Extension cords don’t last forever. They may not last even as long as the appliances you connect to them. And using one that’s past its prime could be something you’ll live to seriously regret.

Extension cords cause roughly 3,300 home fires each year, killing 50 people and injuring about 270 more, according to Electrical Safety Foundation International.

Old, worn-out cords are part of the problem.

Cords wear out from physical use and abuse, not from electricity running through them, says John Drengenberg, consumer safety director for UL, a global product safety certification organization. “If you hit a cord often enough with the vacuum cleaner, it’s going to start to wear out,” he says. “There’s no time limit on them, although as cords get older they may get more brittle.”

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Overheating is a major cause of worn extension cords. What causes overheating? Overloading the cord or connecting appliances, such as space heaters and hair dryers, that consume more watts than the cord can handle, according to the Consumer Product Safety Commission (CPSC).

Older, worn extension cords and those with frayed insulation and weakened wires are more likely to overheat, the CPSC warns.

To prolong the life of your extension cords, never run them under carpets or furniture or through doorways. Foot traffic will wear down the wiring and crack the insulation, while running cords under furniture presents a fire hazard. (If the cord is hidden, you can’t see or feel it to tell if it’s getting hot or wearing out.) And always unplug them when you’re not using them, advises the CPSC.

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When to replace your old cords

Here’s how to tell when your old extension cords need tossing, courtesy of UL’s Drengenberg and the CPSC.

Problem #1: They have cracked insulation. “Check the cords regularly for signs of insulation damage and replace them if you notice any cuts, cracks or nicks in the protective covering,” says Drengenberg. “Get rid of them if the insulation is damaged in any way. If the wires are showing, that means it’s time for replacements. If you nick a cord with your hedge trimmers, for example, don’t tape it or try to repair the cord. Throw it out.”

Problem #2: A loose cord. If you tend to yank your extension cord out of the wall by the cord and not the plug, you may notice that it’s not attached securely to its plug after a while. If so, replace the cord.

Problem #3: A too-short cord. “If the cord’s too short, there may be a temptation to daisy-chain them by connecting several cords,” Drengenberg says. “This is hazardous and cords wear out faster. Get a cord that’s the right length for your job.”

Problem #4: A cord with the wrong electrical rating for the job. Use a cord that has the rating for the appliance you’ve plugged it into. “Talk to your hardware store or home center people” if you’re not sure what rating you need, says Drengenberg.

Problem #5: An indoor cord you’re using outside. If you’re using a cord outside, “only use one that’s marked for outdoor use,” Drengenberg says. “They’re usually orange, yellow or green. Indoor cords are usually brown or white, with a rib down the center of the cord.” Indoor cords used outside may become a shock or fire hazard.

Problem #6: The cord is nailed or stapled to the wall. Bad idea, says Drengenberg. Nailing or stapling cords can damage them. Instead of using a cord in these cases, have outlets installed where you need them by a licensed electrician, says Drengenberg.

Problem #7: The cord is hot to the touch. “That means it’s probably being loaded beyond its capacity and its overheating,” Drengenberg says.

Problem #8: Cords running under carpets and through windows. If it really seems necessary to run extension cords through windows or under carpets, it’s time to get new outlets installed, says Drengenberg.

Problem #9: Old extension cords that are missing important safety features, like easy plugging and unplugging and a large plug face that covers the outlet’s slots. The cord’s plugs should have polarized blades — one blade is slightly wider than the other and the plug can be inserted only one way into the outlet. Also, extension cords should be at least 16 AWG, unless they are 18 AWG with fuse protection. (AWG stands for "American Wire Gauge," a measurement that determines a wire's current-carrying capacity. The AWG wire size is stamped on the cord’s surface.)

Problem #10: Cords that lack safety certification. For maximum peace of mind, buy cords that are UL certified. “If you get a bargain-basement cord, it probably doesn’t have the proper amount of wire in it, and it could be dangerous,” Drengenberg says. UL-certified extension cords have a holographic label with the UL mark printed on it. "UL uses this type of label to make it more difficult for counterfeiters to trick consumers," says Drengenberg.

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Steve Evans, MA, is an award-winning journalist with more than 20 years experience in daily news, investigative, health and business journalism. Among other jobs, he has served as managing editor of the Central Virginia Newspaper Group, as a senior writer for SNL Financial and as a staff writer for The Progress Index and the Richmond Times-Dispatch.