Electrical Hazards and Physiological Effects

Introduction and Context

Electricity is indispensable in generation, transmission, manufacturing, buildings, transportation, health care, and information systems. The same electrical energy that performs useful work can injure or kill when it passes through the human body, releases heat uncontrollably, or produces an ignition source. Electrical safety therefore requires more than identifying energized conductors. It requires analysis of the source, the fault path, the exposed person, the environment, and the duration and magnitude of exposure.

Electrical injury is influenced by voltage, available fault current, current path, frequency, contact area, skin condition, body impedance, and protective-device operating time. A relatively low voltage can be dangerous in wet conditions, while a high-voltage system can produce severe burns and arc-flash injuries without direct contact. Engineers must also consider indirect hazards, including fire, explosion, falling, involuntary muscular reactions, and electromagnetic interference with medical or control equipment.

This chapter develops the physical basis of these hazards and connects it to practical risk assessment, design, maintenance, and regulatory control. Numerical values are indicative engineering guidance rather than universal safe limits. Applicable national regulations, electrical codes, equipment standards, and organizational procedures always govern actual work.

Learning Objectives

By the end of this chapter, a student should be able to:

1. Explain electric shock, electrical burns, arc flash, arc blast, fire, explosion, electromagnetic, and static-electricity hazards.
2. Analyze current paths through the body and explain the effects of touch voltage and step voltage.
3. Use Ohm’s law and a simplified body-impedance model to estimate shock current under stated assumptions.
4. Explain how current magnitude, exposure duration, frequency, and contact conditions affect physiological response.
5. Distinguish shock protection, arc-flash protection, and ignition-control measures.
6. Apply a structured approach to an electrical-hazard case and identify appropriate engineering and administrative controls.

1. Electrical Hazard as an Energy-Transfer Problem

An electrical hazard exists when electrical energy can be transferred to a person, object, or environment in a way that causes injury, damage, or ignition. The relevant source may be a utility system, battery, capacitor, photovoltaic array, motor drive, generator, or static charge. Stored energy remains hazardous after isolation unless it has been discharged and verified.

A useful engineering model is:

Source → conductive or radiated path → exposed person or material → physiological, thermal, mechanical, or ignition effect.

The source determines available voltage, current, fault energy, frequency, and duration. The path determines whether energy travels through the body, across an air gap, through equipment, or over a surface. The receiving body determines the resulting effect. Controls can interrupt any part of this chain.

Important distinctions are:

• Shock hazard: possible injury from body current caused by contact with energized parts or conductive surfaces.
• Arc-flash hazard: thermal energy and intense light produced by an electrical arc.
• Arc-blast hazard: pressure, sound, molten metal, and mechanical forces associated with rapid arc energy release.
• Electrical burn: tissue injury caused by current heating, an arc, or contact with hot electrical equipment.
• Secondary injury: injury caused by reaction to electricity, such as falling, being struck by moving equipment, or losing control of a vehicle.

2. Voltage, Current, and Exposure Duration

For a simple resistive path, Ohm’s law is:

where I is current in amperes (A), V is voltage across the person in volts (V), and Z is the total body-and-contact impedance in ohms (Ω). For direct-current or purely resistive approximation, Z may be replaced by resistance R. In alternating-current systems, impedance includes resistive and reactive effects and may depend on frequency.

The equation is useful only when the voltage is the voltage actually applied across the body and the impedance model is appropriate. It does not by itself predict injury, because physiological response also depends on current path, time, waveform, and individual conditions.

The energy converted to heat in a resistive body path can be approximated by:

where E is thermal energy in joules (J), I is current in amperes, R is resistance in ohms, and t is time in seconds. This relation assumes a constant current and resistance. Human tissue resistance can change during exposure, especially when skin is damaged, so the equation is an approximation.

Exposure duration is critical. A protective device that clears a fault quickly can substantially reduce energy, muscular contraction time, and the likelihood of ventricular fibrillation. Conversely, a person who cannot release a conductor may remain connected until the circuit is interrupted.

3. Human Body Impedance and Contact Conditions

Human-body impedance is not a single constant. It includes skin impedance, internal tissue impedance, contact resistance, footwear, clothing, and the impedance of the return path. Dry, intact skin may provide substantial resistance; wet, cut, abraded, or electrically punctured skin provides much less. Large metal contact areas reduce contact resistance, while a small sharp contact may increase local current density and heating.

Relevant variables include:

• contact voltage and frequency;
• skin moisture, temperature, and condition;
• contact area and pressure;
• current path through the body;
• footwear, gloves, floor, and enclosure conditions;
• duration of contact; and
• whether an arc has bypassed or punctured the skin.

