Electrical Shock Hazards

Ask most people what makes electricity dangerous and they will say "voltage" — a 12,000 V power line sounds far scarier than the 13.8 V on your shack's bench supply. That instinct is incomplete in a way that matters: it is current, not voltage, that actually damages tissue and disrupts the heart's electrical rhythm. Voltage is simply the pressure that pushes current through a path, and the path through your own body — specifically how much resistance that path offers — determines how much current a given voltage will actually produce. This lesson explains that relationship precisely, with the numbers every ham should know before opening up a piece of equipment.

The same 120 V touch voltage produces a barely-perceptible 1.2 mA through dry skin, but a likely-lethal 120 mA through wet skin.

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Why Current, Not Voltage, Is the Injury Mechanism

Every shock hazard ultimately comes down to one application of Ohm's Law: I = V / R. Your body is a resistor connected between two points of different potential, and the current that flows through that resistor is what causes injury — it disrupts the electrical signals your heart uses to beat in a coordinated rhythm, it can cause muscles to contract uncontrollably, and at high enough levels it generates enough heat to burn tissue from the inside out. Voltage matters only because it is the driving force — for a fixed body resistance, a higher voltage drives more current, but the same voltage can produce a current that ranges from completely unnoticeable to fatal depending entirely on how much resistance is in the path.

This is why a 9 V battery touched to dry, intact skin produces nothing you can even feel, while line voltage (120 V) under the wrong conditions can be lethal, and why high-voltage equipment inside a tube amplifier (covered in detail in M22B) demands the most serious respect of all. The voltage sets the ceiling on how much current is possible; your body's resistance at that moment determines how much of that ceiling is actually reached.

Body Resistance: Wet vs. Dry Skin

Human skin is actually a fairly effective insulator when dry and intact — dry skin resistance is commonly cited at approximately 100,000 Ω (100 kΩ) for a hand-to-hand path. This is the main reason most people can touch a 120 V circuit briefly under dry conditions and feel only a tingle or a startle, rather than being seriously injured: at 100 kΩ, 120 V drives only about 1.2 mA, which is at or just above the threshold of perception.

Wet skin is a dramatically different story. Moisture — sweat, humidity, a spilled drink, rain coming through an open shack window, or simply standing on a damp concrete garage floor — can lower skin resistance to roughly 1,000 Ω (1 kΩ), a hundredfold reduction. At 1 kΩ, that same 120 V now drives 120 mA, which sits well within the range where ventricular fibrillation is likely and often fatal (see the reference chart below). This hundredfold difference between wet and dry conditions is the single most important practical fact in this lesson: many shock fatalities involve perfectly ordinary household or shack voltages under wet or sweaty conditions, not exotic high-voltage equipment.

This is why working on a station or amplifier with sweaty hands, in a damp basement shack, or shortly after handling a beverage near the bench is measurably more dangerous than the exact same task performed with dry hands in a dry environment — not as a vague caution, but as a hundredfold difference in the current a given voltage will drive through you.

Current Pathway Through the Body

Where current enters and exits your body matters as much as how much current flows, because the injury that matters most — disruption of the heart's rhythm — depends on whether the current path actually crosses the heart. A hand-to-hand path (for example, touching a live terminal with your right hand while your left hand rests on a grounded chassis) sends current directly across the chest, through or very near the heart, and is the most dangerous common pathway. A hand-to-foot path (touching a live point while standing on a grounded floor) also crosses the torso and carries serious risk, though the exact path through the body depends on which hand and which foot. A foot-to-foot path (current entering one foot and leaving the other, as can happen from a step-and-touch voltage gradient near a downed power line or a faulty ground system) is generally less likely to pass directly through the heart, though it remains dangerous and can still cause severe injury, especially at higher currents.

AC vs. DC Shock Differences

Alternating current (AC) at the standard US line frequency of 60 Hz is generally considered more dangerous, milliamp for milliamp, than direct current (DC) at the same RMS current level, for two specific physiological reasons. First, AC at frequencies in this range is particularly effective at disrupting the heart's own rhythmic electrical signals, making ventricular fibrillation possible at lower currents than an equivalent DC exposure typically requires. Second, AC at 60 Hz tends to cause sustained, tetanic muscle contraction — the "can't let go" effect described in the reference chart below — because the current itself is repeatedly re-triggering the muscle fibers faster than they can relax, often locking the victim's hand around the very conductor that is shocking them. DC, by contrast, more often causes a single strong muscular contraction that can throw the person away from the source (which is itself dangerous in other ways, such as a fall) rather than holding them in continuous contact.

This does not make DC "safe" by any means — high-voltage DC, such as that found in some capacitor banks and amplifier power supplies (see M22B and M22C), is just as capable of delivering a lethal or severely injurious shock, and DC shocks are particularly associated with significant burns at the contact points due to localized heating and electrolytic tissue effects. The practical takeaway is that AC at line frequency is uniquely effective at the specific mechanism (fibrillation and inability to release) that makes low-level shocks so dangerous, which is one reason line-voltage hazards are treated with such consistent seriousness throughout this module.

Reference: Current Levels and Physiological Effects

The table below lists the generally accepted physiological effects of 60 Hz AC current passing through the chest, from imperceptible to fatal. These are approximate population-level thresholds — individual sensitivity varies with body size, health, current path, and duration of exposure — but they represent the consensus figures used throughout electrical safety literature and are the numbers every ham should have memorized.

Look again at the worked example above: wet-skin contact with ordinary 120 V line voltage produced 120 mA — squarely in the "usually fatal" row of this table. This is precisely why the wet-versus-dry distinction is not a minor footnote but the central fact of this lesson.

Calculator: Capacitor Discharge Time to a Safe Voltage

Many of the worst shock scenarios in ham radio do not involve a live, powered circuit at all — they involve a capacitor that was charged while the equipment was on and has not yet been safely discharged, sometimes long after the power switch was turned off (the full physics of this is covered in M22C). The calculator below estimates how long a given bleed resistor takes to discharge a capacitor from its initial voltage down to a chosen safe threshold.

If a Shock Accident Happens

1. Make the scene safe: de-energize the circuit before approaching, if at all possible.
2. Call 911 (or have someone else call) immediately for any shock beyond a minor tingle, even if the person seems alright afterward — cardiac arrhythmias from electrical shock can be delayed.
3. If the person is not breathing or has no pulse and you are trained, begin CPR immediately and continue until help arrives or an AED is available.
4. Treat visible burns as you would a thermal burn, but do not delay calling for emergency help to do so — electrical burns are frequently more severe internally than they appear on the skin surface.
5. Keep the person still and warm, and monitor breathing and responsiveness until emergency responders arrive, even if they insist they feel fine.