The 'Amps Kill, Volts Push' Reality
You have likely heard the workshop adage: 'It's not the volts that kill you, it's the amps.' This is a dangerous half-truth. To understand why, we need to look at Ohm's Law (I = V / R). Current (I) is the actual flow of electrons tearing through tissue and scrambling the sinoatrial node in your heart. Voltage (V) is simply the electromotive force—the electrical pressure—available to push those electrons. Resistance (R) is what opposes that flow. Think of voltage as water pressure in a pipe, and current as the actual volume of water flowing out. A massive water tower might have immense pressure (voltage), but if the valve is barely open (high resistance), only a trickle (low current) escapes. Conversely, a wide-open fire hydrant at low pressure might flow hundreds of gallons a minute, but it won't cut through steel. In electrical shock, your body is the resistor. Dry, intact human skin has a surprisingly high resistance—often between 50,000 and 100,000 ohms. At 12 volts, that resistance is so high that the resulting current is measured in microamps, entirely harmless. But if the voltage is high enough to break down the skin's dielectric layer, or if the skin is wet, resistance plummets, and the voltage pushes a lethal volume of current through the internal organs.Lethal Thresholds and Human Body Resistance
To understand what changes in a real circuit when a human becomes part of it, we must look at the physiological thresholds defined by the Canadian Centre for Occupational Health and Safety (CCOHS) and the IEC 60479-1 standard. The effect of current on the human body is highly non-linear and depends heavily on whether the source is alternating current (AC) or direct current (DC). AC at standard mains frequencies (50/60Hz) is roughly three to five times more dangerous than DC at the same magnitude because the continuous zero-crossing of the waveform perfectly mimics the frequency that triggers muscle tetany and cardiac fibrillation.| Current (mA) | Physiological Effect | AC (50/60Hz) Response | DC Response |
|---|---|---|---|
| 0.5 - 1.0 | Perception Threshold | Mild tingling sensation | Slight warmth at contact point |
| 5.0 - 10.0 | 'Let-Go' Threshold | Muscle spasms; cannot release conductor | Sharp pain; muscles contract but release is possible |
| 15.0 - 30.0 | Respiratory Paralysis | Chest muscles lock; breathing stops | Severe pain; breathing difficulty |
| 30.0 - 50.0 | Ventricular Fibrillation Onset | Heart rhythm disrupted; fatal without immediate defibrillation | Heart rhythm disrupted; severe internal burns |
| 100.0+ | Severe Tissue Damage | Cardiac arrest; deep electrical burns; nerve destruction | Massive thermal burns; tissue carbonization |
Safety Caveat: The 'let-go' threshold is the critical danger zone for electricians. At just 10mA of 60Hz AC, your forearm flexor muscles (which are stronger than your extensors) will involuntarily contract, clamping your hand around the live wire. You physically cannot let go, prolonging the exposure until respiratory paralysis or fibrillation occurs.
Worked Example: 120V Mains vs. 12V Car Battery
The most common confusion regarding what kills you volts or amps stems from misunderstanding available current versus drawn current. People often point to a 12V car battery, which can supply 600 amps to a starter motor, and ask why it doesn't kill you, while a 120V wall outlet that only supplies 15 amps is lethal. Let's run the math using Ohm's Law. Assume you are working in a damp garage, sweating, and you grab a bare conductor with wet hands. Your skin's dielectric barrier is compromised, dropping your internal hand-to-hand body resistance to roughly 1,000 ohms.Scenario A: The 12V Car Battery
The battery has 600A of available capacity, but it only pushes what the circuit's resistance allows. I = V / R I = 12V / 1,000 ohms = 0.012A (or 12mA). Looking at our table, 12mA of DC will cause sharp pain, but you will easily let go. The battery's 600A capacity is irrelevant because the 12V lacks the pressure to push more than 12mA through your 1,000-ohm resistance.Scenario B: The 120V AC Mains Outlet
The breaker is rated for 15A, meaning it will trip if the circuit draws more than 15A. I = V / R I = 120V / 1,000 ohms = 0.120A (or 120mA).120mA of 60Hz AC is more than double the threshold for ventricular fibrillation.
The outlet only needed to supply 0.12A to kill you—well within the 15A capacity of the branch circuit. The 120V provided the necessary pressure to drive that 120mA through your compromised skin resistance. The breaker doesn't trip because 120mA is far below the 15A thermal trip curve, which is exactly why standard breakers do not protect against lethal human shock.
