Learning Objectives

Learning Objectives

By the end of this section, you will be able to do the following:

  • Define thermal hazard, shock hazard, and short circuit
  • Explain what effects various levels of current have on the human body

There are two known hazards of electricity—thermal and shock. A thermal hazard is one where excessive electric power causes undesired thermal effects, such as starting a fire in the wall of a house. A shock hazard occurs when electric current passes through a person. Shocks range in severity from painful, but otherwise harmless, to heart-stopping lethality. This section considers these hazards and the various factors affecting them in a quantitative manner. Electrical Safety: Systems and Devices will consider systems and devices for preventing electrical hazards.

Thermal Hazards

Thermal Hazards

Electric power causes undesired heating effects whenever electric energy is converted to thermal energy at a rate faster than it can be safely dissipated. A classic example of this is the short circuit, a low-resistance path between terminals of a voltage source. An example of a short circuit is shown in Figure 3.24. Insulation on wires leading to an appliance has worn through, allowing the two wires to come into contact. Such an undesired contact with a high voltage is called a short. Since the resistance of the short, r,r, size 12{r} {} is very small, the power dissipated in the short, P=V2/r,P=V2/r, size 12{P = V rSup { size 8{2} } /r} {} is very large. For example, if VV size 12{V} {} is 120 V and rr size 12{r} {} is 0.100Ω,0.100Ω, size 12{0 "." "100" %OMEGA } {} then the power is 144 kW, much greater than that used by a typical household appliance. Thermal energy delivered at this rate will very quickly raise the temperature of surrounding materials, melting or perhaps igniting them.

Part a shows an electric toaster of resistance capital R connected to an A C voltage source. The wires used to connect the toaster to the supply are worn out in one place, allowing them to come into contact with an undesired, lower resistance path, symbolized by lowercase r. Part b of the figure represents the circuit diagram for the electric connection described in part a. The voltage source is connected to two paths in parallel: the toaster with resistance capital R, and the undesired lower resistance p
Figure 3.24 A short circuit is an undesired low-resistance path across a voltage source. (a) Worn insulation on the wires of a toaster allow them to come into contact with a low resistance r.r. size 12{r} {} Since P=V2/r,P=V2/r, size 12{P = V rSup { size 8{2} } /r} {} thermal power is created so rapidly that the cord melts or burns. (b) A schematic of the short circuit.

One particularly insidious aspect of a short circuit is that its resistance may actually be decreased due to the increase in temperature. This can happen if the short creates ionization. These charged atoms and molecules are free to move and, thus, lower the resistance r.r. size 12{r} {} Since P=V2/r,P=V2/r, size 12{P = V rSup { size 8{2} } /r} {} the power dissipated in the short rises, possibly causing more ionization, more power, and so on. High voltages, such as the 480-V AC used in some industrial applications, lend themselves to this hazard, because higher voltages create higher initial power production in a short.

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,P=I2Rw, size 12{P = I rSup { size 8{2} } R rSub { size 8{w} } } {} where RwRw size 12{R rSub { size 8{w} } } {} is the resistance of the wires and II size 12{I} {} the current flowing through them. If either II size 12{I} {} or RwRw size 12{R rSub { size 8{w} } } {} is too large, the wires overheat. For example, a worn appliance cord (with some of its braided wires broken) may have R w =2.00Ω R w =2.00Ω rather than the 0.100Ω0.100Ω size 12{0 "." "100" %OMEGA } {} it should be. If 10.0 A of current passes through the cord, then P=I2Rw=200 WP=I2Rw=200 W size 12{P = I rSup { size 8{2} } R rSub { size 8{w} } ="200"`W} {} is dissipated in the cord—much more than is safe. Similarly, if a wire with a 0.100Ω 0.100Ω 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=1,000W. P=1,000W. Fuses and circuit breakers are used to limit excessive currents. (See Figure 3.25 and Figure 3.26.) Each device opens the circuit automatically when a sustained current exceeds safe limits.

Part a of the figure shows an electric fuse with metal having low melting point enclosed in a case with wires leading to the circuit and voltage source. There is a viewing window in the fuse casing. Part b shows a circuit breaker. There is a movable metal strip at one end from which a connector to the circuit is attached at a fixed contact point. There is a compressed spring and switch gear attached adjacent to each other at the other end of the movable metal strip. The movable metallic strip has a bimeta
Figure 3.25 (a) A fuse has a metal strip with a low melting point that, when overheated by an excessive current, permanently breaks the connection of a circuit to a voltage source. (b) A circuit breaker is an automatic but restorable electric switch. The one shown here has a bimetallic strip that bends to the right and into the notch if overheated. The spring then forces the metal strip downward, breaking the electrical connection at the points.
The diagram shows an electric circuit with an A C voltage source, a fuse or circuit breaker, and a resistance R all connected in series to form a closed circuit.
Figure 3.26 Schematic of a circuit with a fuse or circuit breaker in it. Fuses and circuit breakers act like automatic switches that open when sustained current exceeds desired limits.

Fuses and circuit breakers for typical household voltages and currents are relatively simple to produce, but those for large voltages and currents experience special problems. For example, when a circuit breaker tries to interrupt the flow of high-voltage electricity, a spark can jump across its points that ionizes the air in the gap and allows the current to continue. Large circuit breakers found in power-distribution systems employ insulating gas and even use jets of gas to blow out such sparks. Here AC is safer than DC, since AC current goes through zero 120 times per second, giving a quick opportunity to extinguish these arcs.

