The question of how many volts it takes to stop a person’s heart is complex because voltage alone is not the direct cause of harm. The danger from an electrical shock is determined by the amount of electrical current that flows through the body, particularly through the chest cavity. Voltage provides the potential for this current, but the actual flow is heavily influenced by the body’s resistance, which varies dramatically based on external factors. Understanding the relationship between voltage, current, and resistance is foundational to grasping the true hazard of electricity.
Understanding Voltage, Current, and Resistance
Electrical flow is best understood by separating it into three measurable components: voltage, current, and resistance. Voltage, measured in volts (V), represents the electrical pressure or force that pushes charge through a circuit. An analogy often used is that of water pressure in a pipe. Current, measured in amperes or amps (A), is the flow rate of the electrical charge. In the context of electrical shock, the magnitude of the current directly causes physiological damage, not the voltage. Resistance, measured in ohms (\(\Omega\)), is the opposition to this flow.
These three components are linked by a principle known as Ohm’s Law, expressed by the formula: \(V = IR\) (Voltage equals Current multiplied by Resistance). This relationship shows why voltage is an indirect factor in shock severity. For any given voltage, the resulting current depends entirely on the body’s resistance. If resistance is high, the current will be low and relatively harmless, even with a high voltage. Conversely, if resistance is low, even a moderate voltage can generate a dangerously high current.
How Electricity Disrupts Heart Function
The human heart relies on a highly specialized, intrinsic electrical system to coordinate its rhythmic pumping action. This precise sequence controls the contraction of the heart muscle, ensuring blood is efficiently pumped throughout the body. When an external electrical current passes through the chest, it interferes with these delicate natural signals.
This interference can cause the most dangerous outcome: ventricular fibrillation (V-Fib). V-Fib is an irregular and chaotic heart rhythm where the lower chambers (ventricles) quiver uselessly instead of contracting. This disorganized activity prevents the heart from pumping blood, leading to sudden cardiac arrest. The timing of the electrical shock within the cardiac cycle is important. A shock delivered during the heart’s repolarization phase, specifically the vulnerable T-wave period, is far more likely to induce V-Fib. This short window is when the heart muscle cells are electrically unstable.
Variables That Determine Electrical Hazard
The severity of an electrical shock is modulated by several external and internal factors, not just the source voltage. The primary variable is the total electrical resistance of the body, which directly limits the current flow for any given voltage. The skin provides the vast majority of this resistance, acting as a protective barrier.
Dry, calloused skin can offer resistance as high as 100,000 ohms or more, meaning common household voltage would produce a relatively harmless current. However, when the skin is wet, perspiring, or broken, resistance can plummet to 1,000 ohms or less. Under these low-resistance conditions, a relatively low voltage, such as 120 volts, can push a lethal amount of current through the body.
The path the current takes is another decisive factor. A current path that includes the heart, such as hand-to-hand or hand-to-foot, is significantly more dangerous than a path that bypasses the chest cavity. Finally, the duration of contact plays a role, as longer exposure allows more energy to pass through the body and increases the likelihood of the shock coinciding with the heart’s vulnerable T-wave period.
Specific Lethal Current Thresholds
Physiological harm is quantified by current magnitude, measured in milliamperes (mA), rather than the voltage. For alternating current (AC) at the common frequency of 60 Hz, the minimum current a person can typically sense is about 1 mA. The “let-go” threshold, where involuntary muscle contractions prevent a person from releasing the energized object, is around 10 to 16 mA.
Currents passing through the chest in the range of 75 to 100 mA are considered the threshold for inducing ventricular fibrillation (V-Fib) in humans. This is a remarkably small amount of current, but it is sufficient to disrupt the heart’s rhythm. For direct current (DC), the V-Fib threshold is generally higher, requiring around 300 to 500 mA.
The voltage required to stop a heart is not a fixed number; it is the voltage needed to push 75 to 100 mA through the body. If the body’s resistance is high (e.g., 100,000 ohms with dry skin), thousands of volts are needed to generate a lethal current. If resistance is drastically lowered (e.g., to 1,000 ohms with wet skin), household 120-volt AC is enough to create 120 mA, which is lethal. This demonstrates that low voltage can be lethal under low resistance, while high voltage can be survivable if resistance is high.