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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?

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  • 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
    Commented 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...
    – End Antisemitic Hate
    Commented 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
    Commented 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.
    – manassehkatz-Moving 2 Codidact
    Commented 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
    Commented Feb 19, 2021 at 16:34

3 Answers 3

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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.

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    @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.
    – Harper - Reinstate Monica
    Commented Feb 19, 2021 at 19:12
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    @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
    Commented 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.
    – Harper - Reinstate Monica
    Commented Feb 19, 2021 at 23:08
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    @Tetsujin Oh sorry, NEMA is the manufacturer association responsible for plug designs. NFPA (Fire Protection Association) writes the Electrical Code based on hard data (accident reports). BigClive has documented cases of putting a socket too close to the edge of a device, allowing the plug to be installed reverse with the earth pin reaching out into open air... the US used to have that problem too (particularly with 2-wire extension cords) and now extension cords have a useless nub of plastic solely to reject grounded plugs (which are non-polarized).
    – Harper - Reinstate Monica
    Commented Feb 20, 2021 at 17:47
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    @Tetsujin As for Walsall sockets, turning blades is the USA's secret plan to implement 240V on existing wall wiring. Note how they require the ground pin to assure orientation. (fortunately there's no such thing as a groundless 20A plug). Note that in the US, our sockets all have ground pins, the precious few remaining 2-prong outlets can be trivially swapped to a £12 GFCI, giving a ground pin). So the USA is "mandatory ground ready", just the appliance makers have to stop putting 2-pin plugs on things.
    – Harper - Reinstate Monica
    Commented Feb 20, 2021 at 18:12
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You're not crazy. All our old extension cords don't fit our new appliances. They've definitely changed the standard. Old lamp, radio, and humidifier fit the extension cord. New lamp, footbath and box fan don't fit the old extension cord. The cords are only 10yrs old maximum from the local hardware store. New appliances are from a variety of places in the last 5yrs. It is currently August of 2024.

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An answer from the opposite end of the world. We have three flat pins on the plug here, the earth being wider and longer. Recently I have noticed

  1. A wider pin version of the plug that is rated for a higher current.
  2. Plugs are increasingly difficult to push in to cords and extension boards. You really have to wiggle and push.
  3. The grips on the sockets are further up, so the contact is being made on the insulated bit of the prong. Meaning that there is no current transfer. You have to leave the plug partly out of the socket to work.

The why - in my experience, such stupidity emerges from the United Nations.

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  • UN has nothing to do with electrical standards. Might help better if they did.
    – keshlam
    Commented Aug 4 at 16:00

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