Electric Current, Resistance, and Ohm’s Law
24 Electric Hazards and the Human Body
Learning Objectives
- Define and distinguish between thermal hazards, shock hazards, and short circuits in electrical systems.
- Explain how excessive electric power leads to overheating and potential fire hazards in conductors and circuits.
- Describe how electric current affects the human body, including thresholds for sensation, muscle contraction, respiratory effects, and ventricular fibrillation.
- Identify the key factors that determine the severity of an electrical shock, including current, path, duration, and frequency.
- Use the relationship [latex]I=\frac{V}{R}[/latex] to explain how voltage and body resistance influence shock risk.
- Explain why wet conditions and medical procedures (for example, catheters and pacemakers) increase electrical danger by reducing body resistance and increasing microshock sensitivity.
Electrical Hazards and Safety
Electricity is essential in modern life and healthcare, powering everything from household appliances to life-saving medical equipment. When used properly it is extremely safe, but electrical systems can become hazardous when excessive current produces unwanted heating or when current passes through the human body. Understanding these hazards is especially important in healthcare environments, where patients may have reduced natural protection because of medical devices or invasive procedures.
Electrical hazards are generally classified into two categories. A thermal hazard occurs when electrical energy is converted into heat rapidly enough to damage equipment or ignite surrounding materials. A shock hazard occurs when electric current passes through the body. Electrical shocks range from mildly uncomfortable to life-threatening, depending on the amount of current, the path it follows, the duration of exposure, and the frequency of the current. In this section we examine these hazards quantitatively and explain the physics behind common electrical safety practices. The following chapter discusses the devices and systems used to prevent these hazards.
Thermal Hazards
Electrical power produces heating whenever electrical energy is converted into thermal energy. Normally this heating is controlled and useful—for example, in toasters, electric heaters, or surgical cautery devices. Problems arise when electrical energy is converted into heat faster than it can be safely dissipated.
A classic example is a short circuit, an unintended low-resistance path between the terminals of a voltage source. One example is shown in Figure 24.1. If the insulation on two wires becomes damaged, the conductors may come into direct contact, creating a path with very little resistance. Because the resistance is so small, Ohm's law predicts a very large current, and the electrical power dissipated in the short,
can become enormous. For example, if [latex]V=120~\text{V}[/latex] and the short has a resistance of only [latex]0.100~\Omega[/latex], the power dissipated is
This is hundreds of times greater than the power consumed by a typical household appliance. Such intense heating can quickly melt insulation, damage equipment, or ignite nearby combustible materials.
Short circuits can become even more dangerous because the high temperatures they produce may ionize the surrounding air. Ionized air contains free charged particles that reduce the resistance of the path even further. Since the power dissipated is inversely proportional to resistance, the heating increases even more, creating a positive feedback process that can rapidly produce electrical arcs or fires. This effect is particularly important in high-voltage systems.
Another common thermal hazard occurs when conductors carry more current than they were designed to handle. The power dissipated in a wire is
where [latex]R_{\mathrm{w}}[/latex] is the resistance of the wire. Because the power depends on the square of the current, even a modest increase in current can produce a large increase in heating. For example, if a damaged appliance cord has an unusually high resistance, or if too many high-power devices are connected to the same circuit, the resulting heating may exceed safe limits. To prevent this, electrical circuits are protected by fuses and circuit breakers, which automatically disconnect the circuit whenever the current remains above a safe value for too long.
Health and Safety Connection
Thermal hazards are not limited to electrical fires. In hospitals and laboratories, damaged cables or overloaded equipment can overheat, potentially damaging sensitive instruments or causing burns to patients and healthcare workers. Proper grounding, routine inspection of equipment, and adherence to electrical safety standards help minimize these risks.
Interrupting very large currents presents additional challenges. When a circuit breaker opens a high-voltage circuit, an electrical arc may form across the opening contacts because the air becomes ionized. Power-distribution systems use specialized circuit breakers that contain insulating gases or gas jets to extinguish these arcs. Alternating current provides an advantage in this situation because the current naturally passes through zero every half-cycle (120 times per second for 60-Hz power), making it easier for the arc to extinguish.
Conceptual Check
Why does increasing current dramatically increase the heating produced in a wire? Use the relationship [latex]P=I^2R[/latex] to explain why doubling the current results in much more than double the heating.
