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Safety Grounding
12 questions By Tony R. Kuphaldt
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Question 10 of 12
Special safety devices called Ground Fault Current Interrupters, or GFCI’s, reduce the risk of shock hazard in electrical systems even where there is no ground conductor. Explain the operating principle of a GFCI. How are they able to sense a “ground fault” condition, so as to automatically turn off power to a receptacle?
Reveal answer“GFCI” devices work on the principle of differential current measurement as a means to sense ground faults.
Notes:Be sure to discuss with your students what “differential current measurement” means, rather than assume they all researched the answer in greater detail than what is provided here. This really is a clever way to detect the presence of a ground fault!
You should point out to your students that GFCI power receptacles are commonly installed in “wet” areas of residences, such as in the bathroom and outside, where the hazard of ground-fault electric shock is maximized by the presence of water.
This question also provides an opportunity to discuss why the presence of water at a point of bodily contact increases the severity of electric shock.
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Question 11 of 12
Suppose a GFCI is rated to “trip” with a ground fault current of 5 mA. Given a source voltage of 115 volts, calculate the ground fault resistance range required to trip.
Reveal answerAny resistance value between 0 Ω to 23 kΩ
Notes:This question requires the use of Ohm’s Law, but to calculate a range of resistance values and not just a singular value. All students should be able to obtain the figure of 23 kΩ, but some may be confused as to whether this is a minimum or a maximum resistance value needed to trip the GFCI. Discuss this with your students, asking them to explain how we know 23 kΩ is a maximum figure and not a minimum figure.
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Question 12 of 12
I once watched a television advertisement for an electric drill, in which one of the people selling this product claimed it was “double-insulated,” to which the other person replied, “So it doesn’t get hot in your hand, right?”
What do you think of this comment? Is this what “double-insulated” means?
Reveal answerWhile this advertisement was amusing, it was not very informative. No, the term “double-insulated” does not refer to thermal insulation, but rather electrical insulation.
Notes:One fact of interest here is that double-insulated tools usually do not have three-prong power plugs. In other words, they do not need a “ground” wire to ensure operator safety.
For question 9, how are you supposed to distinguish between ground and neutral using a voltmeter? Is there an assumption that the bottom conductor is ground?