The generally accepted threshold for ventricular fibrillation—the primary mechanism of fatal electric shock—is 100 milliamps (0.1 Amps) of alternating current (AC) passing across the chest. However, under wet conditions, broken skin, or prolonged contact, as little as 30 mA (0.03 Amps) can be fatal. It is a fundamental rule of electrical theory: voltage provides the push, but it is the current (amperage) forced through the body's vital organs that disrupts the heart's electrical rhythm and causes death.
The Physiological Thresholds of Electric Shock
To understand why 100 mA is the critical benchmark, we must look at how the human nervous system reacts to increasing levels of AC current (specifically 50/60 Hz). The following table outlines the physiological effects of current passing hand-to-hand or hand-to-foot, based on classical Dalziel research and modern CDC/NIOSH Electrical Safety documentation.
| Current (AC 60Hz) | Physiological Effect | Lethality Risk |
|---|---|---|
| 1 mA (0.001 A) | Barely perceptible tingle | None |
| 5 mA (0.005 A) | Slight shock felt; not painful but disturbing. Standard GFCI trip threshold. | Low |
| 10 - 20 mA | "Let-go" threshold. Muscle tetany occurs; victim cannot release the conductor. | Moderate (indirect death from falls) |
| 30 mA (0.03 A) | Severe shock, respiratory paralysis begins. Standard EU RCD trip threshold. | High (if prolonged) |
| 100 mA (0.1 A) | Ventricular fibrillation. Heart muscle twitches chaotically, stopping blood flow. | Fatal within minutes without defibrillation |
| 500 mA+ (0.5 A) | Sustained myocardial contraction (heart clamps shut), severe internal burns. | Fatal (though heart may restart if clamp is brief) |
The Formula: Why Voltage and Resistance Dictate the Current
The current that actually flows through your body is not determined by the capacity of the power grid; it is governed by Ohm's Law: I = V / R (Current = Voltage / Resistance). The assumption that fixes the lethal answer is the condition of the skin barrier and the current pathway.
Dry, intact human skin has a high resistance, typically around 100,000 Ω. However, if the skin is wet, sweaty, or punctured by the conductor, resistance plummets to the internal body resistance, which is roughly 300 Ω to 1,000 Ω. Let's substitute these values into the formula for a standard US 120V circuit:
Wet/Broken Skin Scenario: I = 120V / 1,000 Ω = 0.12 A (120 mA) Result: Exceeds the 100 mA fibrillation threshold. Potentially fatal.
This is why a 120V household outlet can be a minor nuisance to an electrician with dry hands pulling a plug, but a lethal hazard to a homeowner standing in a flooded basement touching a faulty sump pump.
How the Lethal Threshold Shifts Across Voltages and Phases
The 100 mA physiological threshold remains constant, but the probability of reaching it shifts dramatically based on the supply voltage and phase configuration. As detailed in OSHA's Controlling Electrical Hazards guide, higher voltages destroy the skin's dielectric barrier almost instantly.
- 120V (Single-Phase US): May not break down dry skin. However, once sweat or moisture lowers resistance below 1,200 Ω, current exceeds 100 mA. This is why NEC code mandates GFCI protection in damp locations.
- 230V/240V (Single-Phase EU/UK/AU & US Split-Phase): The higher electromotive force aggressively breaks down skin resistance upon contact. A 230V shock will almost always push current well past the 300 mA mark through a human body, causing severe internal burns and immediate cardiac arrest.
- 208V/400V+ (3-Phase Industrial): Three-phase systems introduce multiple pathways. A hand-to-hand contact across two phases exposes the victim to the full line-to-line voltage. Furthermore, the 50/60 Hz frequency of 3-phase power perfectly aligns with the vulnerable period of the human cardiac cycle (the T-wave), making fibrillation highly likely even at slightly lower currents.
Decision Tree: Selecting the Right Protective Device
Knowing that 30 mA to 100 mA is the danger zone, we must select overcurrent and ground-fault protection that trips before current reaches those levels. Standard 15A or 20A thermal-magnetic breakers only protect the wiring from melting; they will happily pass 14 Amps through a human body without tripping. Use this decision path to select the correct personnel protection device:
| Environment / Condition | Required Trip Threshold | Concrete Part Pick |
|---|---|---|
| Standard dry indoor receptacles (Bedrooms, Living Rooms) | AFCI protection (Arc Fault); standard 15A/20A thermal trip for overcurrent. | Square D HOM120CAFI (15A AFCI Breaker) |
| Kitchens, Bathrooms, Outdoors, Garages (Wet/Damp risk) | 5 mA Ground Fault Trip (US/Canada NEC mandate) | Square D HOM120GFIC (15A GFCI Breaker) |
| EU/UK/AU Residential Wet Areas (IEC Standards) | 30 mA Residual Current Trip | Siemens 5SU1356-6KA (30mA RCD/RCBO) |
| Industrial 3-Phase Machinery (Personnel Protection) | 30 mA RCD for shock; 300mA for fire prevention. | Eaton FAZ-D32/3-N (with 30mA RCD module) |
When Current Calculations Become Meaningless
Applying Ohm's Law to determine shock hazard becomes meaningless when the power source is strictly current-limited or operates at extreme high-frequency/high-voltage parameters where skin effect and capacitive coupling dominate.
For example, a neon sign transformer might output 10,000V, but its internal impedance limits the maximum current to 30 mA. While painful, it physically cannot push the 100 mA required for fibrillation. Similarly, an electrostatic discharge (ESD) from walking across a carpet can generate 30,000V, but the total charge is measured in micro-coulombs, resulting in a current pulse of microamps that lasts for nanoseconds. Conversely, a 12V car battery can supply 500 Amps to a starter motor, but 12V lacks the electromotive force to push even 1 mA through intact human skin (I = 12V / 100,000 Ω = 0.12 mA). The hazard is not the source's capacity to deliver current; it is the voltage's ability to force current through your specific resistance.
FAQ: Common Safety Misconceptions
Q: Is DC current less dangerous than AC current?
A: Yes, at lower thresholds. DC current does not cause the continuous muscle tetany (the "let-go" effect) that 50/60 Hz AC does, meaning a victim is more likely to be thrown clear of a DC source. Furthermore, DC requires roughly 3 to 5 times the amperage to induce ventricular fibrillation compared to AC. However, high-voltage DC (like in solar arrays or EV batteries) is extremely lethal due to sustained arcing and severe thermal burns.
Q: If a 15A breaker trips at 15 Amps, why do I need a 5mA GFCI?
A: A 15A thermal-magnetic breaker is designed to protect copper wire from melting and starting a house fire. It will not trip at 100 mA (0.1 A). If you become the path to ground, 100 mA will flow through your heart, killing you, while the 15A breaker remains completely unaware because 0.1 A is well below its 15 A trip curve. A GFCI monitors the imbalance between hot and neutral, tripping in milliseconds if it detects as little as 5 mA leaking to ground (i.e., through you).
Q: Can a 120V shock really be fatal?
A: Absolutely. According to the NFPA 70 (National Electrical Code), 120V is responsible for the majority of residential electrocutions. If your skin is wet, or if the current path crosses the chest (e.g.,左手 to right foot), 120V will easily push 100+ mA through the body, triggering fatal ventricular fibrillation.
