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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?

RockPaperLz- Mask it or Casket
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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 Feb 19 '21 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... – RockPaperLz- Mask it or Casket Feb 19 '21 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 '21 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 Feb 19 '21 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 '21 at 16:34
  • I have some extension cords that you have to wiggle and push hard when you plug things in, or that happens. I assume they are low quality. – izzy Feb 19 '21 at 16:39
  • @izzy could be they are better quality if they make tighter connection. Electricity and loose connections not good. – crip659 Feb 19 '21 at 16:56
  • Known quality expensive machine. J random no-name extension cord. Are you asking us to bet on which one is the problem? – Harper - Reinstate Monica Feb 19 '21 at 18:47
  • @dandavis wrote: "*Aside: why can't you rotate the mattress 180 so the plug is by the wall?*" I don't want the pump's ***big hard*** dial control to destroy the drywall when there is another type of pumping going on. ;) – RockPaperLz- Mask it or Casket Feb 20 '21 at 09:35

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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.

jay613
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  • +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 '21 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. – Harper - Reinstate Monica Feb 19 '21 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 Feb 19 '21 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 Feb 19 '21 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 '21 at 23:15
  • @jay613 Correct. Since I plugged the extension cord into itself, and its male end is polarized, the female end must also be polarized. Now regarding your assumption that I am male, well, I can't say whether or not you are correct on that one! ;) – RockPaperLz- Mask it or Casket Feb 20 '21 at 09:43
  • @RockPaperLz-MaskitorCasket I'm guilty. For what it's worth (not much) I was being lazy not presumptuous. – jay613 Feb 20 '21 at 16:26
  • @Harper-ReinstateMonica - I don't honestly know what standards the US supposedly practices, nor who NEMA is without googling, but had they made the earth pin compulsory some time in the past century none of this would now be possible to get wrong, at consumer level. You can't get a 3-pin plug the wrong way round no matter how you try. – Tetsujin Feb 20 '21 at 17:18
  • @Tetsujin wrote *You can't get a 3-pin plug the wrong way round no matter how you try."* It all depends on how much you bend the pins and how good you are with a sledgehammer! ;) – RockPaperLz- Mask it or Casket Feb 20 '21 at 17:24
  • @RockPaperLz-MaskitorCasket - Remind me to change all your sockets to [Walsall](https://en.wikipedia.org/wiki/AC_power_plugs_and_sockets:_British_and_related_types#Variant_pin_configurations) next time I'm round your house :P (it's a UK sparky trick, Walsalls look like regular 13A to casual inspection, but the pins are rotated 90°. Usually used on computer installations so the cleaners don't plug the Hoover in.) – Tetsujin Feb 20 '21 at 17:39
  • @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 Feb 20 '21 at 17:47
  • @Harper-ReinstateMonica - Thanks for that. Still sounds like a solvable problem. If the live & neutral gates cannot be opened without the **compulsory** earth pin to open them, then inversion is impossible, whatever the distance to the edge. Again if earth is compulsory, 2-wire extension cables are illegal, as is anything relying on just 2 pins. – Tetsujin Feb 20 '21 at 17:57
  • @Tetsujin As for Walsall sockets, turning blades [is the USA's secret plan to implement 240V on existing wall wiring](https://i.stack.imgur.com/r3cqw.png). 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 Feb 20 '21 at 18:12
  • @Harper-ReinstateMonica - even those 'specialist' plugs still look horribly wobbly & still seem to have no gate structure to prevent incorrect insertion (inc 2-pin plugs), plus you can always insert a flat pin in a T-shaped hole… and it's a bit ["one more standard"](https://xkcd.com/927/)… and they're really flimsy-looking… but it's a start ;) – Tetsujin Feb 20 '21 at 18:26