"Don't touch that — it's 10,000 volts!" Sounds dangerous, and it can be. But voltage alone doesn't kill — current through your body does. A static shock from a doorknob can be 30,000 volts and harmless. A 120V outlet is potentially lethal. Here's why.

What kills in an electrical shock

Current passing through your body interferes with normal cellular function. Specifically:

  • 0.001 A (1 mA): imperceptible.
  • 0.005 A (5 mA): noticeable tingle.
  • 0.010 A (10 mA): grasp reflex — muscles contract; can't let go of the source.
  • 0.030 A (30 mA): respiratory paralysis. If sustained, fatal.
  • 0.050 A (50 mA): ventricular fibrillation. Heart stops pumping effectively. Highly lethal.
  • 0.100 A (100 mA): almost certain death without immediate intervention.
  • 0.500 A (500 mA): heart stoppage, severe burns.

The "lethal threshold" depends on path through body, duration, and individual variation. Roughly 50–100 mA across the chest for ~3 seconds is typically fatal.

Why voltage doesn't directly kill

Voltage is the "push." Current is what flows in response. The resistance of your body determines how much current actually flows for a given voltage.

Body resistance varies:

  • Dry skin, hand to hand: 100,000 Ω+ resistance.
  • Wet skin, sweaty palm: 1,500–5,000 Ω.
  • Wet hands, in water: as low as 500 Ω.
  • Internal body (no skin): ~300 Ω. Very low.

So 120V on dry hands: I = V/R = 120/100,000 = 1.2 mA. Below feel threshold.

120V on wet hands: 120/2,000 = 60 mA. Lethal.

Same voltage, vastly different outcomes depending on body resistance.

The 9V battery paradox

A 9V battery has plenty of voltage to feel — but you barely feel it on dry skin. Why? Body resistance is too high. 9 / 100,000 = 0.09 mA. Below threshold.

Lick a 9V battery (low resistance) and you feel a noticeable tingle. About 0.5 mA through your tongue (~18,000 Ω).

Skin acts as a powerful insulator until it gets wet or breaks.

The static shock paradox

Walking on carpet builds up static charge of 5,000–30,000 volts. When you touch a doorknob, you discharge it. The voltage is enormous; the current is tiny because:

  • Total charge: very small (microcoulombs).
  • Discharge time: nanoseconds.
  • Average current: enough to feel; not enough to harm.

The math: 30,000V across 5,000 Ω wet hand = 6 A peak — but only for nanoseconds. Total energy delivered is tiny.

Static shocks are uncomfortable but not dangerous because the duration is so short.

Why high-voltage power lines are dangerous

Compared to static, the power grid maintains high voltage continuously. Touch a 7,200V distribution line and you're connected to a near-infinite source of current. The line will deliver whatever current your body can carry until you let go (or are forced off).

Even brief contact at high voltage causes severe burns from the heat of current passing through tissue. Sustained contact is rapidly fatal.

Plus: high voltage can arc through air. You don't have to touch the wire — getting too close can complete the circuit through ionized air. This is why distribution lines have specific clearance distances.

Path matters

Current entering one hand and leaving via the other passes through your chest — including the heart. This is the most dangerous path because:

  • Heart muscle is sensitive to even small currents.
  • 50 mA across the chest can cause ventricular fibrillation.
  • Even brief exposure (less than a second) can be fatal.

Current entering and leaving the same hand (hand-to-elbow on the same side) avoids the chest. Less dangerous; severe burns possible.

Current through the head can affect brain function. Less common in accidents but worth understanding.

The duration factor

Brief exposure to lethal current may not actually kill. The "let-go threshold" is the current level below which you can voluntarily let go (around 10 mA for most adults).

Above 10 mA, your forearm muscles contract involuntarily; you can't release. So you stay in contact with the source, accumulating exposure.

This is why GFCI outlets matter so much. They detect imbalance between hot and neutral (current going somewhere it shouldn't, like through you) and trip in 1/40 of a second. Faster than electrocution.

What protects you

Multiple layers of safety:

  • Insulated wiring prevents accidental contact.
  • Grounded systems let stray current flow to earth instead of through people.
  • Circuit breakers trip if current exceeds rated amount (typically 15A or 20A on home circuits).
  • GFCI outlets detect imbalanced current and trip in milliseconds. Required in bathrooms, kitchens, outdoors.
  • AFCI outlets detect arcing and trip. Required in bedrooms.

Modern electrical codes are designed around current paths, not voltage. Lower the available current = lower the danger.

Voltage classifications

  • Extra-low voltage (under 50V): generally safe under most conditions.
  • Low voltage (50–1000V): household and small commercial. Lethal under wet conditions.
  • Medium voltage (1–35 kV): distribution lines. Always lethal on contact.
  • High voltage (35–230 kV): regional transmission. Arcing distance is meters.
  • Extra-high voltage (230 kV+): long-distance transmission. Special safety distances.

Practical safety rules

  1. Don't work on live circuits. Turn off the breaker.
  2. Use one hand when probing live circuits (limits current path through chest).
  3. Stand on insulating material (rubber mat, dry wood).
  4. Verify the circuit is dead with a tester before working.
  5. For high-voltage equipment, use proper PPE (insulated gloves, hot sticks).
  6. Install GFCI in wet locations.

If someone is being shocked

  1. Don't touch them directly. You'll join the circuit and become a victim.
  2. Cut the power at the breaker if you can identify it.
  3. Use insulated tool (broom handle, dry rope) to push the victim away from the source if possible.
  4. Call 911. Even if the person seems okay, internal injuries from electrical shock can be severe.
  5. Check breathing and pulse. CPR if needed.

Calculate the math

Our Ohm's law calculator shows how voltage, current, and resistance interact. Useful for understanding why specific scenarios are dangerous (or not) — and why GFCI protection makes a meaningful difference.