Effects of current on the body

You will hear: “It’s not the voltage that kills, it’s the current.” That is correct — current through the body causes injury and death. Do not turn that into “voltage is safe.” Voltage is the force that pushes current through resistance, like pressure pushing water through a pipe.

There is no single answer to “How much current at 120 V?” It depends on body resistance, which changes with:

• sweat and moisture (salt water conducts well — hot days lower resistance),
• what you ate or drank,
• path of current (hand-to-hand vs hand-to-foot),
• contact area and pressure,
• broken skin.

Two people at the same voltage can have very different outcomes.

Effects vary person to person, but technicians memorize these general levels:

These numbers explain why GFCI devices trip around 5 mA of ground-fault current — far below the let-go zone.

The voltage considered most dangerous to work with in common field talk is often 120 V, because for many people it drives roughly 100–200 mA through the body — the fibrillation range.

Higher voltages are absolutely lethal too, but technicians encounter 120 V constantly: receptacles, lighting, control circuits, “minor” troubleshooting. Complacency is the enemy.

Large currents also cause internal burns that may look minor on the skin but are severe inside. Do not judge injury by appearance alone.

The worst path for survival is often hand to hand because current flows through the chest and heart. Hand to foot can still kill, but the same current may sometimes be less immediately fatal than hand-to-hand.

That is why safety training pushes one-hand work and insulated tools when you must be near energized parts. It is also why wet floors, metal ladders, and grounded equipment increase risk — they complete paths you did not plan.

Between 20 and 30 mA, muscles may contract so the victim cannot release the conductor. You cannot expect them to “just let go.” Shut off the source if you can do so safely; use a nonconductive rescue tool if trained and policy allows — never become the second victim by grabbing them with bare hands.

Above 30–40 mA, paralysis and breathing problems appear quickly. This ties back to not working alone and knowing CPR.

When someone says “It’s only 120,” your brain should answer with milliamperes, not comfort:

1. Can this path cross my heart?
2. Am I sweaty or on a conductive surface?
3. Is GFCI or LOTO in play?
4. Do I have a partner and CPR-capable help nearby?

Numbers turn bravado into procedure.

Above 200 mA, the heart may clamp shut rather than fibrillate. When current is removed, normal rhythm sometimes returns — the same principle behind defibrillator use for fibrillation (different problem, different treatment).

You are not an EMT because you read this — but you should understand why immediate shutdown of current and CPR matter while waiting for medical help. Time and path decide survival as much as voltage on the label.

Situation. On a humid attic, a technician contacts an energized bare splice with both hands while kneeling on a metal joist. They cannot let go until the breaker trips. Coworkers find them conscious but unable to breathe normally.

How to think with this lesson.

• Sweat lowered resistance; hand-hand path crossed the heart.
• Current was likely in the let-go and breathing difficulty range before the trip.
• Working alone would have removed immediate help.

Learning conclusion: treat 120 V and milliamperes seriously; control path, moisture, and backup help.