Electric Hazards and the Human Body
Learning Objectives
By the end of this section, you will be able to:- Define thermal hazard, shock hazard, and short circuit.
- Explain what effects various levels of current have on the human body.
Thermal Hazards

Another serious, but less dramatic, thermal hazard occurs when wires supplying power to a user are overloaded with too great a current. As discussed in the previous section, the power dissipated in the supply wires is P = I2Rw, where Rw is the resistance of the wires and I the current flowing through them. If either I or Rw is too large, the wires overheat. For example, a worn appliance cord (with some of its braided wires broken) may have Rw = 2.00 Ω rather than the 0.100 Ω it should be. If 10.0 A of current passes through the cord, then P = I2Rw = 200 W is dissipated in the cord—much more than is safe. Similarly, if a wire with a 0.100-Ω resistance is meant to carry a few amps, but is instead carrying 100 A, it will severely overheat. The power dissipated in the wire will in that case be P = 1000 W. Fuses and circuit breakers are used to limit excessive currents. (See Figure 1 and Figure 2.) Each device opens the circuit automatically when a sustained current exceeds safe limits.


Shock Hazards
- The amount of current I
- The path taken by the current
- The duration of the shock
- The frequency f of the current (f=0 for DC)

Current (mA) | Effect |
---|---|
1 | Threshold of sensation |
5 | Maximum harmless current |
10–20 | Onset of sustained muscular contraction; cannot let go for duration of shock; contraction of chest muscles may stop breathing during shock |
50 | Onset of pain |
100–300+ | Ventricular fibrillation possible; often fatal |
300 | Onset of burns depending on concentration of current |
6000 (6 A) | Onset of sustained ventricular contraction and respiratory paralysis; both cease when shock ends; heartbeat may return to normal; used to defibrillate the heart |
Very small currents pass harmlessly and unfelt through the body. This happens to you regularly without your knowledge. The threshold of sensation is only 1 mA and, although unpleasant, shocks are apparently harmless for currents less than 5 mA. A great number of safety rules take the 5-mA value for the maximum allowed shock. At 10 to 20 mA and above, the current can stimulate sustained muscular contractions much as regular nerve impulses do. People sometimes say they were knocked across the room by a shock, but what really happened was that certain muscles contracted, propelling them in a manner not of their own choosing. (See Figure 3(a).) More frightening, and potentially more dangerous, is the “can’t let go” effect illustrated in Figure 3(b). The muscles that close the fingers are stronger than those that open them, so the hand closes involuntarily on the wire shocking it. This can prolong the shock indefinitely. It can also be a danger to a person trying to rescue the victim, because the rescuer’s hand may close about the victim’s wrist. Usually the best way to help the victim is to give the fist a hard knock/blow/jar with an insulator or to throw an insulator at the fist. Modern electric fences, used in animal enclosures, are now pulsed on and off to allow people who touch them to get free, rendering them less lethal than in the past.
Greater currents may affect the heart. Its electrical patterns can be disrupted, so that it beats irregularly and ineffectively in a condition called “ventricular fibrillation.” This condition often lingers after the shock and is fatal due to a lack of blood circulation. The threshold for ventricular fibrillation is between 100 and 300 mA. At about 300 mA and above, the shock can cause burns, depending on the concentration of current—the more concentrated, the greater the likelihood of burns.
Very large currents cause the heart and diaphragm to contract for the duration of the shock. Both the heart and breathing stop. Interestingly, both often return to normal following the shock. The electrical patterns on the heart are completely erased in a manner that the heart can start afresh with normal beating, as opposed to the permanent disruption caused by smaller currents that can put the heart into ventricular fibrillation. The latter is something like scribbling on a blackboard, whereas the former completely erases it. TV dramatizations of electric shock used to bring a heart attack victim out of ventricular fibrillation also show large paddles. These are used to spread out current passed through the victim to reduce the likelihood of burns.
Current is the major factor determining shock severity (given that other conditions such as path, duration, and frequency are fixed, such as in the table and preceding discussion). A larger voltage is more hazardous, but since I = V/R, the severity of the shock depends on the combination of voltage and resistance. For example, a person with dry skin has a resistance of about 200 kΩ. If he comes into contact with 120-V AC, a current I = (120 V)/(200 k Ω) = 0.6 mA passes harmlessly through him. The same person soaking wet may have a resistance of 10.0 kΩ and the same 120 V will produce a current of 12 mA—above the “can’t let go” threshold and potentially dangerous.
Most of the body’s resistance is in its dry skin. When wet, salts go into ion form, lowering the resistance significantly. The interior of the body has a much lower resistance than dry skin because of all the ionic solutions and fluids it contains. If skin resistance is bypassed, such as by an intravenous infusion, a catheter, or exposed pacemaker leads, a person is rendered microshock sensitive. In this condition, currents about 1/1000 those listed in Table 1 produce similar effects. During open-heart surgery, currents as small as 20 μA can be used to still the heart. Stringent electrical safety requirements in hospitals, particularly in surgery and intensive care, are related to the doubly disadvantaged microshock-sensitive patient. The break in the skin has reduced his resistance, and so the same voltage causes a greater current, and a much smaller current has a greater effect.


