Why Do Electrical Prongs Have Holes in Them?

electrical plug
Why do the prongs on the electrical plug have holes in them? Yellow Cat/Shutterstock

If you unplug any appliance in your house, there's a 98 percent chance than the two flat prongs have holes in them.

There are three reasons for the holes:

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  • If you were to take apart an outlet and look at the contact wipers that the prongs slide into, you would find that they have have bumps on them. These bumps fit into the holes so that the outlet can grip the plug's prongs more firmly. This detenting prevents the plug from slipping out of the socket due to the weight of the plug and cord. It also improves the contact between the plug and the outlet.
  • 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 the prong holes. Construction projects or industrial safety requirements may require this type of sealing. For example, a manufacturer might apply a plastic band through the hole and attach it to a tag that says, "You must do blah blah blah before plugging in this device." The user cannot plug in the device without removing the tag, so the user is sure to see the instructions.
  • There also is a small savings in raw materials (metal) 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 it is done with a bump and springy copper contacts.

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Originally Published: Jun 30, 2009

Prongs on Plugs FAQ

What is a 3 prong plug called?
A three-pronged receptacle are known as a grounding receptacle. Unlike two-prong receptacles, three-prong receptacles are connected to a ground that protects the electrical appliance that is plugged into it from being damaged in case of a short circuit.
What is a 2-prong plug called?
Originally called Nema 1-15 when it was invented by Harvey Hubbell II, two-pronged plugs are now known as Type-A plugs. These types of ungrounded plugs are pretty common in North and Central America.
Which side of a 2-prong plug is positive?
Because we use A/C current, prongs don't have have a positive and negative. Instead, the two prongs have a 'hot' and 'neutral' side. The wider prong connects to the neutral wire and the smaller prong smaller prong is the hot side of the circuit.
What happens if the third prong breaks?
If one of the prongs or the wires inside them becomes dislodged, electricity won't flow as efficiently through the device. This misdirected current could, at times, ruin appliances or may even shock the user because it is no longer grounded.