What equipment is used to prevent damage to the cable on the manhole frame?

The correct choice is the feeding tube, as it serves a critical purpose in safeguarding the cable during installation and maintenance processes involving the manhole frame. The feeding tube acts as a protective conduit through which cables can be drawn, minimizing the risk of abrasion, kinks, or other forms of damage that might occur if cables were to make direct contact with rough edges or surfaces of the manhole frame.

In the context of the other options, while sheaves can facilitate cable movement and reduce friction, they are more often used for routing cables and not specifically for protection against damage at the frame. Insulators, on the other hand, are primarily used to prevent electrical currents from unintended paths but do not contribute to physical protection of the cable itself. Guide rails assist in directing equipment but do not offer any protective functionality concerning the cable's integrity. Thus, the feeding tube is uniquely suited for the role of protecting cables in this scenario.