Over 100 anti-theft devices for manhole covers primarily employ six methods: 1) chain connection; 2) hinge connection; 3) reverse rotation technique; 4) bolt plugging technique; 5) lock and key method; and 6) lever joint linkage method.
Of these, the first and second methods cannot lock the manhole cover and frame together, making them easy to open; the chain or hinge can be pried off with a steel chisel. The third method only requires reverse rotation to open and remove the cover. The fourth method uses bolts and nuts to plug the hook hole, but the cover can be opened and removed by inserting the ends of a bent wire into the two grooves under the bolt head. While the fifth and sixth methods can lock the cover and frame together, they require high precision and space, necessitating the casting of new covers for compatibility. Existing covers cannot be modified, resulting in significant costs and failing to solve the problem of rusting. After about six months, the cover becomes unusable and cannot be opened for work.
Modern manhole cover anti-theft technology has been incorporated into a more comprehensive standard system. According to the "Technical Specification for Installation and Maintenance of Municipal Inspection Manhole Covers" (DB 4205/T 94-2022), manhole covers should possess functions such as anti-theft, anti-fall, anti-slip, anti-displacement, noise reduction, and easy opening. Specifically, inspection manhole covers should preferably use built-in pins or hinges for anti-theft and should be equipped with corrosion-resistant locking devices.
For example, ductile iron anti-theft manhole covers, by adding fixing devices, can not only effectively prevent theft but also prevent the cover from being washed away when water accumulates. Furthermore, the anti-theft function of common "five-proof manhole covers" is usually achieved through structures such as anti-theft hidden boxes and opening flaps under the manhole ring, replacing the earlier anti-theft chains.
