Section 5.2: Hazardous Voltages #

What Are Hazardous Voltages? #

A common rule of thumb in ham radio is to treat anything above 30 volts as potentially hazardous — though OSHA’s formal threshold is 50 volts. Either number is a starting point, not a safety guarantee. What actually causes injury is current flowing through you, and how much current a given voltage can push depends on your skin resistance, how wet things are, and how much current the source can supply in the first place.

🔥 A real-world reminder from the primary author of this book (KD7BBC): while installing a circuit breaker on a 24V battery bank, his wrench contacted the silver ring on his finger, which touched the metal trailer wall. The wall was bonded to DC ground — so the battery was now shorted through the ring. The current melted the ring enough to partially fuse it to the wrench and caused near third-degree burns. Twenty-four volts isn’t classed as “hazardous,” yet he still has a circumference scar on that finger. Never assume a voltage is safe just because it isn’t officially “hazardous.”

Where Hazardous Voltages Show Up #

Hazardous voltages can show up in several places around a ham station:

Power Supplies: That 13.8V power supply for your radio converts 120V AC to DC internally. One wrong move inside the case and you could be in for a shock.
Antenna Systems: Some antennas develop high voltages at their feed points during transmission, especially at higher power levels. Never touch an antenna while it’s transmitting.
Older Equipment: Vintage tube gear can have voltages in the hundreds or even thousands of volts. If you’re lucky enough to be working with it, be extra cautious.
Capacitors: Capacitors don’t care whether the equipment is unplugged. They can hold dangerous charges for hours or even days after power is removed.
Test Leads: When you’re measuring hazardous voltages, the probes are at those voltages too — easy to overlook when focused on taking a reading.

Staying Safe Around Hazardous Voltages #

Remove metal jewelry first: Rings, watches, and bracelets are excellent conductors. A wedding ring across a high-current source can short a circuit through your finger before you even know what’s happening (as the author can attest).
Assume everything is “hot”: Use a voltmeter to verify before you touch anything.
One hand rule: Keep one hand in your pocket when working around high voltages. If you do get shocked, this prevents current from flowing across your chest — and across your heart.
Discharge capacitors properly: Before opening equipment that might have charged capacitors, discharge them through a resistor of appropriate value. Never short them directly.
Use properly insulated tools: Tools with insulated handles rated for the voltage you’re working with are marked with voltage ratings (often 1000V). Check that the insulation isn’t cracked or damaged before using them.
Keep your workspace dry: Water and electricity don’t mix. Make sure your work area is dry and you’re not standing on a damp floor or concrete (which can be conductive).
Never work alone: When working on high-voltage equipment, have someone nearby who could help in an emergency.
When in doubt, don’t touch it: There’s no shame in asking for help or leaving it to the experts.

If you’re regularly working around high-voltage equipment, it’s worth taking a first aid course that covers electrical shock response. This isn’t the place for us to walk you through that procedure — shock first aid has real subtleties that deserve hands-on training.

Electrical shock is only one way equipment can hurt you. The next section covers the devices specifically designed to stop things from going wrong in the first place: fuses and circuit breakers.