Shock Hazards

Shock Hazards

Electrical currents through people produce tremendously varied effects. An electrical current can be used to block back pain. The possibility of using electrical current to stimulate muscle action in paralyzed limbs, perhaps allowing paraplegics to walk, is under study. TV dramatizations in which electrical shocks are used to bring a heart attack victim out of ventricular fibrillation (a massively irregular, often fatal, beating of the heart) are more than common. Yet most electrical shock fatalities occur because a current put the heart into fibrillation. A pacemaker uses electrical shocks to stimulate the heart to beat properly. Some fatal shocks do not produce burns, but warts can be safely burned off with electric current (though freezing using liquid nitrogen is now more common). Of course, there are consistent explanations for these disparate effects. The major factors upon which the effects of electrical shock depend are as follows:

  1. The amount of current II size 12{I} {}
  2. The path taken by the current
  3. The duration of the shock
  4. The frequency ff size 12{f} {} of the current (f=0f=0 size 12{f=0} {} for DC)

Table 3.3 gives the effects of electrical shocks as a function of current for a typical accidental shock. The effects are for a shock that passes through the trunk of the body, has a duration of 1 s, and is caused by 60-Hz power.

Part a of the diagram shows a person working on an electrically hot wire with a metal tool. The next step shows that he is a victim of electric shock and is thrown backward with his arms and legs stretched. The metal tool also falls off his hand. Part b of the diagram shows a person holding the electrically hot wire with his hands. The person is not thrown away. He cannot let go of the wire because the muscles that close the fingers are stronger than those that open them.
Figure 3.27 An electric current can cause muscular contractions with varying effects. (a) The victim is thrown backward by involuntary muscle contractions that extend the legs and torso. (b) The victim can't let go of the wire that is stimulating all the muscles in the hand. Those that close the fingers are stronger than those that open them.
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
6,000 (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
Table 3.3 Effects of Electrical Shock as a Function of Current3

Our bodies are relatively good conductors due to the water in our bodies. Given that larger currents will flow through sections with lower resistance (to be further discussed in the next chapter), electric currents preferentially flow through paths in the human body that have a minimum resistance in a direct path to Earth. Earth is a natural electron sink. Wearing insulating shoes, a requirement in many professions, prohibits a pathway for electrons by providing a large resistance in that path. Whenever working with high-power tools (drills), or in risky situations, ensure that you do not provide a pathway for current flow (especially through 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 10-20 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 3.27(a). More frightening, and potentially more dangerous, is the can't let go effect illustrated in Figure 3.27(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 100–300 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,I=V/R, size 12{I = ital "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 200kΩ. 200kΩ. If he comes into contact with 120-V AC, a current I=(120 V)/(200 k Ω) = 0.6 mA I=(120 V)/(200 k Ω) = 0.6 mA size 12{I = \( "120 V" \) / \( "200 k " %OMEGA \) " = 0" "." "6 mA "} {} passes harmlessly through him. The same person soaking wet may have a resistance of 10.0 kΩ10.0 kΩ size 12{"10" "." 0" k" %OMEGA } {} 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 1/1,000 those listed in Table 3.3 produce similar effects. During open-heart surgery, currents as small as 20 μA20 μA size 12{"20" μ"A"} {} 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.

The graph of average values for the threshold of sensation and the Can't let go current as a function of frequency, with current in milliamperes verses frequency in hertz. The current is plotted along the vertical axis and frequency along the horizontal axis. The plot has two curves. The curve for Can't let go current starts off at a value nearly eighteen milliamps on the vertical axis. The curve is smooth and dips until frequency equals about one hundred hertz and then rises for values of frequency above
Figure 3.28 Graph of average values for the threshold of sensation and the can't let go current as a function of frequency. The lower the value, the more sensitive the body is at that frequency.

Factors other than current that affect the severity of a shock are its path, duration, and AC frequency. Path has obvious consequences. For example, the heart is unaffected by an electric shock through the brain, which treats disorders in the chemistry of the brain. And it is a general truth that the longer the duration of a shock, the greater its effects. Figure 3.28 presents a graph that illustrates the effects of frequency on a shock. The curves show the minimum current for two different effects, as a function of frequency. The lower the current needed, the more sensitive the body is at that frequency. Ironically, the body is most sensitive to frequencies near the 50- or 60-Hz frequencies in common use. The body is slightly less sensitive for DC (f=0),(f=0), size 12{f=0} {} mildly confirming Edison's claims that AC presents a greater hazard. At higher and higher frequencies, the body becomes progressively less sensitive to any effects that involve nerves. This is related to the maximum rates at which nerves can fire or be stimulated. At very high frequencies, electrical current travels only on the surface of a person. Thus a wart can be burned off with very high frequency current without causing the heart to stop. (Do not try this at home with 60-Hz AC!) Some of the spectacular demonstrations of electricity, in which high-voltage arcs are passed through the air and over people's bodies, employ high frequencies and low currents. (See Figure 3.29.) Electrical safety devices and techniques are discussed in detail in Electrical Safety: Systems and Devices.

Photograph of an electric arc produced between two multi stranded wires close to each other but not in contact.
Figure 3.29 Is this electric arc dangerous? The answer depends on the AC frequency and the power involved. (Khimich Alex, Wikimedia Commons)


  • 3 For an average male shocked through trunk of body for 1 s by 60-Hz AC. Values for females are 60–80 percent of those listed.