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Photos courtesy of Fran Gardino. More manhole covers can be seen at **Manhole Covers Etc.**

Description:

“Ever see a manhole cover in any other shape than round? That's not by accident. If covers were any other shape, like square, it would be possible to accidentally drop one thru the hole when removing it.” Steve VanLiew
I have successfully searched the web for the existence of non-round manhole covers, but if I were in the sewer below them, I would most certainly watch out for falling manhole covers!

Counterexamples:

The city of Nashua, New Hampshire has long had triangular manhole covers, which it has begun only recently to phase out.

The Rouleaux triangles mentioned in this article are indeed used as covers (e.g., this one from Govind Steel Company Ltd).

Categories:


Mistake-proofing category: Mistake prevention
Setting function: Physical
Regulatory function: Shutdown
Source: Off-the-shelf

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Secondary Problems (side effects, weakness or concerns) with this example:

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{create links here to other examples that are comparable}