I purchased a somewhat expensive name brand Beautyrest air mattress. Its electronic pump is at the foot of the bed, and its electrical cord is shorter than the length of the bed, making an extension cord mandatory (ugh). The plug is polarized.

I grabbed a UL-listed polarized extension cord and tried plugging the air mattress into the extension cord. Much to my surprise, the plug does not go in all the way. It stops going in with about 1cm of the prongs exposed.

Thinking perhaps one of the extension cord receptacles had something stuck in it, I tried the other two receptacles available at the head of the extension cord. Same problem.

I then tried plugging the extension cord's polarized plug into itself: no problem at all.

In all my years, I don't think I recall this ever happening before. Am I not thinking of something obvious? Is this likely a manufacturing defect, or is there something going on that I'm not considering? Is it safe to use a Dremel to grind down the air mattress's plug just a little so that it is still polarized, but slightly more narrow?

  • Did you compare the extension plug length with that of the mattress plug length and others plugs around? They all should be the same, within mm, not cm. Would say the one that is different is defective.
    – crip659
    Feb 19, 2021 at 16:02
  • @crip659 Yes, I did compare them. Eyeballing them, they look the same. I think the airbed's "polarized" prong may be about 1mm wider than the extension cord's "polarized" prong. My micrometer is missing from my workbench, so I can only eyeball it. The cat and I are going to have a little discussion about the missing micrometer... Feb 19, 2021 at 16:12
  • I've seen those "extra polarized" plugs before. A dremel remedy is fine, or you could just lop the head off and extend the cord with a new pigtail. Aside: why can't you rotate the mattress 180 so the plug is by the wall?
    – dandavis
    Feb 19, 2021 at 16:22
  • Its electronic pump is at the foot of the bed, and its electrical cord is shorter than the length of the bed OMG! The engineer who got the memo from UL saying "cords now limited to 6 feet" didn't tell the engineers building the rest of the bed to move the cord exit point (and possibly include a few extra feet of cord inside the bed. Feb 19, 2021 at 16:28
  • The only problem I think in making a bit smaller is maybe removing some protective coating on plug, if any.
    – crip659
    Feb 19, 2021 at 16:34

1 Answer 1


Since the question is "why" I'll try to answer that at the risk of many irate downvotes .. because one, or the other, or both, is not quite to spec! I guess that's obvious.

This question reminds me of the vast difference in design and engineering between American and British plugs and sockets. Two ends of the spectrum. The British ones with their longer ground pin, little plastic doors on the sockets (live terminals blocked until ground pin inserted), individual switches, individual fuses, finger knurls, 90 degree exits (it's impossible to yank one out by pulling the cord), and physical strength that easily holds even the largest PSU snug to the wall. There is more design and engineering in them than in some cars. And the American ones .... your question says it all. Crappy ill-fitting plugs dangling loosely from walls with people taking dremels to them just to get them to fit.

So much for "why". I've frequently filed down an oversized neutral spade on a power plug to get it to fit in a socket. Use a hand file and be very very subtle. As soon as it fits, stop. Then try to insert it backwards ... if it fits, you've gone too far. Cut it off and install a better plug.

  • +1 I'm with you on the general hokeyness of the US system. I can imagine the committee meeting… "Our plugs are gonna kill someone, what should we do?" "Drop it to 110v & we'll get fewer complaints" "You mean we have to split 3-phase down from 240, feeding 110 up each pole?" "Yup, that ought to do it, we get two chances to not kill people that way". "Cool, let's go for it… oh, what about fuses & earth?" "It's only 110v, what could possibly go wrong?"
    – Tetsujin
    Feb 19, 2021 at 18:02
  • @Tetsujin It's because America was an early adopter of electricity. "DC, with multiple taps" is a standard GE trick of the time. By the time Tesla won the war of the currents, Edison had already built out a lot of DC distribution. That was wired +100V / common / -100V to reduce voltage drop (no transformers in DC) and allow 200V for motors (bumped to 110/220 before DC gave up). Power companies were trying to transition to AC as cheap as possible with least disruption. Feb 19, 2021 at 19:12
  • 1
    @Harper-ReinstateMonica - they've still had the best part of a century to rationalise those appallingly unsafe & apparently unregulated (judging by the OP) plugs & sockets. The UK had a massive restructure from the old 'round pin' days to the modern 13A + earth structure. It can be done, it just requires some leadership. My general rule… if you can wobble a plug in its socket, it should really, really have been designed differently.
    – Tetsujin
    Feb 19, 2021 at 19:31
  • @Tetsujin I thimk the issue at hand is a Chinese extension cord where they polarized the plug but not the socket. If NEMA plugs are so dangerous, why isn't NFPA up in arms about it? And we did find a safer outlet, it is called GFCI. And no, RCD is not that. Feb 19, 2021 at 23:08
  • @Harper-ReinstateMonica I may read too much between the lines of OP's question but I think he would have noticed if the socket end of the extension was not polarized. There is too much specificity for him to have just overlooked that including plugging it into itself. And ... I can't help myself here ... NEMA plugs are safe because they fall out, and/or don't make contact at all. What could be safer than a self-disconnecting plug? :)
    – jay613
    Feb 19, 2021 at 23:15

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