Electrical safety in the shack

This is the part of the syllabus where getting it wrong has consequences beyond the exam.

What actually harms you is current, not voltage

A shock's severity depends on the current flowing through your body, and that follows Ohm's Law: current equals voltage divided by resistance.

Your body's resistance is mostly skin resistance, and it varies enormously:

• Dry skin: tens of thousands of ohms
• Wet or sweaty skin: a few thousand ohms
• Broken skin: very low

This is why mains voltage contacted with wet hands while standing on a damp floor is dramatically more dangerous than the same voltage touched with dry hands on a dry floor. Same voltage, far less resistance, far more current.

Currents in the tens of milliamperes across the chest can cause ventricular fibrillation. It does not take much.

Fuses

A fuse protects the equipment and the wiring, not you. It goes in series with the live supply conductor, so it carries the full current and can break the circuit when that current becomes excessive.

Never fit a fuse in the earth conductor, and never replace a fuse with a higher rating "because it keeps blowing". A fuse that blows repeatedly is reporting a fault.

Earthing

A proper earth connection gives fault current a low-resistance path back to source, so protective devices operate quickly and exposed metalwork does not become live.

Its purpose is safety. Any RF benefit is a separate matter and a secondary one.

The capacitor trap

A power supply can remain lethal after it is switched off and unplugged.

Large filter capacitors store energy. In a high-voltage supply — a valve amplifier, for example — that stored charge can kill you minutes after the mains has been removed.

The discipline:

1. Switch off and unplug.
2. Wait for bleeder resistors to do their job.
3. Verify with a meter that the capacitors are discharged.
4. Only then work on the equipment.

Never assume. Measure.

Working habits that matter

• Keep one hand in your pocket when probing live circuits. Current across the chest, hand to hand, is the dangerous path.
• Do not work alone on high-voltage equipment.
• Use insulated tools and keep the bench dry.
• Fit a residual current device on the shack supply.
• Route mains and RF cabling separately and keep both tidy.

RF burns

RF is a different hazard from mains shock. High voltage exists at the ends of a radiating antenna, and contact causes a deep RF burn — painful and slow to heal because the damage is under the skin surface rather than on it.

The rule is simple: do not touch an antenna while transmitting.