A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why are there holes in the prongs of electrical plugs?

December 20, 1985

Dear Cecil:

I've always wanted to know why there are holes in the prongs in electrical plugs. My guess is that a small rod, inserted through the holes, could lock an appliance into the socket. This could prevent such domestic disasters as having to get behind the fridge to plug it back in when the plug falls out. However, I have never seen this put into practice. What's the straight dope?

Cecil replies:

You will be distressed to learn this, Dan, but I dunno. Engineers have been sticking holes in the prongs of plugs for so long — there is an undercurrent of sexual imagery here that I am studiously going to ignore — that they have forgotten why they started. An expert at Underwriters Laboratories says the holes are a by-product of the manufacturing process: they're needed to hold the prongs in place while the plastic part of the plug is molded around them. A guy from General Electric, however, says the purpose of the holes is to dissipate the heat generated by the flow of electricity. The UL guy says the GE guy is nuts. A third engineer from a smaller electrical manufacturing firm agrees with the UL guy. He says originally there were little nubbins inside the — ahem — "female" connector (i.e., the socket) that clicked onto the holes when the plug was shoved in to keep it from pulling out. (UL won't OK a socket if a plug comes out with a pull of less than three pounds.) But nubbins aren't needed anymore because of improved socket design, and now the holes are just a manufacturing convenience. Two-thirds of the voters thus opt for convenience, so I guess we'll have to go with that. I've heard of Heisenberg's uncertainty principle before, but this is ridiculous.

The Teeming Millions stick in their two cents' worth

Dear Cecil:

Just to set you straight when you're stumped: the holes in the prongs of electrical plugs are a safety feature. They are there so you can slip a small lock through one of them and keep the key, thus preventing unauthorized persons from plugging in the appliance, e.g., small children from using an electric carving knife, iron, or dangerous power tool.

Simple logic tells me that the GE guy is nuts. Prongs of electric plugs used in other countries, with sometimes higher voltages (e.g., 220 volts in Germany), do not have holes. Hope I have made your day.

Dear Cecil:

I don't know the engineering of this, but isn't it the case that punching holes in strip metal increases its strength? I seem to remember from my model carmaking days that the holes in dragster frames, in addition to reducing weight, increased the frame's rigidity.

Theory #2: Can one melt and reuse the punched-out slugs, thus saving about one-quarter the raw materials cost per unit, which, multiplied by the krillions of plugs made each day, would result in substantial savings? Just a thought.

Cecil replies:

Both of your "thoughts," Bob, reduced my engineer buddy to hysterics. 'Nuff said.

As for you, Sibylle, you're a little closer to the mark. It is in fact possible to buy a little locking gizmo that slips over an electrical plug and engages the holes, thus preventing the appliance from being used. But that's not why the holes are there in the first place. The holes have been around as long as anybody I've talked to on this subject can remember — at least 20 or 30 years. The earliest anybody can remember the locks appearing on the scene is about 10 years ago.

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