Electric Shock

Basic Electricity

• Question 1

It is sometimes said regarding electrical safety that, Ït’s not the voltage that will hurt you, it’s the current.” Why then are there signs reading Danger: High Voltage near electrical substations and on large pieces of electrical equipment, rather than signs reading Danger: High Current?

• Question 2

Why is this sign a joke?

• Question 3

Using Ohm’s Law, explain why personal protective equipment such as insulating gloves and boots help reduce the risk of electric shock.

• Question 4

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?

• Question 5

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:

• Question 6

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:

• Question 7

Briefly answer the following safety questions:

• What does electric current do to muscle tissue?
• What can electric current do to motor muscles (fingers, arms, legs)?
• What can electric current do to heart and lungs?
• Why should you use one hand while working on live power circuits? Which hand?
• Why is water dangerous when working around electric power?
• Why is metal jewelry dangerous to wear when working on electric circuits?
• What kinds of tools are best for working on live power circuits?
• What kind of immediate medical attention does an electric shock victim require?
• What footwear is appropriate when working around electric power?
• What footwear is not appropriate when working around electric power?
• Question 8

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 wire-to-hand contact resistance of 1500 Ω, 4400 Ω of resistance for foot-to-ground contact, 50 Ω internal body resistance, 200 Ω of resistance through the soil from the person’s location to the earth ground point, and a female victim, calculate the amount of voltage necessary to achieve each of the listed shock conditions (threshold of perception, pain, etc.) for the following circuit:

• Question 9

All other factors being equal, which possesses a greater potential for inducing harmful electric shock, DC electricity or AC electricity at a frequency of 60 Hertz? Be sure to back up your answer with research data!

• Question 10

Determine whether or not a shock hazard exists for a person standing on the ground, by touching any one of the points labeled in this circuit:

• Point “A”
• Point “B”
• Point “C”
• Point “D”
• Point “E”
• Question 11

The following recommendations came from a flyer published by an electric power utility. Read and comment on their instructions regarding downed power lines:

Assume any downed line is an energized power line. If a power line falls on your car while you are driving, slowly drive on until you are completely clear of the line (but do not drive over it). If your car is immobilized, stay in it until help arrives. Call for help from a cell phone if you have one. If you need to escape from a vehicle, such as for a car fire, jump clear of the car. Electricity is not only traveling through the vehicle, but may also be traveling in the ground around the area. Keep your arms crossed over your chest while you jump, and both feet together. Do NOT touch the vehicle and the ground at the same time. Once you land on the ground, shuffle clear of the area, keeping both feet together, on the ground, and touching at all times. Continue shuffling until you’re at least 30 feet from the accident site.

Why do you suppose the following actions were recommended?

• Do not drive over a downed power line.
• Stay in the car if possible.
Do not touch the car and the ground at the same time.
• Shuffle away from the car (rather than walk), with both feet together.
• Question 12

The ignition system in a spark-ignition automobile engine produces voltages in the range of tens of thousands of volts: greater than the voltage levels typically used to distribute electric power through neighborhoods. Although this is capable of producing very painful electric shocks, the actual shock hazard it poses to a person is minimal. Why is this?

• Question 13

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?

• Question 14

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?

• Question 15

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?

• Question 16

Determine whether or not a shock hazard exists for a person standing on the ground, by touching any one of the points labeled in this faulted circuit:

• Point “A”
• Point “B”
• Point “C”
• Point “D”
• Point “E”
• Question 17

What sort of immediate medical attention does a victim of severe electrical shock require? Explain why this attention is necessary.