Each year, roughly 3,300 home fires originate in extension cords, killing 50 people and injuring 270 more. Extension cords can overheat and cause fires when used improperly, so keep these 12 important tips in mind to protect your home and workplace.

1. Don’t attempt to plug extension cords into one another.
2. Make sure extension cords are properly rated for their intended use—indoor or outdoor—and meet or exceed the power needs of the device being used.
3. Keep all outdoor extension cords clear of snow and standing water.
4. Do NOT overload extension cords.
5. A heavy reliance on extension cords is an indication you have too few outlets to address your needs. Have additional outlets installed where you need them.
6. Inspect cords for damage before use. Check for cracked or frayed sockets, loose or bare wires, and loose connections.
7. Do NOT nail or staple extension cords to walls or baseboards.
8. Do NOT run extension cords through walls, doorways, ceilings, or floors. If a cord is covered, heat cannot escape, which may result in a fire hazard.
9. Never use three-prong plugs with outlets that only have two slots. Never cut off the ground pin to force a fit, which could lead to electric shock.
10. Buy only cords that have been approved by an independent testing laboratory.
11. Do NOT substitute extension cords for permanent wiring.
12. Do NOT use an extension cord or power strip with heaters or fans, which could cause cords to overheat and result in a fire.

Source: https://www.esfi.org/resource/reaching-to-safety-use-extension-cords-properly-542