A large percentage of manholes are usually flooded, and some corrosive pollutants containing in the flooded manhole may corrode the metal cable closure and therefore cause severe problem in the telephone cable plant.

• 由于大多数的人孔经常充满来自于周围环境的地下水，若有侵蚀性物质意外流入人孔，可能导致人孔内之铅管接头加速腐蚀而影响其长期使用的安定性。