Basic Electricity

Electric Shock

17 questions By Tony R. Kuphaldt

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Question 4 of 17

In the late 1700’s, an Italian professor of anatomy, Luigi Galvani, discovered that the leg muscles of a recently deceased frog could be made to twitch when subjected to an electric current. What phenomenon is suggested by Galvani’s discovery? In other words, what does this tell us about the operation of muscle fibers in living creatures? More importantly, what practical importance does this have for people working near electric circuits?

Reveal answer

Essentially, muscle fibers are “activated” by electrical signals. I’ll let you figure out what practical importance this effect has for you!

Notes:
This question presents an excellent opportunity to discuss one of the important aspects of electrical safety: involuntary muscle contraction.
Question 5 of 17

An American researcher named Charles Dalziel performed experiments with both human and animal subjects to determine the effects of electric currents on the body. A table showing his research data is presented here:

Important Note: Dalziel’s human test subjects were men and women in good health, with no known heart conditions or any other abnormalities that would have compromised their safety. In other words, these data points represent best-case scenarios, and do not necessarily reflect the risk to persons in poorer states of health.

Assuming a skin contact resistance of 600 Ω for a sweaty hand, 1000 Ω of resistance for foot-to-ground contact, 50 Ω internal body resistance, 70 Ω of resistance through the soil from the person’s location to the earth ground point, and a male victim, calculate the amount of voltage necessary to achieve each of the listed shock conditions (threshold of perception, pain, etc.) for the following circuit:

Reveal answer

• Slight sensation at point(s) of contact: 0.69 volts

• Threshold of bodily perception: 1.9 volts

• Pain, with voluntary muscle control maintained: 15.5 volts

• Pain, with loss of voluntary muscle control: 27.5 volts

• Severe pain and difficulty breathing: 39.6 volts

• Possible heart fibrillation after three seconds: 172 volts

Notes:
Not only does this question introduce students to the various levels of shock current necessary to induce deleterious effects in the (healthy) human body, but it also serves as a good exercise for Ohm’s Law, and for introducing (or reviewing) the concept of series resistances.

For the morbidly curious, Charles Dalziel’s experimentation conducted at the University of California (Berkeley) began with a state grant to investigate the bodily effects of sub-lethal electric current. His testing method was as follows: healthy male and female volunteer subjects were asked to hold a copper wire in one hand and place their other hand on a round, brass plate. A voltage was then applied between the wire and the plate, causing electrons to flow through the subject’s arms and chest. The current was stopped, then resumed at a higher level. The goal here was to see how much current the subject could tolerate and still keep their hand pressed against the brass plate. When this threshold was reached, laboratory assistants forcefully held the subject’s hand in contact with the plate and the current was again increased. The subject was asked to release the wire they were holding, to see at what current level involuntary muscle contraction (tetanus) prevented them from doing so. For each subject the experiment was conducted using DC and also AC at various frequencies. Over two dozen human volunteers were tested, and later studies on heart fibrillation were conducted using animal subjects.

Given that Dalziel tested subjects for the effects of a hand-to-hand shock current path, his data does not precisely match the scenario I show in the schematic diagram (hand-to-foot). Therefore, the calculated voltages for various hand-to-foot shock conditions are approximate only.
Question 6 of 17

Explain why birds do not get shocked when they perch on a power line, even if both of their feet touch the wire. Explain why birds become electrocuted if their wings bridge between two different power conductors:

For a schematic view of both scenarios, see this diagram:

Reveal answer

Perhaps the easiest way to explain the relative danger of these two scenarios is by way of voltage. The “safe” bird will not be shocked because there is negligible voltage between its feet (both feet resting on the same wire). Points connected directly together with low-resistance (conductive) wire are said to be electrically common, and should never have appreciable voltage between them.

The dead bird got that way because there is full source voltage between the two points of contact (each wing-tip touching a different wire).

Follow-up question: a safety device called a Faraday cage protects anyone inside from electrical shock. A Faraday cage is nothing more than a cage made of closely-spaced metal bars (or alternatively, made of metal wire mesh):

Explain how the principle of electrically common points protects the person inside the Faraday cage from being shocked by the outside source of high voltage. Discuss how this principle might apply to metal-frame automobiles and aircraft.

Notes:
Very vivid demonstrations have been performed with Van de Graaff generators and Tesla coils showing the protective nature of a Faraday cage. When students understand that substantial voltage cannot exist between electrically common points (at least at frequencies below RF!), not only are they prepared to understand the purpose of safety grounding in electrical systems, but they are also equipped with an extremely important concept for use in electrical troubleshooting.