex]
= 171 mA

A current of this magnitude can easily cause heart failure.
Additional protection would be provided if the circuit breaker were a ground fault interrupter.

Question 9.18: Suppose that a man is working on the roof of a mobile home w...

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