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Fowler Associates Labs

 

 

Static Fire Stories Articles & Technical Papers Current News

Response to Steve Fowler's Paper "Electrical Hazard"

by Bill Klein

Electrical Hazard
click here to view Steve's original paper

Your technical note entitled "Electrical Hazard" contains useful compilation of information regarding shock hazards. Unfortunately, it contains an error which also is significant from a technical standpoint, although probably not of much practical significance. You stated, quoting directly from the Merck Manual that body resistance to the flow of electricity can be expressed as a function of skin resistance, which is essentially true since most of the resistance of the body resides in the skin, but went on to say the unit of resistance is ohms per square cm. This is wrong. The usually quoted handbook figure for skin (body) resistance is based on a contact area of 1cm2 but to be dimensionally correct it must be expressed as ohms-cm2 not as ohms/cm2. Clearly, a larger area has lower resistance than a smaller one, other things being equal, which of course they almost never are. We might alternately use conductivity rather than resistivity, expressing conductivity as Siemens/cm2 in that most engineers are more comfortable with resistivity.

All of this is rather academic since skin contact areas are not usually very well defined and do not vary widely (not counting sitting in the bath tubs) but the range of inherent skin conductivities varies tremendously, a factor of thousands as pointed out in your article. Earlier editions of the Merck Manual apparently recognized this and spoke of skin and membrane resistances in ohms without explicitly considering contact areas. Later editions added the terms ohms/cm2, possibly as a result of criticism for incomplete definition but mixed up the dimensions to replace a somewhat vague characterization with an incorrect one. This is inexcusable for MDs but not EEs.

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