T0A

Power, fuse, battery and lightning safety

Spot the shock, fire and battery hazards in a station, and know where fuses, arresters and grounds belong.

12 pool questions1 on the exam

Syllabus: Power circuits and hazards: hazardous voltages, fuses and circuit breakers, grounding, electrical code compliance; Lightning protection; Battery safety

Current through you hurts; stored charge still bites

Current through the body heats tissue, disrupts cells and clenches muscles. Every one of those is a hazard.

Four ways to stay safe: three-wire cords, one common ground, discharged capacitors, rated meters.

Do all of these: use three-wire cords, bond equipment to a common safety ground, and discharge capacitors before working inside. A power supply's filter capacitors keep their charge after switch-off. When measuring high voltage, use a meter and leads rated for that voltage.

RememberOff is not safe: capacitors hold charge. Ground everything. Rate your meter.

Fuses go in the hot wire, sized to the wiring

In US 120 V cable black is hot, white is neutral, green is the equipment ground. A fuse or breaker goes in series with the hot wire only, and removes power when current is too high.

Break the hot wire and the whole circuit is dead. The fuse protects the wire and the equipment.

Use the rated fuse. A 20 A fuse won't blow at currents that already overheat 5 A wiring, so the wire overheats: fire.

The wiring can take 5 A. A fuse must open before the wire overheats, so never use a larger fuse.

Current drawn (fault)15 A

Fuse5A

RememberFuse = in series, hot wire, correct rating. Oversized fuse = fire.

Batteries: never short them, never rush them

A 12 V battery without protection circuitry can deliver huge current. Shorting the terminals gives burns, fire or an explosion. Rapid charge or discharge overheats it and makes it out-gas.

Batteries hold a lot of energy. Short circuits and rapid charge or discharge turn it into heat and gas.

Arrester at the entry, and bond every ground

Divert the surge to ground before it enters the house, and tie every ground together so none rises above the rest.

Put the lightning arrester on a grounded panel where feed lines enter the building. Then bond all external ground rods together with heavy wire or strap.

RememberArrester at the entry panel. All grounds bonded.

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Compendium articles on the ideas in this lesson, beyond what the exam asks.

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Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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