A manhole cover is a removable plate forming the lid over the opening of a manhole, to prevent anyone or anything from falling in, and to keep out unauthorized persons and material.
Manhole covers date back at least to the era of ancient Rome, which had sewer grates made from stone.
Manhole covers are often made out of cast iron, concrete or a combination of the two. This makes them inexpensive, strong, and heavy, usually weighing more than 50 kilograms (110 lb). The weight helps to keep them in place when traffic passes over them, and makes it difficult for unauthorised people not having suitable tools to remove them.
Manhole covers may also be constructed from glass-reinforced plastic or other composite material (especially in Europe, or where cover theft is of concern). Because of legislation restricting acceptable manual handling weights, Europe has seen a move towards lighter weight composite manhole cover materials, which also have the benefits of greater slip resistance and electrical insulating properties.
A manhole cover sits on metal base, with a smaller inset rim which fits the cover. The base and cover are sometimes called "castings", because they are usually made by a casting process, typically sand-casting techniques.
The covers usually feature "pick holes", into which a hook handle tool is inserted to lift them. Pick holes can be concealed for a more watertight lid, or can allow light to shine through. A manhole pick or hook is typically used to lift them, though other tools can be used as well, including electromagnets.
Although the covers are too large to be easily collectible, their ubiquity and the many patterns and descriptions printed on them has led some people to collect pictures of covers from around the world.
The question of why manhole covers are typically round (in some countries) was made famous by Microsoft when they began asking it as a job-interview question. Originally meant as a psychological assessment of how one approaches a question with more than one correct answer, the problem has produced a number of alternative explanations, from the tautological ("Manhole covers are round because manholes are round.") to the philosophical.
Reasons for the shape include:
- A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it were inserted diagonally in the hole. The existence of a "lip" holding up the lid means that the underlying hole is smaller than the cover, so that other shapes might suffice. (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.)
- Round tubes are the strongest and most material-efficient shape against the compression of the earth around them, and so it is natural that the cover of a round tube assume a circular shape.
- A round manhole cover has a smaller surface than a square one, thus less material is needed to cast the manhole cover, meaning lower cost.
- The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using a lathe.
- Circular covers do not need to be rotated to align with the manhole.
- A round manhole cover can be more easily moved by being rolled.
- A round manhole cover can be easily locked in place with a quarter turn (as is done in countries like France), which makes them hard to open without a special tool. Lockable covers do not have to be made as heavy, because traffic passing over them cannot lift them up by suction.
Other manhole shapes can be found, usually squares or rectangles. Nashua, New Hampshire, may be unique in the United States for having triangular manhole covers that point in the direction of the underlying flow. The city is phasing out the triangles, which were made by a local foundry, because they are not large enough to meet modern safety standards and a manufacturer for larger triangles cannot be found. Some manhole covers in Hamilton, Bermuda, are triangular, and hinged. Some triangular water-main covers also exist in San Francisco.
Security and safety
Because of concerns about unauthorized access to underground spaces, manhole covers may be locked down, or even temporarily spot-welded in place. This practice has become routine in some locales, as advance preparation for official parades and similar events attracting large crowds or important people.
In urban areas, stray voltage issues have become a significant concern for utilities. In 2004, Jodie S. Lane was electrocuted after stepping on a metal manhole cover, while walking her dog in New York City. As result of this and other incidents, increased attention has been focused on these hazards, including technical conferences on stray voltage detection and prevention.
Interaction with race cars
Because of their aerodynamic design, some modern racing cars create enough vacuum to lift a manhole cover off its recess. During races on city streets, manhole covers must therefore be welded or locked down to prevent injury. In 1990, during the Group C World Sportscar Championship race at Circuit Gilles Villeneuve (located in a public park in Montréal, Quebec), a Brun Motorsport Porsche 962 struck a manhole cover that was lifted by the ground effect of the car he was following, a Courage C24 Porsche. This caused the trailing Porsche to catch fire, and safety issues ended the race shortly afterwards.
Propelled into space
According to urban legend, a manhole cover was accidentally launched from its shaft during an underground nuclear test in the 1950s, at great enough speed to achieve escape velocity. The myth is based on a real incident during the Operation Plumbbob nuclear tests, where a 900 kilograms (1,984 lb) steel plate cap was blasted off the test shaft at an unknown velocity, and appears as a blur on a single frame of film of the test; it was never recovered. A calculation before the event gave a predicted speed of six times Earth escape velocity, but the calculation is not likely to have been accurate. After the event, Dr. Robert R. Brownlee described the best estimate of the cover's speed from the photographic evidence as "going like a bat out of hell!"
A robotics research paper in 2011 suggested that robots could examine the shapes of specific manhole covers and use them to calculate their geographic position, as a double-check on GPS data.
The manufacturing process of metal manhole covers consists of six steps:
- Design and simulation
- Quality control. 
Trondheim manhole cover, Trondheim, Norway
Prague manhole cover
Manhole cover in Tel-Aviv, Israel
Standard Berlin, Germany manhole cover with infills of concrete.
Triangular, hinged manhole cover in Hamilton, Bermuda.
Venezuelan manhole cover from Caracas.
One of the many manhole covers in Singapore.
Manhole cover in Taiwan.
Manhole cover in Pima County, Arizona.
- Coal hole
- List of New York City manhole cover abbreviations
- Public utility
- Storm drain
- Stray voltage
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