Principle of the Manhole Cover

Thirty-four years ago, while commissioning a control system in a factory, I walked about with my customer Claud George of the Sam P. Wallace Company. Claud was one of my earliest on-the-job tutors for understanding building construction. He carried a seemingly endless "punch list" of discrepancies that required correction before the architect would certify a finished building.

"Cover in hole" appeared frequently. It meant that hundreds of small cast-iron manhole covers had fallen into the aperture they were intended to guard. Without exception, every manhole featured an elliptical cover.

"Do you know why manhole covers became round?” asked Claud, somewhat angrily. No, I replied. "Manhole covers were made round, because a round cover cannot fall into the hole it covers!” he exclaimed.

Claud described to me what could be characterized as a cycle of ignorance. Over years, young engineers forget why manhole covers were made round and they will specify other shapes - square, rectangular and elliptical.

That lesson was not lost on me. Over the years I have worked in the building controls business, I never forget to challenge new designs and new products. Are these new things the products of sound engineering? How do they stand up to history?

I learned, too, to test new products to discover their limitations and their practical use. At Rockwall Controls, our motto is, "We test it on us before we test it on you."