A simplified series model is:

Here, Z_total is the complete loop impedance, Z_body is internal body impedance, Z_contact,1 and Z_contact,2 are contact impedances at the two body terminals, and Z_return represents the external return path. The model is conceptual; actual impedance is frequency-dependent and nonlinear.

Worked Example: Estimated Shock Current

A worker contacts an energized conductor at 120 V rms while standing on a conductive floor. Assume, for a preliminary calculation, that the total loop impedance is 1,500 Ω.

An estimated current of 80 mA is potentially fatal, particularly if it travels from hand to hand or hand to foot through the chest. The calculation is not a guarantee of outcome: actual impedance may be higher or lower, and the current waveform and clearing time matter. The example demonstrates why ordinary low-voltage systems must not be treated as inherently safe.

4. Current Paths and Physiological Effects

The current path is often more important than the source voltage alone. A hand-to-hand path may cross the chest and heart. A hand-to-foot path may also cross the thorax. A foot-to-foot path may primarily affect the legs but can still cause burns, muscular contraction, and falls.

The principal physiological mechanisms are:

1. Perception and startle: small currents may produce tingling or surprise, causing a person to move into danger or fall.
2. Muscular contraction: increasing current can prevent release from a conductor. This is often called the let-go problem, but thresholds vary widely.
3. Respiratory interference: sustained current through the chest can impair breathing.
4. Ventricular fibrillation: electrical disruption of the heart’s rhythm can be fatal unless rapidly treated.
5. Cardiac arrest or tissue damage: very high current can stop the heart or cause extensive thermal injury.
6. Secondary trauma: falls, fractures, burns, and impact injuries may occur even when current is not fatal.

Alternating current at power frequency is particularly hazardous because repeated stimulation can cause sustained muscular contraction and interfere with cardiac rhythm. Direct current often produces a single strong contraction at connection or disconnection, although it can also cause severe burns and cardiac injury. High-frequency currents may produce intense surface heating with different cardiac effects; frequency must therefore be considered rather than assuming that all amperes have the same consequence.

No universal current threshold guarantees safety. Published physiological ranges are statistical and depend on path, duration, frequency, person, and contact conditions. Safety design should assume vulnerable conditions rather than rely on a favorable threshold.

5. Touch Voltage and Step Voltage

Touch Voltage

Touch voltage is the voltage difference between an object touched by a person and the person’s feet or another point of contact. During a fault, a metal enclosure may rise above true earth potential because current flows through grounding conductors, bonding connections, soil, or structural steel.

A simplified touch-current relationship is:

where V_touch is the voltage between the hand-contact point and the feet, and Z_body-loop includes the body and all return impedances. Equipotential bonding, low-impedance protective conductors, rapid disconnection, insulating barriers, and restricted access reduce touch risk.

Step Voltage

Step voltage is the voltage difference between two points on the ground separated by approximately one human step. It is significant near substation ground faults, fallen overhead conductors, lightning discharge points, and energized utility equipment. Current may enter one foot and leave the other.

A person near a ground fault should not run normally, because a large stride may span a larger voltage difference. Where procedures permit movement, personnel are commonly instructed to shuffle with small steps or hop with feet together, while avoiding contact with other objects. Such actions are emergency guidance, not a substitute for isolation and rescue procedures.

Ground grids, buried conductors, surface layers of high-resistivity material, equipotential bonding, and automatic fault clearing are important controls. Grounding does not make every point in the surrounding soil equal to zero volts; it manages potential gradients and provides a fault-current path.

6. Electrical Burns and Thermal Injury

Electrical burns may be:

• Contact burns, caused by current entering and leaving the body;
• Internal burns, caused by resistive heating in tissue;
• Arc burns, caused by radiant heat, plasma, and molten metal; and
• Flame burns, caused when clothing or nearby materials ignite.

The skin may show small entry and exit wounds while deeper muscles, nerves, blood vessels, and organs are severely damaged. Electrical burns therefore require professional medical assessment even when external injury appears limited.

Local heating is affected by current density. For a given current, a smaller contact area produces greater current density and more concentrated heating. Wet skin lowers impedance and increases current, while perspiration can make gloves, clothing, and surfaces conductive.

7. Arc Flash and Arc Blast

An arc is a sustained electrical discharge through ionized gas. It can occur when insulation fails, a tool bridges conductors, a loose connection vaporizes metal, or a conductor is opened under load. Arc-flash injury results mainly from thermal radiation, hot gases, molten metal, intense ultraviolet light, and ignition of clothing. Arc blast involves the pressure wave, sound, shrapnel, equipment doors, and movement caused by rapid vaporization and expansion.

