Which of the following tools is NOT commonly used when installing down conductors?

The torque wrench is the tool that is not commonly used when installing down conductors in lightning protection systems.

Down conductors are typically secured to structures, and the installation involves fastening them using screws or bolts, for which a screwdriver or pliers would be essential. A hammer might also be helpful if stakes or ground rods are being driven into the ground as part of the grounding system.

While a torque wrench is valuable in many mechanical applications where specific tension or torque settings are required, it is not a standard tool for the installation of down conductors, as these installations usually do not necessitate precise torque measurements. Instead, they rely on proper fastening techniques to ensure the conductors are securely attached. Thus, in the context of lightning protection system installations, the use of a torque wrench is not essential.