Do The Holes In Electrical Plug Prongs Actually Have A Secret Purpose?

In 1904, back when Thomas Edison was alive and Tesla was just his rival's last name, Harvey Hubbell patented the electric plug. Before that, electrical connections were hardwired. Hubbell's plug was detachable, and it revolutionized wiring. To prevent those plugs from detaching all too easily, Hubbell's design included "guide-holes" in the metal prongs. As he explained in his patent application, guide holes ensured "immediate and certain engagement of the contact-posts with the locking contact springs as soon as the cap is pushed into place ... so that there were no imperfect connections and no sparking."

Today, simple friction keeps plugs in place, and guide holes are for the most part unnecessary. Neither polarized nor nonpolarized power cords in the United States need them. Nor do plugs in many other parts of the world have them. The National Electrical Manufacturers Association (NEMA), which sets electrical code standards for the United States, does not require guide holes in plugs. Other than for a specific locking purpose, guide holes have (per NEMA) "no other mechanical, electrical, or thermal function." Sorry,  there is no secret purpose for holes in plugs. They're just a legacy from the early days of electricity manufacturing.

This doesn't mean holes in plugs can't be useful. They're a perfect example of making good use out of an outdated product feature. Manufacturers have repurposed Hubbell's original invention by creating NEMA locking-blade plugs, which use the guide holes as a safety feature to keep the plug in a locked position and prevent unintended or malicious disconnections of vital equipment or devices. 

Guide holes are sometimes used for just the opposite reason. As part of the "lockout-tagout" (LOTO) safety procedure, guide holes that lock are often used on industrial sites to make sure that electrical equipment is only used by the right person at the right time. Homeowners and DIYers can use guide holes for the same purpose. Parents can slip a LOTO padlock through the two holes of a plug to prevent unauthorized use of equipment. Or you can simply insert a twist tie through the two holes as a reminder to check that a tool or device is in the "off" position before plugging it in, or to make sure you're using the correct type of electrical extension cord. Harvey Hubbell would be proud.