What actually hurts people
Not RF. The injuries in amateur radio come overwhelmingly from mains voltage, stored charge, batteries, and falling — in roughly that order. This lesson covers the first three; towers get their own.
Current is what injures, not voltage
Voltage is what pushes; current through the body is what does the damage. The effects scale with how much flows and where it goes:
- A few milliamperes: perceptible tingle.
- Around 10 mA: muscles contract and may not let go.
- Around 100 mA across the chest: ventricular fibrillation.
The pool asks what hazard current through the body poses and answers “all these choices” — burns, muscle paralysis, cardiac arrest — because all of them are real at different levels.
The practical consequence is that low voltage is not automatically safe. A 12 V battery cannot push meaningful current through dry skin, but it can push hundreds of amperes through a wrench, and that is a different kind of injury.
Batteries
A 12 V storage battery without internal overcurrent protection is a hazard because shorting the terminals can cause burns, fire, or an explosion. There is no current limit worth speaking of; a lead-acid battery will deliver hundreds of amperes into a short until something melts.
Charging and discharging rapidly brings a second hazard: overheating or out-gassing. Lead-acid cells vent hydrogen, which is explosive in air, and lithium cells fail more energetically still.
Practical habits: fuse the battery lead within a few inches of the positive terminal, remove metal jewellery before working near terminals, and never rest a tool across a battery top.
Fuses and breakers
A fuse exists to remove power in the event of an overload. Note what it does not do: it does not protect you, and it does not protect the equipment from most faults. It protects the wiring from becoming a heater.
Two rules follow:
Never fit a larger fuse than specified. Replacing a 5 A fuse with a 20 A one means the wire, sized for 5 A, now carries four times that before anything interrupts. Recall that heat goes as the square of current: sixteen times the dissipation in the same wire. Excessive current could cause a fire — and that is the pool’s answer, in those words.
Fuse the hot conductor. In a 120 V AC circuit, the fuse or breaker goes in series with the hot conductor only. A fuse in the neutral can open while the equipment remains connected to the hot side — still live, and now with no obvious sign of it.
In US 120 V three-wire practice: black is hot, white is neutral, green (or bare) is safety ground.
Stored charge
A power supply is dangerous immediately after it is switched off, because its filter capacitors hold charge. A large supply can hold a lethal charge for minutes.
Before working inside anything with a mains-powered supply: switch off, unplug, wait, and then verify with a meter that the capacitors have discharged. Bleeder resistors are supposed to handle this and sometimes fail open.
Measuring high voltages
When measuring high voltage with a voltmeter, ensure the meter and its leads are rated for the voltage being measured. Standard probe sets are commonly rated for a few hundred volts and will flash over above that, with your hand on the probe.
Keep one hand in a pocket while probing live circuits. It sounds like folklore; it prevents current crossing the chest.
Guarding against shock
The pool asks for a good way to guard against shock and answers “all these choices are correct”. Those choices amount to:
- Use three-wire cords and plugs for all AC-powered equipment.
- Connect all AC-powered equipment to a common safety ground.
- Install mechanical interlocks in high-voltage equipment, so opening the cabinet removes the power.
Grounding and lightning
Two rules cover most of it.
Bond everything. All external ground rods and earth connections must be bonded together with heavy wire or conductor, including the electrical service ground. Separate, unbonded grounds are more dangerous than one: a surge raises one relative to another, and the difference appears across whatever bridges them — normally your equipment, normally while you are touching it.
Arrest at the entry point. A lightning arrester belongs on a grounded panel near where the feed lines enter the building, not at the antenna. The point is to divert the surge to earth before it gets inside.
Check yourself
- Why is fitting a 20 A fuse where a 5 A fuse belongs dangerous?
- Your power supply has been off for ten seconds. Is it safe to open?
- You add a ground rod for the antenna system. What must you also do?
Answers
- The wiring is sized for 5 A. At 20 A it dissipates sixteen times the heat before anything opens — a fire risk, which is exactly what the pool says.
- No. Filter capacitors hold charge after switch-off. Unplug, wait, and verify with a meter.
- Bond it to every other ground rod and to the electrical service ground with heavy conductor.