The old workshop adage says, "It's the volts that jolt, but the amps that kill," but this is only half the story. Electric shock lethality is determined by the amount of current (amps) flowing through the body, but voltage (volts) is the necessary pressure required to overcome the skin's resistance and push that fatal current. You cannot separate the two in a real-world shock scenario: current dictates the biological damage, while voltage dictates whether that current can actually breach your skin to reach your vital organs.
When makers and DIYers ask "do volts or amps kill," they are usually trying to reconcile why a 20,000-volt static shock is harmless, while a 120-volt wall outlet can be fatal. The answer lies in the intersection of Ohm's Law, human tissue impedance, and the energy delivery capacity of the source. Here is the exact physics of what happens when electricity meets the human body.
The Physiological Thresholds of Electric Current
Current (measured in milliamperes, mA) is the physical mechanism of injury. When electrons flow through biological tissue, they disrupt the electrochemical signals in your nervous system and generate resistive heat. The most immediate cause of death in standard AC mains shocks is not burning, but ventricular fibrillation—where the heart's sinoatrial node is overridden, causing the muscle to quiver uselessly instead of pumping blood.
The table below outlines the physiological effects of 60Hz AC current passing across the chest (hand-to-hand pathway). These thresholds are derived from the foundational Dalziel research and are codified in modern safety standards like NFPA 70E.
| Current (60Hz AC) | Physiological Effect | Real-World Consequence |
|---|---|---|
| 0.5 – 1 mA | Perception Threshold | Faint tingle; barely noticeable. Safe. |
| 1 – 5 mA | Slight Shock | Startling, but not painful. Most people can easily let go. |
| 6 – 16 mA | "Let-Go" Threshold | Painful. Muscles contract involuntarily. You may be unable to release the conductor. |
| 17 – 99 mA | Respiratory Arrest | Extreme pain, severe tetanic muscle contractions, breathing stops. |
| 100 – 5,000 mA (0.1 – 5 A) | Ventricular Fibrillation | Heart rhythm disrupted. Death is highly likely without immediate defibrillation. |
| > 5 A | Cardiac Arrest & Burns | Heart clamps completely. Severe internal tissue and skin burning. |
Why Voltage is the Gatekeeper (The Math of Skin Resistance)
If current is the bullet, voltage is the gunpowder. The human body's internal resistance (blood, muscle, nerves) is remarkably low—roughly 300 to 500 ohms. However, your skin acts as a high-resistance dielectric barrier. To push a lethal 100 mA through your internal organs, the voltage source must first overcome the skin's impedance.
Let's run a worked numeric example using Ohm's Law (I = V / R) on a standard 120V AC North American branch circuit, referencing OSHA's electrical safety guidelines regarding environmental conditions.
Scenario A: Dry, Intact Skin
- Source Voltage: 120V AC
- Skin Resistance: ~100,000 Ω (dry, unbroken epidermis)
- Current Flow: 120V / 100,000 Ω = 0.0012 A (1.2 mA)
- Result: You feel a mild tinge. You are well below the let-go threshold.
Scenario B: Wet or Broken Skin
- Source Voltage: 120V AC
- Skin Resistance: ~1,000 Ω (sweaty hands, standing in a puddle, or a wire piercing the skin)
- Current Flow: 120V / 1,000 Ω = 0.120 A (120 mA)
- Result: You have crossed the 100 mA ventricular fibrillation threshold. The shock is potentially lethal.
This is why voltage matters. A 12V car battery can supply 800 Cold Cranking Amps (CCA), but 12V lacks the electrical "pressure" to push current through 100,000 ohms of dry skin. Conversely, a 400V DC electric vehicle battery pack easily breaches the skin barrier, driving lethal current through the body even if the contact area is small.
Where You Meet This in Practice
Understanding the V/I/R relationship of the human body explains several seemingly contradictory phenomena on the bench and the jobsite.
Static Electricity (High Volts, Micro-Amps)
Walking across a carpeted room in winter can generate 20,000 volts of static potential. When you touch a doorknob, that voltage easily breaches your skin. So why doesn't it kill you? Because the total charge (and therefore the available current over time) is microscopic, and the internal resistance of the source (your shoes and the carpet) is astronomically high. The current spikes for a fraction of a microsecond and dissipates before it can disrupt your nervous system.
Arc Welding (Low Volts, High Amps)
A typical stick welder outputs 80V Open Circuit Voltage (OCV) and can deliver 200+ amps to the arc. If the machine can push 200 amps, why can you safely hold the ground clamp? Because the 200A rating is for the arc, which has a near-zero resistance once established. If you touch the electrode, the 80V OCV attempts to push current through your skin. Using our 100kΩ dry skin math, 80V / 100,000 Ω = 0.8 mA. You might feel a buzz, but the machine isn't forcing 200A through you; it's only delivering what the resistance allows. (Note: Wet conditions or leaning against a grounded metal workbench drastically lowers your resistance, making welding shocks highly dangerous).
Bench Power Supplies (Current Limiting)
When designing or testing circuits, makers often use bench supplies rated for 30V and 5A. If you accidentally touch the 30V terminals, the supply's 5A rating is irrelevant. 30V cannot push 5A through a human body. However, if you are working with high-voltage flyback transformers or neon sign power supplies (e.g., 5,000V at 30mA), the voltage is more than sufficient to push that 30mA through your skin, landing you squarely in the respiratory arrest zone.
AC vs DC and Common Confusions
One of the most dangerous misconceptions in electrical theory is treating AC and DC shocks as physiologically identical. They are not. According to data compiled by the CDC and NIOSH, 60Hz AC is roughly three to five times more dangerous than DC at the same voltage levels.
Why? Because 60Hz alternating current perfectly overlaps with the natural electrical frequencies of the human nervous system. AC causes tetanic contraction—your muscles lock up violently, often freezing your hand around the live conductor. DC, on the other hand, typically causes a single, violent muscle spasm that often throws the victim clear of the source (though this secondary impact can cause severe trauma).
Furthermore, the threshold for ventricular fibrillation in DC is much higher than in AC. It generally takes about 300-500 mA of DC to induce fibrillation, compared to just 100 mA of 60Hz AC. This is a critical distinction when sizing safety gear and calculating arc flash boundaries for 48V DC solar arrays versus 120/240V AC split-phase panels.
Ultimately, asking whether volts or amps kill is like asking whether the speed of a car or its mass causes the damage in a crash. The kinetic energy (current) breaks the bones, but the velocity (voltage) is what ensures the car actually reaches you. Respect both.