Arc severity depends on system voltage, bolted fault current, arcing current, equipment enclosure, electrode configuration, gap, conductor geometry, protective-device clearing time, and working distance. Available short-circuit current alone is not sufficient: an arc’s current may differ from the bolted fault current, and longer clearing time can increase incident energy.

Incident energy is commonly expressed in joules per square centimetre or calories per square centimetre. The exact calculation method depends on the applicable standard and equipment configuration. A generic energy concept is:

where E_incident is incident energy, V is system voltage, I_arc is arcing current, t is arc duration, and d is working distance. This proportional relationship is conceptual, not a universal design equation. Formal studies must use the method required by the governing standard and verified equipment data.

Controls include de-energization, remote operation, current-limiting devices, faster protection, arc-resistant equipment, insulated tools, barriers, restricted approach boundaries, correct enclosure maintenance, and arc-rated clothing and face protection selected through a documented assessment. Personal protective equipment reduces injury; it does not eliminate the hazard.

8. Fire and Explosion Hazards

Electrical fires arise from overloads, short circuits, loose connections, insulation deterioration, undersized conductors, poor maintenance, blocked ventilation, or abnormal equipment operation. A connection with high resistance can generate heat even when current is below the protective-device trip level. For a resistance fault, heating follows:

where P is power dissipated as heat in watts, I is current in amperes, and R is resistance in ohms. The relationship assumes a resistive element under the stated operating conditions.

Explosions require fuel, oxygen, and an ignition source in suitable concentration and energy conditions. Electrical ignition sources include arcs, sparks, hot surfaces, static discharge, and overheated components. Flammable gases, vapors, combustible dusts, fibers, and liquids require classification of hazardous locations and equipment suitable for that classification.

Controls include elimination or substitution of flammable materials, ventilation, enclosure integrity, bonding and grounding, intrinsically safe circuits, explosion-protected equipment, dust control, hot-work permits, gas testing, and ignition-source isolation. Equipment must not be installed in a hazardous location merely because it is labelled “industrial”; its certification and installation method must match the environment.

9. Electromagnetic and Static-Electricity Hazards

Electromagnetic fields are produced by electric charges and currents. At high field strengths, induced currents can stimulate nerves, heat tissue, or interfere with implanted medical devices. Rapidly changing fields can also induce voltages in nearby conductors. Control systems, instrumentation, communication networks, and medical environments may be affected even when physiological exposure is not severe.

Engineering controls include shielding, separation, grounding, filtering, twisted-pair or fiber-optic communication, controlled access, and compliance with exposure limits established by competent authorities. Risk assessment must distinguish electric-field exposure, magnetic-field exposure, radiofrequency energy, and electromagnetic compatibility.

Static electricity is an imbalance of charge that accumulates when materials separate, rub, flow, or contact and separate. A person can accumulate charge while walking on an insulating floor and then discharge to a grounded object. The discharge may be startling and can ignite flammable vapors or dusts.

Stored electrostatic energy can be approximated by:

where E is stored energy in joules, C is capacitance in farads, and V is voltage in volts. Although the energy may be small, the discharge can have a short rise time and high local temperature. Bonding and grounding, humidity control where appropriate, antistatic materials, conductive footwear, and controlled transfer rates reduce static risk. Grounding a person is not sufficient if containers, hoses, and process equipment are not bonded together.

10. Hierarchy of Controls and Regulatory Practice

Electrical safety programs should apply the hierarchy of controls:

1. Elimination: remove the need to work near electricity or de-energize the equipment.
2. Substitution: use lower voltage, remote systems, or less hazardous materials where feasible.
3. Engineering controls: guarding, insulation, interlocks, grounding, bonding, current limitation, automatic disconnection, and arc-resistant designs.
4. Administrative controls: training, permits, approach boundaries, job planning, inspection, and supervision.
5. Personal protective equipment: gloves, footwear, eye and face protection, hearing protection, arc-rated clothing, and fall protection.

De-energization is generally the preferred control for exposed energized parts. A sound procedure includes identification of all sources, opening disconnecting devices, preventing re-energization, testing for absence of voltage with a properly rated instrument, discharging stored energy, and applying temporary grounding where required. Test instruments must be proved on a known source before and after testing, consistent with the governing procedure.

Regulatory systems differ by jurisdiction, but typically address electrical installations, live work, hazardous locations, personal protective equipment, lockout/tagout, grounding, equipment maintenance, and emergency response. Standards should be treated as a coordinated system rather than as isolated checklists. Employers and engineers must also consider the ethical duty to protect workers, communicate residual risk honestly, and stop work when conditions are uncertain.

