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8 questions By Tony R. Kuphaldt
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Question 1 of 8
In this motor control circuit, how would you ensure that there is no danger of electric shock prior to touching either of the motor terminals (shown as points A and B in the schematic diagram)? Describe both the action required to secure the power, and the means by which you would check for the presence of hazardous voltage at the motor:
Reveal answerFollow these steps:
- Open the circuit breaker.
- Lock the circuit breaker in its “open” position so no one can close it.
- Try to start the motor by turning the On/Off switch “on”.
- Leave the On/Off switch in the “off” position.
- Test for hazardous voltage (both AC and DC) between A and B, between A and ground, and between B and ground.
Notes:For each of the steps given in the answer, discuss the rationale with your students. Why is it important we do each one of those steps, in the order shown? How many voltage checks must we do with the voltmeter, total?
Also, be sure to ask your students how they would know whether or not their voltmeter was functioning properly prior to using it to check for the presence of hazardous voltage. What types of faults in the meter could cause it to not indicate voltage when there really was voltage?
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Question 2 of 8
Linemen working on high-voltage conductors do not simply rely on open disconnect switches to isolate sections of power lines from sources of electricity during maintenance. They also attach “grounding” cables from line to line, and then to earth ground like this:
Explain why this decreases the risk of electric shock for the linemen, based on what you know about electrically common points in a circuit.
Reveal answerBy connecting the three wires together, you make them electrically common to each other. This prevents any substantial voltage (potential difference) developing between them. Likewise, connecting the three wires to the earth makes them electrically common to the earth, preventing any substantial voltage from developing between any of the wires and ground. Know more about grounding.
Follow-up question: after the linemen are done with their work, they remove the grounding wires from the power lines before they close the disconnect switches. Explain why this is done, by describing the catastrophic consequences of closing the disconnect switches with the grounding wires still in place.
Notes:A physicist would describe such a “grounded” system as being one large equipotential surface. This is an important concept for students to grasp, not only for safety but also for the purpose of better understanding where voltage drops should and should not be in working circuits.
Some students may (wisely) ask how any voltage at all could be developed between the isolated conductors in the absence of grounding wires, since the disconnect switches are open at all points. Although it may be premature to discuss with your students how capacitive coupling with nearby (energized) conductors could cause voltages to appear between non-grounded conductors and ground (depending on their level of electrical understanding), you can still answer the question by appealing to a general sense of safety conservatism. With the wires all made electrically common to each other and to earth ground, there is still some measure of protection even in the event of one or more of the disconnect switches accidentally closing, a lightning strike, or a bird landing between the open poles of a disconnect switch.
If your students have not yet studied three-phase AC systems, they may (wisely) ask why three power line conductors are necessary instead of two. You may tell them that this is irrelevant to the safety question: that all they need to know is that there will be high voltage present between each wire pair (A and B, B and C, A and C) and between each wire and ground, when the power lines are in operation.
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Question 3 of 8
Describe what the phrase zero energy state means for a system, in terms of safety for those performing maintenance work on it.
Reveal answer“Zero energy state” means that all sources of energy, either conducted from an outside source or stored within components of the system, have been secured so as to pose no danger to the people working on the system. Learn about shock current path.
Identify some sources of energy in a system that could pose a safety hazard to people working on it.
Notes:When discussing this with your students, be sure to emphasize that “zero energy” is not just limited to electrical energy, but to all forms of energy that could cause harm. This includes mechanical and thermal energy as well.
