Monday, November 14, 2011

Why Do Electrical Plugs Have Holes In Them?

This is one of those questions that's always low-level bothered me, but never enough for me to actually expend effort finding out what the answer was (in cases like this I'm usually content to just assume the answer is "some dumb legacy thing" and get on with my life, but I started this blog partially so I'd stop doing things like that).  At least here in the US, 99% of consumer-grade electrical plugs have ~2mm wide holes bored into the hot and cold prongs.  See the technical diagram below if you don't know what I'm talking about:




They don't serve any purpose that I can think of, but since it must cost more to manufacture plugs with them than without them they must have at least a mildly compelling reason to exist, right?  And that reason probably isn't "to make it easy to attach wires to them for incredibly unsafe DIY electrical work" either, although they are handy for that.

You would think that being a person who gets paid to do things involving electronics I'd have taken apart at least one electrical outlet in my life, or at least smashed it open to see what was inside (my standard method for doing science from ages 8-31).  Apparently not though, because if I had I'd know there were little leaf-spring-loaded knobs inside modern electrical sockets that lightly latch into the plug-holes when you plug something in.  They don't really "lock" as much as "add a little extra force holding the plug in," so you can do things like plug into a ceiling socket without gravity unplugging you.  The contacts inside an electrical socket are also tapered so they'll "grab" the plug blades a little bit, but that will eventually wear out.  If you've ever tried to keep a hole-less plug plugged into an older/crappier outlet, or even a holed plug plugged into an outlet so old it doesn't have the knob assembly, you'll probably notice that they fall out much more easily (I just tested this a second ago; handily, most AC USB chargers don't seem to be made with holes in the plugs these days, probably because they're light and have no hardwired cord).

Amazingly, even Wikipedia didn't know this one.  I had to go to some random techie message board to figure it out, unfortunately depriving me of the opportunity to smash open an outlet and empirically determine the answer.  I might do it anyway though, just to be sure.

17 comments:

  1. The answer is that there exist outlets in which there are balls that cling onto those holes for a more secure hold.

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    Replies
    1. How to remove once they cling onto those holes? Actually I am not able to remove.

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  2. Isn't that what he just said, fuckwit?

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  3. Have a very good Switches and Sockets blog which is really interesting and knowledgeable. I really appreciate you for this good work keeps it up. You can also find a very good blog at Switches and Sockets by Meteor Electrical

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  4. I wish there were travel adapters that would use the hole to lock the travel adapter to the plug. I live in Europe so every device bought in the us needs a travel adapter. However the prongs tend to come loose quickly and then I'll hear the sparkles.

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  5. Excellent piece of information. Basic standards living along us from ages and unfortunately mostly it is hard to find why a certain standard exists or even if it is some standard.

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  6. The reason they have those holes is so you can install a small lock to prevent usage. For example: To keep kids from plugging in electrical items or to lock out electrical appliances for repair. This was mandated back in 1957 for safety, Lock Out-Tag Out. Many years ago you could purchase a small lock at your local grocery store that supported those cord ends. Now not so available. GP

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  7. You can do that same trick with small zipties. That's how I LOTO 120V devices at work, at least.

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  8. A small padlock could be used to prevent someone from using an appliance. For example your teenager is grounded, so you lock the cord on the tv, or game console. Also like mentioned above you could tag out an appliance for service.

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  9. Wow, no. The holes are there because UL requires certain tests to be done in order for power cords to be "certified". Those holes are only there because one of the UL tests involves sliding a rod through those holes to perform the pull test. If you have a plug with no holes, it is a non certified knock off. ie...Chinese fake.

    (I am a power cord manufacturer in North Carolina.)...and I love random trivia.

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  10. Interesting. Now that i think about it, all the hole-less plugs I have are things like USB wall chargers, e.g. no cord, so maybe they're not subject to cord pull tests?

    There are definitely leaf springs that mesh with the plug holes inside an outlet, I took one apart awhile ago out of curiosity. I wonder if they came first or they took advantage of the existing holes for the UL test.

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  11. Interesting. Now that i think about it, all the hole-less plugs I have are things like USB wall chargers, e.g. no cord, so maybe they're not subject to cord pull tests?

    There are definitely leaf springs that mesh with the plug holes inside an outlet, I took one apart awhile ago out of curiosity. I wonder if they came first or they took advantage of the existing holes for the UL test.

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  12. Do you know if you can permanently plug prongs into a outlet? I assume you couldn't because tripping over the cord could cause damage to an outlet. I've done a light search and find anything in codes or UL. Can anyone point me to a code or legal restriction against "locking" into a socket?

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  13. Not sure Aaron as to why you would want to do that (epoxy will do the trick of you really want to go ahead), as you're then committing that outlet to the plug in question. If anything were to happen where the plug or cord needs to be replaced (such as a frayed cord), you then have to replace the whole outlet in addition to the plug.

    Legally, it depends on where you are as to what code violation (if any), such a task would constitute. Best to ask a locally licensed electrician, although odds are that they will try to talk you out of it for the sake of practicality regarding the outlet.

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  14. Wow very nice post its all information is really great for electrical adapters.

    15A to 20A adapter

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