Megger test your extension cord at 500V.

Yes, you can — but only with the cord completely disconnected from power and any load. Here’s the deal:

Unplug the cord from everything. If it has molded ends, you’ll need to access the conductors either at a damaged cut or by probing the prongs (carefully — the megger puts out high voltage). Connect one lead to a conductor (hot, neutral, or ground) and the other lead to a different conductor or to the outer jacket (ground). Press test. The megger applies 500V or 1000V and reads the insulation resistance in megohms.

What’s good? Anything above 1 MΩ is technically passable, but I’d want to see 100 MΩ or more on a healthy cord. Lower than 1 MΩ? Toss it or repair it.

A few gotchas: moisture in the cord will tank the reading, so test it dry. Don’t touch bare wires while testing — 500V won’t kill you but it’ll wake