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Electric Shock
17 questions By Tony R. Kuphaldt
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Question 13 of 17
If you scuff your feet across a carpeted surface on a dry day, you will produce an electric potential resulting from a static electric charge that may range in the order of tens of thousands of volts! Can this pose a danger to you, at least in principle?
Reveal answerStatic electric charges rarely pose any shock hazard, because the actual charge quantity (measured in coulombs) is so small that the resulting current upon discharge can only last a very brief moment in time.
Notes:This question is a good starting point for a discussion on time as a variable in determining electric shock hazard. There is more to determining hazard to the human body than a simple assessment of volts, amps, and ohms!
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Question 14 of 17
Suppose the foot-ground contact resistance of a person standing barefoot is 3 kΩ, the contact resistance between a person’s hand and a held wire is 10 kΩ, and the person’s hand-to-foot internal body resistance is 500 Ω. How much danger would a person be in, given these conditions, if the voltage between the wire they were holding and the ground they were standing on was 120 volts, 60 Hz alternating current?
Reveal answerAccording to research done by Charles F. Dalziel, the amount of current that would go through this individual would be sufficient to cause pain, but not enough to cause loss of voluntary muscle control.
Notes:This question is not only a practical exercise of Ohm’s Law, but also a stimulus for researching shock current levels and their effects on the human body. Be sure pursue a line of questioning here during discussion time to explore the effects of electric current on the human body for a wide range of currents. In other words, don’t just leave the answer at “causes pain, but muscle control maintained,” but rather use this time to investigate what levels of current cause more (and less) serious effects on the human body.
Also, be sure to point out that the health of the individual being shocked is important to consider. Dalziel’s work was based on healthy individuals, who had been medically screened for this testing. A person in ill health may not fare as well in the same scenario.
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Question 15 of 17
Suppose you are an electrician, and you need to disconnect the power conductors from a large electric motor. What steps should you take to ensure no shock hazard exists prior to touching the bare conductors?
Reveal answer- • Turn the motor off (using the start/stop switch).
- • Shut off the circuit breaker (or disconnect switch) providing electrical power to the motor.
- • Lock and tag the circuit breaker (or disconnect switch) so no one else will turn it on in your absence.
- • Check the operation of a voltmeter by connecting it to a known source of voltage.
- • Check for the presence of voltage at the conductor terminations with a voltmeter.
- • Re-check the operation of a voltmeter by connecting it to a known source of voltage.
- • As a final step, touch the bare conductor with the back of your right hand, before touching it in any other way.
Follow-up questions:
- • Why check the voltmeter both before and after testing for voltage at the motor connections?
- • Why touch the conductor with the back of your hand? What does it matter which side of your hand should touch the bare conductor?
- • Why use your right hand? Why not your left?
Notes:A lot of electrical safety principles are covered in this one question. Be sure to spend adequate time with the follow-up questions, to be sure students understand exactly why the given steps are necessary to ensure safety.
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