Shock Hazards
When electric current passes through the human body, it can produce a wide range of physiological effects. Some applications intentionally use electricity for medical treatment—for example, cardiac pacemakers regulate heart rhythm, defibrillators restore normal heart rhythms during cardiac emergencies, and electrical stimulation is used in pain management and rehabilitation. However, unintended electrical currents can also cause injury or death. Understanding how electric current interacts with the body is therefore essential for anyone working in healthcare or laboratory environments.
The severity of an electrical shock depends on four primary factors:
- The magnitude of the current, I
- The path the current takes through the body
- The duration of the exposure
- The frequency of the current (for direct current, f = 0)
Among these factors, current is usually the most important. Table 24.1 summarizes the typical physiological effects of a 60-Hz current passing through the trunk of the body for approximately one second.
| Current (mA) | Typical Effect |
|---|---|
| 1 | Threshold of sensation |
| 5 | Maximum generally harmless current |
| 10–20 | Onset of sustained muscle contraction ("can't let go"); breathing may become difficult |
| 50 | Severe pain |
| 100–300+ | Ventricular fibrillation possible; often fatal without immediate treatment |
| 300 | Burns become likely depending on current concentration |
| 6000 (6 A) | Sustained muscle contraction and respiratory paralysis; may reset the heart rhythm |
Currents below about 1 mA generally cannot be felt. Around 5 mA, a person experiences a noticeable but usually harmless shock. As the current increases to approximately 10–20 mA, muscles begin to contract involuntarily. One particularly dangerous effect is the can't let go phenomenon shown in Figure 24.4(b). Because the muscles that close the hand are stronger than those that open it, a person may be unable to release an energized conductor, increasing the duration of the shock.
Larger currents can interfere with the normal electrical activity of the heart. Between roughly 100 and 300 mA, ventricular fibrillation becomes possible. During ventricular fibrillation, the heart no longer pumps blood effectively, making the condition rapidly fatal unless normal rhythm is restored with prompt medical treatment, often using a defibrillator. At still higher currents, severe burns may occur because large amounts of electrical energy are converted into thermal energy within the tissues.
Very large currents produce different physiological effects. Rather than causing fibrillation, they can force the heart and respiratory muscles into sustained contraction. In some cases, normal heart rhythm resumes after the current stops. This principle is used therapeutically in defibrillation, where carefully controlled, high-current electrical pulses reset the heart's electrical activity.
Although current determines the immediate physiological effects, the current itself depends on both the applied voltage and the body's electrical resistance according to Ohm's law:
Most of the body's electrical resistance is provided by the skin. Dry skin has a relatively high resistance, whereas wet skin has a much lower resistance because water and dissolved salts provide additional pathways for charge to move. For example, a person with dry skin may have a resistance of approximately [latex]200~\text{k}\Omega[/latex]. Contact with a 120-V source would produce a current of only
which is typically below the threshold of sensation. If the skin is wet and the resistance decreases to about [latex]10.0~\text{k}\Omega[/latex], the same voltage produces
a current large enough to produce the dangerous "can't let go" effect.
Current follows all available conductive paths, with larger currents flowing through paths of lower resistance. Wearing insulating shoes or gloves increases the resistance between the body and ground, reducing the current that can flow during accidental contact with energized equipment.
Health and Bioscience Connection
Patients undergoing invasive medical procedures can be far more vulnerable to electrical shock than healthy individuals. Catheters, pacemaker leads, and other conductive devices bypass the skin, greatly reducing the body's resistance. In these situations, extremely small currents—sometimes only a few tens of microamperes—can reach the heart directly. This phenomenon, known as microshock, is one reason why hospitals enforce stringent electrical safety standards for operating rooms, intensive care units, and cardiac monitoring equipment.
Current magnitude is the primary determinant of injury, but the path through the body, the duration of exposure, and the frequency of the current also influence the outcome. Alternating current at frequencies near 50–60 Hz is particularly hazardous because it efficiently stimulates nerves and muscles. At much higher frequencies, the body becomes progressively less sensitive to nerve stimulation, and the current tends to remain near the surface of the body. This property allows high-frequency currents to be used safely in some medical and surgical procedures, such as electrocautery.
Safety Reminder
Never touch a person who is in contact with a live electrical source. Disconnect the power if it can be done safely, or use a dry, nonconductive object to separate the person from the source before providing assistance. Once the electrical hazard has been removed, seek emergency medical help immediately.
Conceptual Check
Why is wet skin significantly more dangerous than dry skin during an electrical shock? Use Ohm's law, [latex]I=\frac{V}{R}[/latex], to explain how reducing the body's resistance increases the current that flows for the same applied voltage.
