20.6 Electric Hazards and the Human Body

Electric Hazards

Thermal vs. Shock Hazards

There are two main categories of electrical danger: thermal hazards and shock hazards.

Thermal hazards happen when electric current generates enough heat to cause burns or start fires. This can result from overloaded circuits, short circuits, or faulty wiring. The heat melts insulation, ignites nearby materials, or damages components.

Shock hazards happen when electric current passes through the human body, disrupting normal functions like heart rhythm and breathing. The severity of a shock depends on the current's path through the body, how long the exposure lasts, and the frequency of the current.

Effects of Current on the Body

The human body conducts electricity, and even small currents can be dangerous. Here's how different current levels affect the body:

• 1 mA or less: Generally not perceptible.
• 1–5 mA: Slight tingling sensation, usually no lasting harm.
• 5–10 mA: Noticeable shock with possible involuntary muscle contractions.
• 10–20 mA: Painful shock and loss of muscle control. At this level, you may be unable to let go of the energized object because the current forces your hand muscles to grip tighter.
• 20–100 mA: Severe pain, difficulty breathing, and potentially fatal ventricular fibrillation (uncoordinated heart contractions that prevent the heart from pumping blood). This range requires immediate medical attention.
• 100–200 mA: Ventricular fibrillation is very likely, along with severe burns and muscle damage. High risk of death.
• Above 200 mA: Severe burns, internal organ damage, cardiac arrest, and respiratory paralysis. Usually fatal without immediate intervention.

Electrical burns can occur both at the contact points on the skin and internally along the path the current travels through the body.

Factors Influencing Shock Severity

Current path is one of the biggest factors. The path depends on where the body contacts the circuit:

• A hand-to-hand or hand-to-foot path is especially dangerous because the current may pass through the heart.
• Contact across a small area on the same body part is generally less severe.

Duration of exposure matters a great deal. Longer contact with a current source increases the risk of injury or death. Even relatively low currents become dangerous with prolonged exposure.

Frequency of the current also plays a role:

• AC (alternating current) is generally more dangerous than DC (direct current) at the same voltage. AC causes repeated muscle contractions that can lock your grip onto the source, making it harder to let go.
• At higher frequencies (above about 100 kHz), the current is less likely to cause ventricular fibrillation, though it can still cause burns.

Other factors include:

• Voltage: Higher voltages drive more current through the body. By Ohm's law ($V = IR$), for a given body resistance, doubling the voltage doubles the current.
• Skin resistance: Dry, calloused skin can have a resistance of $100{,}000 \; \Omega$ or more, while wet or broken skin may drop to $1{,}000 \; \Omega$ or less. This is why water and electricity are such a dangerous combination.
• Overall health: People with heart conditions or pacemakers are more susceptible to shock injuries.

Electrical Safety Measures

Several devices and practices are designed to prevent electrical accidents:

• Grounding provides a low-resistance path for excess current to flow safely to the earth instead of through a person. If a fault occurs in a grounded appliance, the current takes the easier path to ground rather than through you.
• Insulation prevents direct contact with live electrical parts, reducing the risk of both shocks and short circuits.
• Circuit breakers automatically interrupt the flow of electricity when they detect an overload or short circuit. They protect wiring from overheating and causing fires, but they typically trip at currents (15–20 A) far above what can be fatal to a person.
• Ground Fault Circuit Interrupters (GFCIs) detect small differences (as little as 5 mA) between the current flowing out on the hot wire and returning on the neutral wire. If current is "leaking" through an unintended path (like your body), the GFCI shuts off power within milliseconds. GFCIs are required in bathrooms, kitchens, and outdoor outlets for this reason.
• Electrical safety standards are guidelines and regulations set by organizations (such as the National Electrical Code) to ensure safe electrical installations and practices.