Safety
Safety is competency 1.0 for a reason — it comes first. The Associate CET exam expects you to know how electricity injures, how to work on equipment safely, the fire classes, and how to respond to an incident. These are high-yield, memorizable facts; this page collects the ones the exam draws from.
The single most important safety concept: current, not voltage, is what harms you. Voltage is what drives current through your body, but it's the amount of current and its path that determines the danger. As little as 100 mA through the chest can cause ventricular fibrillation — the most common cause of death by electric shock.
| Current through body | Typical effect |
|---|---|
| ~1 mA | Barely perceptible tingle |
| ~5 mA | Slight shock, not painful; startle reaction can cause injury |
| 10–20 mA | "Can't let go" — muscles contract on the conductor |
| ~50 mA | Severe pain, breathing difficulty |
| 100–300 mA | Ventricular fibrillation — likely fatal |
Because current depends on the path and on body resistance, the practical rules all aim to keep current from flowing through you:
One hand behind your back when probing live circuits keeps current from crossing your chest hand-to-hand. Discharge capacitors before working — a charged filter capacitor in a power supply can hold a lethal charge long after power is removed. Use a GFCI on bench outlets, keep one hand in your pocket, remove rings and watches, and never work alone on dangerous equipment.
Lockout/Tagout is the procedure for making sure equipment stays de-energized while it's being serviced. You physically lock the energy-isolating device (a breaker or disconnect) in the off position and attach a tag identifying who locked it and why. Only the person who applied the lock removes it.
- Notify everyone affected that equipment is being shut down for service.
- Shut down the equipment by its normal stopping procedure.
- Isolate the energy source (open the disconnect/breaker).
- Lock and tag the isolating device in the off position with your own lock.
- Release stored energy — discharge capacitors, bleed pressure, block parts that could move.
- Verify zero energy: test that the equipment won't start and measure that voltage is actually gone.
A technician must know which extinguisher fits which fire — using the wrong one can make things worse (water on an electrical fire is a shock and spread hazard).
Not all safety is about protecting people — Electrostatic Discharge (ESD) protects the parts. A static spark you can't even feel (hundreds of volts) can destroy or weaken sensitive devices like CMOS chips and MOSFETs. Control it with a grounded wrist strap, an ESD mat, anti-static bags, and a humidity-controlled bench.
Do
Cut the power first — the disconnect, breaker, or unplug. If you can't, use a non-conductive object to separate the victim from the source. Then call for help and begin CPR if trained and needed.
Don't
Never touch a person who is still in contact with a live source — you'll become a second victim. Don't use anything conductive (or wet) to move them. Don't assume "low" voltage is safe.