Section Summary
- The two types of electric hazards are thermal (excessive power) and shock (current through a person).
- Shock severity is determined by current, path, duration, and AC frequency.
- Table 1 lists shock hazards as a function of current.
- Figure 5 graphs the threshold current for two hazards as a function of frequency.
Conceptual Questions
- Using an ohmmeter, a student measures the resistance between various points on his body. He finds that the resistance between two points on the same finger is about the same as the resistance between two points on opposite hands—both are several hundred thousand ohms. Furthermore, the resistance decreases when more skin is brought into contact with the probes of the ohmmeter. Finally, there is a dramatic drop in resistance (to a few thousand ohms) when the skin is wet. Explain these observations and their implications regarding skin and internal resistance of the human body.
- What are the two major hazards of electricity?
- Why isn’t a short circuit a shock hazard?
- What determines the severity of a shock? Can you say that a certain voltage is hazardous without further information?
- An electrified needle is used to burn off warts, with the circuit being completed by having the patient sit on a large butt plate. Why is this plate large?
- Some surgery is performed with high-voltage electricity passing from a metal scalpel through the tissue being cut. Considering the nature of electric fields at the surface of conductors, why would you expect most of the current to flow from the sharp edge of the scalpel? Do you think high- or low-frequency AC is used?
- Some devices often used in bathrooms, such as hairdryers, often have safety messages saying "Do not use when the bathtub or basin is full of water." Why is this so?
- We are often advised to not flick electric switches with wet hands, dry your hand first. We are also advised to never throw water on an electric fire. Why is this so?
- Before working on a power transmission line, linemen will touch the line with the back of the hand as a final check that the voltage is zero. Why the back of the hand?
- Why is the resistance of wet skin so much smaller than dry, and why do blood and other bodily fluids have low resistances?
- Could a person on intravenous infusion (an IV) be microshock sensitive?
- In view of the small currents that cause shock hazards and the larger currents that circuit breakers and fuses interrupt, how do they play a role in preventing shock hazards?
Problems & Exercises
1. (a) How much power is dissipated in a short circuit of 240-V AC through a resistance of 0.250 Ω? (b) What current flows?
2. What voltage is involved in a 1.44-kW short circuit through a 0.100-Ω resistance?
3. Find the current through a person and identify the likely effect on her if she touches a 120-V AC source: (a) if she is standing on a rubber mat and offers a total resistance of 300 kΩ; (b) if she is standing barefoot on wet grass and has a resistance of only 4000 kΩ.
4. While taking a bath, a person touches the metal case of a radio. The path through the person to the drainpipe and ground has a resistance of 4000 Ω. What is the smallest voltage on the case of the radio that could cause ventricular fibrillation?
5. Foolishly trying to fish a burning piece of bread from a toaster with a metal butter knife, a man comes into contact with 120-V AC. He does not even feel it since, luckily, he is wearing rubber-soled shoes. What is the minimum resistance of the path the current follows through the person?
6. (a) During surgery, a current as small as 20.0 μA applied directly to the heart may cause ventricular fibrillation. If the resistance of the exposed heart is 300 Ω, what is the smallest voltage that poses this danger? (b) Does your answer imply that special electrical safety precautions are needed?
7. (a) What is the resistance of a 220-V AC short circuit that generates a peak power of 96.8 kW? (b) What would the average power be if the voltage was 120 V AC?
8. A heart defibrillator passes 10.0 A through a patient’s torso for 5.00 ms in an attempt to restore normal beating. (a) How much charge passed? (b) What voltage was applied if 500 J of energy was dissipated? (c) What was the path’s resistance? (d) Find the temperature increase caused in the 8.00 kg of affected tissue.
9. Integrated Concepts A short circuit in a 120-V appliance cord has a 0.500-Ω resistance. Calculate the temperature rise of the 2.00 g of surrounding materials, assuming their specific heat capacity is 0.200 cal/g ⋅ ºC and that it takes 0.0500 s for a circuit breaker to interrupt the current. Is this likely to be damaging?
10. Temperature increases 860ºC. It is very likely to be damaging.
11. Construct Your Own Problem Consider a person working in an environment where electric currents might pass through her body. Construct a problem in which you calculate the resistance of insulation needed to protect the person from harm. Among the things to be considered are the voltage to which the person might be exposed, likely body resistance (dry, wet, …), and acceptable currents (safe but sensed, safe and unfelt, …).
Glossary
- thermal hazard:
- a hazard in which electric current causes undesired thermal effects
- shock hazard:
- when electric current passes through a person
- short circuit:
- also known as a "short," a low-resistance path between terminals of a voltage source
- microshock sensitive:
- a condition in which a person’s skin resistance is bypassed, possibly by a medical procedure, rendering the person vulnerable to electrical shock at currents about 1/1000 the normally required level
Selected Solutions to Problems & Exercises
1. (a) 230 kW (b) 960 A3. (a) 0.400 mA, no effect (b) 26.7 mA, muscular contraction for duration of the shock (can't let go)
5. 1.20 × 105 Ω
7. (a) 1.00 Ω (b) 14.4 kW