Ohm’s Law (again!)

Chapter 3.4 Ohm’s Law (again!)

Lessons In Electric Circuits Volume I – DC Book
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Lessons In Electric Circuits Volume I – DC Book

  • 86CHAPTER 3. ELECTRICAL SAFETY• Special, insulated shoes and mats are made to protect persons from shock via groundconduction, but even these pieces of gear must be in clean, dry condition to be effective.Normal footwear is not good enough to provide protection from shock by insulating itswearer from the earth.• Though dirt is a poor conductor, it can conduct enough current to injure or kill a humanbeing.3.4Ohm’s Law (again!)A common phrase heard in reference to electrical safety goes something like this: ”It’s not volt-age that kills, its current!” While there is an element of truth to this, there’s more to understandabout shock hazard than this simple adage. If voltage presented no danger, no one would everprint and display signs saying: DANGER – HIGH VOLTAGE!The principle that ”current kills” is essentially correct. It is electric current that burnstissue, freezes muscles, and fibrillates hearts. However, electric current doesn’t just occur onits own: there must be voltage available to motivate electrons to flow through a victim. Aperson’s body also presents resistance to current, which must be taken into account.Taking Ohm’s Law for voltage, current, and resistance, and expressing it in terms of currentfor a given voltage and resistance, we have this equation:Ohm’s LawI =ERCurrent =VoltageResistanceThe amount of current through a body is equal to the amount of voltage applied betweentwo points on that body, divided by the electrical resistance offered by the body between thosetwo points. Obviously, the more voltage available to cause electrons to flow, the easier theywill flow through any given amount of resistance. Hence, the danger of high voltage: highvoltage means potential for large amounts of current through your body, which will injure orkill you. Conversely, the more resistance a body offers to current, the slower electrons will flowfor any given amount of voltage. Just how much voltage is dangerous depends on how muchtotal resistance is in the circuit to oppose the flow of electrons.Body resistance is not a fixed quantity. It varies from person to person and from time totime. There’s even a body fat measurement technique based on a measurement of electricalresistance between a person’s toes and fingers. Differing percentages of body fat give providedifferent resistances: just one variable affecting electrical resistance in the human body. Inorder for the technique to work accurately, the person must regulate their fluid intake forseveral hours prior to the test, indicating that body hydration is another factor impacting thebody’s electrical resistance.Body resistance also varies depending on how contact is made with the skin: is it from hand-to-hand, hand-to-foot, foot-to-foot, hand-to-elbow, etc.? Sweat, being rich in salts and minerals,is an excellent conductor of electricity for being a liquid. So is blood, with its similarly highcontent of conductive chemicals. Thus, contact with a wire made by a sweaty hand or openwound will offer much less resistance to current than contact made by clean, dry skin.