11. Illustrative Case Study: Arc Flash During Switchboard Maintenance

Background

A maintenance team planned to replace a circuit-breaker mechanism in a low-voltage switchboard. The board was supplied from a transformer and also connected to a standby generator. The work order identified the utility source but did not identify the generator backfeed.

Problem

The team opened one disconnect and assumed the compartment was dead. A technician removed a barrier and contacted a still-energized bus connection. A tool slipped, producing an arc flash. The technician suffered burns and hearing injury; a second worker fell while retreating.

Analysis

The immediate cause was contact with an energized part. Contributing causes included incomplete source identification, inadequate isolation, failure to verify absence of voltage, and insufficient control of the work boundary. The event also demonstrates that shock and arc-flash hazards can coexist. Even if a worker avoids direct contact, a tool bridge can initiate an arc.

Approach and Solution

A revised method required a single-line diagram review, identification of normal and alternate sources, isolation of utility and generator supplies, lockout of each source, discharge and verification, and a documented energized-work justification if de-energization was impossible. The employer also performed an arc-flash study using verified protective-device settings, installed warning labels, improved barriers, and trained workers in test-instrument use and approach boundaries.

Outcome

The revised procedure reduced the probability of unexpected energization and ensured that any remaining energized work was justified, supervised, and performed with appropriate equipment. The principal improvement was not a new item of PPE; it was a better isolation and verification process.

Learning Points

• A circuit is not safe because one disconnect is open.
• Alternate sources, stored energy, and backfeed must be identified.
• Absence of voltage must be tested, not assumed.
• Arc-flash assessment and shock assessment are related but distinct.
• Incident prevention depends on design, procedures, competence, and organizational learning.

12. Contemporary Developments and Professional Practice

Modern systems introduce additional sources of risk. Battery energy-storage systems may maintain high fault current or dangerous DC voltage after the utility supply is disconnected. Photovoltaic arrays can remain energized in daylight. Power electronic converters can produce non-sinusoidal waveforms and unexpected backfeed. Digital protective relays, remote switching, thermal imaging, and continuous insulation monitoring improve detection but do not replace physical verification.

Artificial intelligence can assist with predictive maintenance by identifying abnormal temperature, partial-discharge patterns, or changing load behavior. However, AI outputs must be validated, data quality must be assessed, and automated recommendations must not bypass established isolation and authorization procedures. Safety decisions remain accountable to competent engineers and responsible organizations.

Best practice combines sound design, accurate drawings, preventive maintenance, calibrated instruments, competent personnel, clear communication, emergency planning, and a learning-oriented safety culture. The central professional principle is simple: treat uncertainty as a hazard and resolve it before exposure occurs.

Chapter Summary

Electrical hazards result from uncontrolled transfer of electrical, thermal, mechanical, chemical, or electromagnetic energy. Shock current depends on the applied voltage and the complete body-and-return impedance, but injury also depends on current path, waveform, frequency, exposure duration, and individual condition. Hand-to-hand and hand-to-foot paths are especially dangerous because they may cross the chest.

Touch voltage describes the potential difference between a contacted object and a person’s feet; step voltage describes the difference between two ground points. Grounding, bonding, rapid disconnection, insulation, barriers, and controlled movement reduce these risks. Electrical burns may be deep even when external marks are small.

Arc flash produces intense heat, light, molten metal, and toxic products; arc blast produces pressure, sound, and mechanical hazards. Fire and explosion arise when electrical heating or discharge ignites combustible materials or atmospheres. Electromagnetic fields can affect people and equipment, while static electricity can cause shocks and ignite flammable atmospheres.

Effective safety practice follows the hierarchy of controls, prioritizes de-energization, verifies absence of voltage, identifies all sources, and uses PPE as the final layer rather than the primary solution. Regulations, engineering standards, competent work practices, and ethical responsibility must operate together.

Key Terms

Shock hazard: Risk of injury from current passing through the body.

Touch voltage: Voltage between a contacted conductive object and a person’s feet or another contact point.

Step voltage: Voltage difference between two points on the ground separated by a step.

Body impedance: Frequency- and condition-dependent opposition to current through the body.

Let-go current: Current level at which muscular contraction may prevent release from a conductor.

Ventricular fibrillation: Disorganized heart rhythm that can prevent effective circulation.

Electrical burn: Tissue injury caused by current, an arc, or electrical heating.

Arc flash: Thermal and luminous energy released by an electrical arc.

Arc blast: Pressure, sound, molten material, and mechanical effects of an arc event.

Incident energy: Energy received per unit area at a specified working distance.

Bolted fault current: Prospective current for a low-impedance short circuit.

