Electrical Safety
The Electronics School safety rules, and the science behind every one of them.
The question
If a 9 V battery is safe to touch, why is the 120 V outlet on your wall so dangerous?
What you’ll discover: the safety rules for Electronics School, and the reasons behind each one.
The Electronics School rules
- Batteries only. Every project here runs on AA batteries, or on a USB cable for the microcontroller projects. Never use wall outlets, extension cords or anything that plugs into the wall. The one exception is the optional soldering iron (Lesson 18), which an adult sets up.
- Never open things that plug in, or anything with a flash. Old TVs, microwaves, power supplies, and disposable cameras or flash units (even battery-powered ones) can hold a dangerous charge inside long after they’re switched off or unplugged.
- No short circuits. Never connect a wire straight across a battery. Always have a load, and a resistor with any LED.
- Disconnect before you change. Disconnect the battery pack, or switch it off, before moving wires.
- Check for heat with your eyes and nose, never your fingers. If something smells hot, smokes or a battery swells, disconnect it, step back and tell an adult. Don’t touch it to check.
- Batteries the right way round. Match + to + every time. Never mix old and new batteries, or different types, in one pack.
- Never take batteries apart, and never use damaged ones. Leave lithium battery packs and loose lithium cells alone.
- Store batteries safely. Keep them where their ends can’t touch keys, coins or other metal, and put tape over the terminals of any 9 V battery. Never try to recharge ordinary alkaline batteries, and never put any battery in a fire.
- Recycle used batteries. Don’t put them in the garbage. Find a drop-off near you through Call2Recycle Canada.
- Outdoors, stay well away from overhead power lines, and never touch an electric fence.
- Small parts, small children. Keep LEDs, resistors, transistors and batteries away from children under 3. They’re a choking hazard.
- Grown-ups nearby. Under about 10: an adult at the table. Older builders: an adult at home who has read this page. Soldering is never required here, and when anyone does solder, it’s with an adult, good ventilation and eye protection.
Button batteries: a special warning
Keep button batteries away from young children. Health Canada warns that a swallowed button battery can cause severe internal burns in as little as two hours, and even used batteries are dangerous. If you think a child has swallowed one, go to the nearest emergency department immediately.
None of the Electronics School projects use button batteries.
Why low voltage is safer
BeginnerIt’s about how much current gets through you
What harms people is current flowing through the body. Dry skin has a fairly high resistance, so a small battery’s low voltage can’t push a dangerous current through it. You might not feel a 9 V battery at all, except on your tongue (don’t try that).
A household outlet in Canada supplies 120 V, and some appliances use 240 V. That’s enough push to drive a dangerous, even deadly, current through a person. That’s why we never experiment with wall power.
IntermediateLow voltage can still burn
Low voltage protects you from shocks, but it doesn’t stop heat. A short circuit on even a small battery can drive a large current, making wires and batteries hot enough to burn skin or melt plastic. That’s why the “no short circuits” rule matters as much as the “batteries only” rule.
AdvancedWhy current, not voltage, is the danger
By Ohm’s law, the current through your body is I = V ÷ Rbody. Body resistance varies widely: dry, intact skin can be many thousands of ohms, but wet or broken skin is far lower, and the inside of the body is lower still. A few milliamps through the body is noticeable; currents in the tens of milliamps across the chest can disrupt the heart. Mains voltage can easily drive currents in that range, especially with wet hands, which is why newer Canadian homes must have GFCI outlets in bathrooms, near sinks and outdoors. These cut the power when current leaks to ground. Older homes may not have them.
Quick check
Why don’t we ever use wall outlets in Electronics School?
Household power in Canada is 120 V (and 240 V for some appliances), enough to push a dangerous current through a person.
A battery pack starts to feel warm and smell hot. What do you do?
Disconnect it if you can do that safely, step back and tell an adult. Don’t touch it to check how hot it is.
Can a low-voltage battery still be dangerous?
Yes. A short circuit can make wires and batteries hot enough to burn, and button batteries are very dangerous if swallowed.
Words to know
- Low voltage
- A small electrical push, like the 1.5 V of an AA battery. Safer, but it can still cause heat.
- Mains power
- The electricity from wall outlets: 120 V in Canadian homes. Never experiment with it.
- GFCI
- Ground fault circuit interrupter: an outlet that cuts power when current leaks where it shouldn’t.
Learn more
- Health Canada: Button battery safety
- Parachute: Electrical safety at home: Canada’s injury-prevention charity
- Next: LEDs and resistors
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.