An electric shock occurs when a person becomes part of an electrical circuit, allowing current to flow through the body and disrupt normal physiological functions. Determining the minimum voltage required to cause a serious shock is complex because voltage is only one factor governing the outcome. The severity of the injury is not determined by the voltage, but by the amount of electrical current driven through the body’s tissues.
Why Current is the Measure of Danger
The actual threat to the human body is the flow of electrons, known as current, measured in amperes (A) or milliamperes (mA). Current directly causes muscle contraction, burns tissue, and disrupts the heart’s rhythm. Voltage, measured in volts (V), is the electrical pressure that attempts to push the current through the body.
The relationship between these factors is defined by Ohm’s law: current equals voltage divided by resistance. The human body acts as a resistive component, meaning its resistance limits the current flow for any given voltage. Therefore, the voltage required to cause harm depends entirely on the body’s resistance.
Key Physiological Thresholds of Electrical Harm
The body responds to current in a sequence of increasingly hazardous thresholds, measured in milliamperes. The perception threshold, the minimum current a person can feel, is typically around 1 mA for alternating current (AC) and about 5 mA for direct current (DC). The sensation is a faint tingling or warming, but it is not harmful.
As the current increases, it reaches the “let-go” threshold. This is the point where the electrical energy causes sustained muscle contraction (tetany), preventing the person from releasing the energized object. This threshold is generally between 9 to 30 mA for adult men and 6 to 25 mA for adult women exposed to AC.
The most dangerous threshold is the current level that causes ventricular fibrillation (V-fib). V-fib is a chaotic quivering of the heart muscle that stops blood circulation. For common AC frequencies, currents between 50 mA and 100 mA passing through the chest can induce V-fib, which is often fatal if not immediately treated.
Variables That Determine the True Danger
Skin Resistance
The resistance of the skin is the most significant variable determining how much current a specific voltage will drive through the body. Dry, intact skin offers substantial resistance, potentially over 100,000 ohms, which severely limits current flow. Wet skin, especially if sweaty or cut, can drastically lower resistance to as little as 1,000 ohms or less, allowing a much greater current to pass at the same voltage.
Path of Current
The path the current takes through the body is a major factor. The hand-to-hand or hand-to-foot path is particularly dangerous because it sends the current directly across the heart.
Duration and Type of Current
The duration of contact is critical, as a brief shock is less likely to induce V-fib than a longer exposure. Alternating current (AC) is generally considered three to five times more dangerous than direct current (DC) at the same voltage. This is because AC at 50 or 60 Hz is more effective at inducing V-fib and causing the sustained muscle contraction that inhibits a person’s ability to “let go.”
Practical Minimum Voltage Scenarios
Because body resistance is highly variable, there is no single minimum voltage that guarantees a serious shock; the danger is entirely contextual. Safety standards are often based on a conservative threshold, with voltages below 50 volts AC commonly considered safe under dry conditions. This level assumes a high body resistance that limits the current to non-lethal levels.
In the worst-case scenario—such as contact with wet hands or bare feet—the body’s resistance can drop dramatically. Under these low-resistance conditions, a voltage as low as 42 to 50 volts AC can be sufficient to drive 50 to 100 mA through the body, enough to cause ventricular fibrillation. Therefore, any voltage above 50V AC must be considered potentially lethal, and low-voltage circuits pose a serious threat when skin resistance is bypassed by moisture.