Clinical Connection
Patients with intravenous catheters, pacemaker leads, or other conductive pathways that bypass the skin are considered microshock sensitive. Because the skin's protective resistance is greatly reduced, currents as small as a few tens of microamperes can reach the heart and interfere with its normal electrical activity. For this reason, medical equipment used in operating rooms, intensive care units, and cardiac care units must meet stringent electrical isolation and grounding standards.
Section Summary
- Electrical hazards are generally classified into two categories:
- Thermal hazards, caused by excessive electrical power that can overheat wires and equipment, potentially leading to fires.
- Shock hazards, caused by electric current passing through the human body.
- Short circuits create very low-resistance paths that can produce extremely large currents and dangerous heating because of the relationship [latex]P=\frac{V^2}{R}[/latex].
- The heating produced in electrical wires increases with the square of the current:
[latex]P=I^2R[/latex]
This relationship explains why overloaded wires can become fire hazards.
- The severity of an electrical shock depends primarily on the magnitude of the current, the path it takes through the body, the duration of exposure, and the frequency of the current.
- Body resistance varies greatly with conditions. Wet skin has much lower resistance than dry skin, allowing much larger currents to flow for the same applied voltage.
- Table 24.1 summarizes the physiological effects of electric current on the human body.
- Figure 24.5 shows how the threshold current for electrical hazards varies with the frequency of alternating current.
- Patients with conductive pathways that bypass the skin, such as intravenous catheters or pacemaker leads, may be microshock sensitive, making extremely small currents potentially dangerous.
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 not to operate electrical switches with wet hands and never to throw water on an electrical fire. Explain the physics behind both recommendations.
- Before working on a power transmission line, linemen sometimes touch the line with the back of the hand as a final check that the voltage is zero. Why use the back of the hand rather than the palm?
- Why is the resistance of wet skin much smaller than that of dry skin, and why do blood and other bodily fluids have relatively low resistance?
- Could a person receiving intravenous (IV) therapy be microshock sensitive? Explain.
- In view of the relatively small currents that can produce dangerous shocks, while circuit breakers and fuses typically interrupt much larger currents, how do these devices still help prevent electrical shock hazards?
Problems & Exercises
- (a) How much power is dissipated in a short circuit of 240-V AC through a resistance of [latex]0.250~\Omega[/latex]? (b) What current flows?
- What voltage is involved in a 1.44-kW short circuit through a [latex]0.100~\Omega[/latex] resistance?
- 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 [latex]300~\text{k}\Omega[/latex]; (b) if she is standing barefoot on wet grass and has a resistance of only [latex]4000~\Omega[/latex].
- 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 [latex]4000~\Omega[/latex]. What is the smallest voltage on the case of the radio that could cause ventricular fibrillation?
- 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?
- (a) During surgery, a current as small as [latex]20.0~\mu\text{A}[/latex] applied directly to the heart may cause ventricular fibrillation. If the resistance of the exposed heart is [latex]300~\Omega[/latex], what is the smallest voltage that poses this danger? (b) Does your answer imply that special electrical safety precautions are needed?
- (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?
- 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.
- Integrated Concepts A short circuit in a 120-V appliance cord has a [latex]0.500~\Omega[/latex] resistance. Calculate the temperature rise of the 2.00 g of surrounding materials, assuming their specific heat capacity is [latex]0.200~\text{cal}/(\text{g}\cdot^\circ\text{C})[/latex] and that it takes 0.0500 s for a circuit breaker to interrupt the current. Is this likely to be damaging?
- 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, ...).
Footnotes
- 1 For an average male shocked through the trunk of the body for 1 s by 60-Hz AC. Values for females are 60–80% of those listed.
Glossary
- thermal hazard
- A hazard caused by excessive electrical heating that can damage equipment, burn tissue, or ignite surrounding materials.
- shock hazard
- A hazard that occurs when electric current passes through a person, potentially causing physiological injury.
- short circuit
- A low-resistance path between the terminals of a voltage source, often producing dangerously large currents and rapid heating.
- microshock sensitive
- A condition in which the skin's protective resistance is bypassed, often by a medical procedure or device, making very small currents potentially dangerous to the heart.
A hazard caused by excessive electrical heating that can damage equipment, burn tissue, or ignite surrounding materials.
A hazard that occurs when electric current passes through a person, potentially causing physiological injury.
A low-resistance path between the terminals of a voltage source, often producing dangerously large currents and rapid heating.
A condition in which the skin's protective resistance is bypassed, often by a medical procedure or device, making very small currents potentially dangerous to the heart.