Arcing current: Current that flows through an electrical arc.

Grounding: Intentional connection to earth or a grounding reference.

Bonding: Connecting conductive parts to maintain electrical continuity and reduce potential differences.

Static electricity: Accumulated electric charge at rest on a material or object.

Equipotential bonding: Arrangement intended to keep accessible conductive parts at nearly the same potential.

Protective device: Device that interrupts abnormal current or fault conditions.

Hazardous location: Area where flammable gas, vapor, liquid, dust, or fibers may create an ignition risk.

Lockout/tagout: Procedure for isolating energy sources and preventing unauthorized re-energization.

Hierarchy of controls: Ordered preference for eliminating or reducing hazards.

Review Questions

Short Answer Questions

1. Define touch voltage and explain why it may exist during a ground fault.
2. List four factors that influence human-body impedance.
3. Distinguish arc flash from arc blast.
4. Why can static electricity be dangerous in a flammable atmosphere?
5. State the preferred control for work on exposed energized parts.

Long Answer Questions

1. Explain how current magnitude, path, frequency, and duration influence physiological injury.
2. Describe the mechanisms of electrical fire and identify appropriate prevention measures.
3. Compare shock protection and arc-flash protection, including the limitations of PPE.
4. Explain grounding, bonding, touch voltage, and step voltage in a substation environment.
5. Discuss how a complete isolation and verification procedure prevents the case-study incident.

Analytical / Application Questions

1. A person contacts 230 V rms and the estimated total loop impedance is 2,000 Ω. Calculate the approximate current using Ohm’s law and explain why the result cannot be interpreted as a guaranteed injury threshold.
2. A switchboard has a possible alternate generator supply. Develop a step-by-step isolation plan before maintenance and identify at least three verification actions.
3. A solvent-transfer area experiences repeated static discharges. Analyze the likely charge-generation mechanisms and propose engineering, administrative, and material-control measures.

Multiple Choice Questions with Answers

Question 1

Which factor most directly increases current through a person for a fixed applied voltage?

A. Increasing total body-and-return impedance

B. Decreasing total body-and-return impedance

C. Increasing working distance from an arc

D. Reducing the fault-clearing time

Correct Answer: B. Decreasing total body-and-return impedance

Question 2

A person contacts 120 V with a total loop impedance of 1,500 Ω. The approximate current is:

A. 0.008 A

B. 0.080 A

C. 8.0 A

D. 80 A

Correct Answer: B. 0.080 A

Question 3

Which current path is generally of greatest concern because it may cross the heart?

A. Foot to foot

B. Hand to hand

C. Elbow to wrist on the same arm

D. Finger to finger on the same hand

Correct Answer: B. Hand to hand

Question 4

Step voltage is best defined as the voltage difference between:

A. A conductor and its insulation

B. A person’s hand and a touched enclosure

C. Two ground points separated by a step

D. Two phases of a transformer

Correct Answer: C. Two ground points separated by a step

Question 5

Which statement about arc flash is correct?

A. It is only a shock hazard and has no thermal effect

B. It can produce intense heat, light, molten metal, and toxic products

C. It occurs only in high-voltage transmission lines

D. It is prevented simply by wearing rubber footwear

Correct Answer: B. It can produce intense heat, light, molten metal, and toxic products

Question 6

For a resistive fault, dissipated power is represented by:

A. P = I²R

B. P = V ÷ I²

C. P = R ÷ I

D. P = I ÷ R²

Correct Answer: A. P = I²R

Question 7

Which action is normally the preferred control before work on exposed energized parts?

A. Selecting thicker clothing only

B. Increasing the work duration

C. De-energizing, isolating, and verifying absence of voltage

D. Standing farther away without testing

Correct Answer: C. De-energizing, isolating, and verifying absence of voltage

Question 8

What is the principal purpose of bonding conductive equipment in a hazardous location?

A. To increase static charge accumulation

B. To reduce potential differences and uncontrolled discharge

C. To increase arc duration

D. To replace all ventilation requirements

Correct Answer: B. To reduce potential differences and uncontrolled discharge

Question 9

In the simplified stored-energy relation E = ½CV², doubling voltage while capacitance remains constant makes stored energy:

A. Half as large

B. Twice as large

C. Three times as large

D. Four times as large

Correct Answer: D. Four times as large

Question 10

Why must alternate sources such as generators be considered during isolation?

A. They can backfeed equipment believed to be de-energized

B. They always reduce the fault current to zero

C. They eliminate the need for voltage testing

D. They prevent all arc-flash events automatically

Correct Answer: A. They can backfeed equipment believed to be de-energized