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Why Do Electric Plugs Have Holes in the Prongs?

As you may have already noticed the two little holes near each prong’s tip in an electric plug. If not, there are two holes mostly in Type-A (NEMA 1-15) and Type-B (NEMA 5-15) plugs which are commonly used in the US for residential and commercial applications. But did you ever think and wonder what are they for? If you are of this age to perceive with curiosity, let’s know what are the main purpose(s) of these tiny holes in the flat prongs of electric plugs?

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NEMA 1-15 (Type – A) plug with two flat parallel prongs was invented by Harvey Hubbell II in 1904-1906. NEMA 5-15 (Type – B) has the same two blades with an additional round (U) shaped prong used for grounding.

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Reasons of Holes in the Prongs of Electric Plugs

The indentations in the prongs in the first design by Harvey Hubbell II (and small holes later) was used as a safety feature. There were small bumps in the electrical outlets (receptacles and socket) which held the prongs tightly and firmly with the help of holes and indents in both plugs and outlets. These rightly fitted prongs would have not slipped back and pull-out easily. Additionally, the prongs with strong grip made perfect contact into the outlet’s slots.
You may have heard about the Lock Out Tag Out (LOTO). As it was not the main intention because there was no concept of such things in the 1900s. But the manufacturer as well as the end user may use it now as a safety measure. For example, you may put a small lock or a plastic tag in the holes to prevent unauthorized operation of an electrical device and appliance. The same rule applies in homes for the safety of children.
The manufacturer may consider that the holes in the prongs may save large metallic raw material based on large scale production.
According to the official regulations by NEMA (National Electrical Manufacturers Association) and ANSI (American National Standards Institute), the holes in the pins and blades of electrical sockets are optional and intended for manufacturing purposes only. But you are free to use a padlock or plastic tag (or any other LOTO) to prevent your kids or any other unauthorized person from powering up certain appliances and devices. This may help to protect both the human and machines in unwanted and unpleasant conditions.
Finally, the main reason for those holes in the prongs of power plugs was an old design with common use in the domestic and industrial applications for decades, the manufacturers now are not going to change the design that’s been around for almost 100 years. Same like the voltage level of 120V and 240V (or frequency of 60Hz) i.e. it won’t make sense with a new design of 99V AC (instead of 120V AC) or 40Hz frequency (instead of 60Hz or 50Hz) and so on.

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