The main difference between manholes and handholes is simply the size of the hole. The so-called manholes are inspection holes set on the laying pipes or wells of communication systems, where this hole can accommodate a person to pass through (either钻过去 or钻进去) during inspections or maintenance of such equipment. In contrast, handholes, being smaller, only allow hands to be inserted for inspections or operations. Moreover, both manholes and handholes are not only required in communication engineering but also in large-scale systems such as HVAC (mainly central air conditioning) systems and power line systems, where such inspection openings are also provided.