Why Do Electrical Prongs Have Holes in Them?

electrical plug
This is an example of a Type A prong, which is not grounded. Notice it has two electrical prongs with holes in them. But why? Yellow Cat/Shutterstock

If you unplug any appliance in your house, there's a 98 percent chance that the two flat prongs on the electrical plug have holes in them. These holes are there for a reason. We'll explain why, but, first let's talk a bit about electrical plugs.

There are two common types in North America: Type A and B plugs (technically known as the NEMA 1-15 and NEMA 5-15, respectively). Type A is an ungrounded plug with two flat parallel prongs, and was invented in 1904 by Harvey Hubbell II. Type B also has two flat blades, but it also has a round or U-shaped pin that grounds the device before the power is connected.


Both Type A and B's flat prongs have holes near the tips. So again, what purpose do these holes serve?

  1. If you were to take apart an electrical outlet and look inside where the prongs slide into, you would see they have bumps on them. These bumps fit right into the holes on the prongs so that the outlet can grip the plug more firmly. This prevents the plug from being pulled out of the socket from the weight of the plug or cord. It also helps improves the contact between the plug and the outlet.
  2. Some electrical devices also come "factory-sealed" or "locked-out" by the manufacturer with a plastic tie inserted through one or both of the prong holes. Construction projects or industrial safety requirements may call for 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 'X' before plugging in this device." The user cannot plug in the device without removing the tag, so they are sure to see the instructions.
  3. There may also be a small savings in raw materials (metal) for the manufacturer of the actual plug prong.

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.


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.

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