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#12824 08/18/02 06:19 AM
Joined: Jul 2002
Posts: 103
jes Offline OP
OK, here's a trivia question.
Why do male attachment plug prongs (some, not all) have round holes through them near the ends??? I DON'T know the answer!!

Joined: Jul 2002
Posts: 680
Lockout, tagout?

Joined: Sep 2001
Posts: 806
Helps hold the plug into the receptacle. There is a corresponding "dimple" in the contact spring, which "locks" into the hole in the plug blade.

Joined: Nov 2000
Posts: 2,236
Likes: 1
And here I thought they were to clean the contacts...?

Makes sense though...

Residential/Commercial Inspector
5 Star Inspections
Member IAEI
Joined: Aug 2001
Posts: 7,520
Don't forget that the holes also make it easy to wrap bare wires for Mexican-style extension cords.... [Linked Image]

Joined: Jul 2002
Posts: 103
jes Offline OP
Interesting comments but:
The holes have been there a LONG time... long before lockout/tagout came on the scene, however I have seen some loto devices that use the holes.
The contact idea is interesting but I've never seen a receptacle with contact strips having dimples...
A cleaning function is interesting...could be!
Still searching!

I was told back in school that a long time ago receptacles had little spring loaded balls that would engage those holes.

Joined: Mar 2002
Posts: 360
My bet is that they are part of the manufacturing process, like maybe how they are hung onto when they mold the plug around them.

Joined: Dec 2001
Posts: 2,498
I've got a 1970ies Sony stereo with NEMA plug and an unswitched outlet. When I push in a plug I can feel that the prongs get locked when fully inserted.
BTW, I was close to using the mexican style cord. I replaced a light switch in a room without a window, so I needed some temporary lighting. The only thing I could find was an imported US table lamp with an adaptor which has been hooked up forever in our living room. When I tried to hook it up I nearly got a fit. The adaptor was as stone aged as the lamp and it wouldn't fit Schuko receptacles! Finally I pulled two (illegal) ungrounded extension cords through the appartment and hooked my lamp up. SO much about being able to stick to codes...

Joined: Jul 2006
Posts: 272
Don't forget that the holes also make it easy to wrap bare wires for Mexican-style extension cords....

I'm not an old timer on this site but I bet there are some pics of this in the violation photo forum. I've seen that practice done plenty of times, unfortunately. [Linked Image]


Luke Clarke
Electrical Planner for TVA.

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