Why do electric plugs have holes?

by | Jul 14, 2009 08:00 PM ET

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Hi, I'm Marshall Brain with today's question.

Why do the two flat prongs on plugs for most electrical appliances have holes in them?

If you unplug any appliance in your house, there is a 98 percent change that the two flat prongs have holes in them. There are three reasons for those holes. First, if you were to take apart an outlet, and look at the contact wipers that those prongs slide into, you would find that they have bumps on them. These bumps fit into the holes, so that the outlet can grip the plugs prongs more firmly. This detenting prevents the plug from slipping out of the socket due to the weight of the plug in the cord. It also improves the contact between the plug and the outlet. Second, electrical devices can be factory sealed, or locked out, by the manufacturer or owner, using a plastic tie, or a small padlock that runs through one or both of those prong holes. For example, a manufacturer might apply a plastic band through the hole, and attach a tag to it that says 'you must do blah, blah, blah, before plugging in this device'. The user can't plug in the device without removing the tag, so the user is sure to see these instructions. Third, there is also a small savings in raw materials for the manufacturer of the actual plug prong. Every little bit helps.

It has been reported that really old outlets used captive ball bearings and coil springs for the detent, but today is done with a bump on a springy copper contact.

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