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The small holes in some plug prongs were originally designed to engage bumps inside older outlets and help keep plugs from slipping out. Modern outlets grip the blades without those bumps; manufacturers may still use the holes during production or to attach tags and locks.
Small holes in electrical plug prongs originally helped secure plugs in older outlets, but that function is no longer needed in modern receptacles, according to the National Electrical Manufacturers Association. A report by Family Handyman says manufacturers may still include the holes to help hold blades during production or to attach safety tags and locks; the holes do not carry electricity or provide grounding.
In older outlet designs, small raised bumps inside the receptacle could fit into the holes in a plug’s metal blades. That arrangement helped keep the plug from slipping out. The design changed as outlet requirements and manufacturing practices evolved: modern receptacles are intended to grip the blades directly, without matching bumps.
NEMA says manufacturers largely stopped adding those internal “detent nibs” because they made it harder for outlets to meet the required maximum force for withdrawing a plug, without helping meet the minimum force requirement. The holes themselves are not required by American plug standards, though NEMA specifies their size and placement when manufacturers include them.
Today, the holes can serve manufacturing purposes, such as holding the metal blades in place while a plug is made. They can also accommodate small safety tags or locks on unplugged equipment. Those uses do not make a hole a substitute for appropriate safety procedures or equipment.
What the Holes Mean for Safety
Knowing what the feature does—and does not do—can help prevent mistaken assumptions about household electrical safety. The holes are not childproofing: tamper-resistant receptacles use internal shutters to block objects from entering the outlet. The Electrical Safety Foundation International says those receptacles work like ordinary outlets, although users may feel the shutters engage when inserting a plug.
The holes also do not indicate that a plug is grounded. On a typical three-prong plug, the round third prong provides the ground connection; the two flat blades carry out their usual functions whether or not they have holes. A plug that slips out of an outlet may point to a worn receptacle, rather than a missing or ineffective hole. The report advises that the outlet may need replacement; electrical repairs should be handled with appropriate safety precautions and by a qualified professional when needed.
For workplace maintenance, a small hole or attached lock is not, by itself, a complete safeguard against equipment starting unexpectedly. OSHA sets requirements for controlling hazardous energy, including lockout/tagout procedures in applicable situations. The tag or lock feature should not be treated as a replacement for those rules.
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From Outlet Bumps to Blade Grip
The holes were part of an older way of securing a plug: a bump inside the outlet fit into an opening in each blade. As outlet designs and testing requirements changed, manufacturers moved away from those bumps. NEMA’s explanation says the bumps increased the force needed to withdraw a plug, while modern receptacles rely on contact pressure against the blades.
Not every plug looks alike. Some have two prongs, while others have a third grounding prong or specialized designs for particular equipment. The holes are a separate feature: they do not determine whether a plug is grounded, child-resistant or suitable for a particular outlet. Family Handyman’s report draws on NEMA, Hubbell Wiring Device-Kellems, OSHA and the Electrical Safety Foundation International.
Why Some Plugs Still Have Holes
The purpose can vary by manufacturer and product. The report identifies blade positioning during production and attachment points for tags or locks as present-day uses, but it does not establish which purpose applies to any particular plug. There is no single required modern function, because American standards do not require the holes. The source material also does not identify a specific product line or manufacturer that uses them for one purpose over another.
Check the Plug and Receptacle
No standards change or new safety alert is described in the report. For consumers, the practical next step is to treat a loose-fitting plug as a possible sign that the receptacle needs attention, rather than relying on the blade holes to hold it in place. If a plug is unusually difficult to insert into a tamper-resistant outlet, the ESFI says the plug may be damaged. Do not force it; inspect the equipment and seek qualified help if the outlet or plug appears faulty.
Key Questions
Do the holes in plug prongs help electricity flow?
No. The holes are not there to improve electrical flow. The flat blades perform their electrical functions whether or not they have openings.
Do the holes make a plug childproof?
No. Child-resistant outlets use internal shutters. The holes in plug blades are not a child-safety feature.
Are the holes how a three-prong plug is grounded?
No. On a typical three-prong plug, the round third prong provides the grounding connection. The holes in the flat blades are unrelated to grounding.
Why do some plugs still have holes?
Manufacturers may use them to hold blades during production or attach safety tags and locks to unplugged equipment. The reason can differ by product.
What should I do if a plug keeps slipping out?
A loose fit may mean the outlet is worn and needs replacement. Do not rely on the holes to secure it; arrange an appropriate inspection or repair, using a qualified electrician when needed.
Source: familyhandyman.com
