TL;DR:
Most robot accidents happen at power-up. A written checklist makes you look at the same things every time.
Choose a fuse above the normal running current and below what your wire can carry. Stall current from V ÷ R tells you the worst case.
Lithium batteries need the right charger, a safe place and a retired-when-damaged rule.
Robots look harmless until a battery gets hot or a gear catches a finger. The dangerous moment is almost always the same: someone switches on power to something that was not checked. A short checklist, done every time, is the cheapest safety equipment you can own.
What this checklist covers
It is for low-voltage robots of the kind built in classrooms and clubs: a battery pack, a motor driver, motors, and a controller board. It is not for anything connected to mains electricity. If your project needs wall power, stop and ask a qualified adult. Follow your school or club's own rules in addition to this list, and the manual of your battery and charger.
Worked example: the checklist for a classroom robot
Say your team robot has two motors and a two-cell lithium polymer pack. Before power goes on, someone reads each line aloud and another person confirms it.
Look at the battery: No puffing, dents, torn wrap or damaged wires. A swollen or damaged pack is retired, not used.
Check the connectors: The plug matches the socket and goes in one way only. Red to red, black to black.
Check wiring for shorts: No bare wire touching another wire or the frame. Strain relief on wires near moving parts.
Check the fuse and switch: A fuse of the right rating sits on the battery positive, and a power switch is within reach.
Secure moving parts: Wheels, arms and gears are tight. Lift the robot so the wheels are off the table for the first power-up.
Clear the area: People back, loose hair and sleeves tied, safety glasses on. Say "power on" out loud.
Test the stop: Before running a program, check that the switch or emergency stop cuts power.
Choosing a fuse: the arithmetic
A fuse is a deliberate weak link that opens when current gets too high. Its rating should sit above the running current but below what the wire can carry.
A two-cell lithium pack has a nominal voltage of
2 × 3.7 V = 7.4 V.A motor with winding resistance of about 3.7 Ω would, if it were stopped (stalled), draw
I = V ÷ R = 7.4 ÷ 3.7 = 2 A.If normal running is around 0.5 A per motor, two motors draw about 1 A. A stall of one motor plus the other running is
2 + 0.5 = 2.5 A.If the wire is rated for 5 A, a 3 A fuse sits above 2.5 A and below 5 A.
The values are an example. Your motors, wires and fuse will differ; use the datasheets. The reasoning carries over: find the worst case from the stall current, then choose a fuse between that and the wire's limit. A fuse that is too big does not protect, and one that is too small blows during normal use.
Fuses: Never replace a blown fuse with a bigger one, a piece of wire or foil. Find out why it blew first.
Lithium batteries
Charge only with a charger made for that battery type and cell count, and never leave it charging unattended.
Charge and store on a non-flammable surface, away from anything that burns. A fire-resistant bag or box is a good idea.
Never puncture, crush, short or heat a cell. A short circuit across a lithium pack can cause intense heat very quickly.
Retire a pack that is swollen, dented, hot while idle, or smells odd. Take it to a proper battery recycling point, not the bin.
Do not run a pack to empty. Use a low-voltage cutoff or alarm if your system has one.
Make the checklist a habit rather than a rule. Print it, keep it with the robot, and let anyone on the team, not just the most senior person, say stop if a line is not satisfied. A new member who spots a loose wire has done the team a favor. Run the list again whenever something changes: a new battery, a rewired motor, a different program. Most accidents happen right after a small change that seemed too minor to check.
Pinch points and moving parts
Look for places where two parts move toward each other: gear meshes, closing grippers, arm joints, belts and chains. If a finger could get in, add a guard or keep hands clear, and never reach into a robot that is powered. Treat a robot arm as able to move without warning when the program starts.
The emergency stop
An emergency stop is a way to cut power quickly that does not depend on the software. A big switch on the battery lead is a good start. Test it often. For a larger robot, a wireless or wired stop that cuts motor power is worth its cost.
After the test
Switch off and disconnect the battery before touching anything.
Feel for warm motors, wires and the driver before packing up. Heat is a clue.
Store batteries as the manual says, not left in the robot.
Frequently asked questions
Is a small robot safe enough to skip the checklist?
The checklist is quick and the habit is what counts. Small robots still pinch fingers and short circuits still get hot.
Can I use a power adapter from the wall to run a robot?
Only an adapter designed for the device, with its output matching what the robot needs, and never a homemade connection to mains. Ask a qualified adult.
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