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Most of the two-prong extension cords I have encountered have two conductors which do not seem double-insulated. This is in contrast with three-prong power strips which usually have three, already insulated, conductors grouped inside another insulation.

I was wondering why is that so? Given both are exclusively run externally and are often trip hazards, shouldn't wires for two-prong extension cords also have another jacket of insulation?

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    Doubt very much if the two prongs are listed for outdoor use. They are mainly for inside use.
    – crip659
    May 12, 2022 at 23:15

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Yes it is safe to use them, they're UL listed and approved. Extension cords are not meant to be installed permanently in most cases. They are not supposed to be installed so they are trip hazards. Some of the two prong ones that are not "double insulated" are the light duty, cheaper ones. They are also available in heavy duty that includes the extra protective layer. Once you get to the three conductor extension cords, it's cheaper to add that extra protective layer than to mold the three together.

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  • "Some of the two prong ones"? I don't recall ever seeing an ungrounded extension cord with an outer jacket. I've seen a couple 3-wire ones w/o jackets, but never 2-wire with...
    – dandavis
    May 13, 2022 at 19:11
  • @dandavis They are still out there. Husky made the last one I purchased. They were readily available before grounding became the buzzword. Heavy duty outdoor extension cords for Christmas lights...
    – JACK
    May 13, 2022 at 21:32

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