What device must be removed when wiring devices are installed in metallic boxes?

When wiring devices are installed in metallic boxes, the insulation pieces must be removed to ensure proper electrical connectivity and safety. The purpose of these insulation pieces is primarily to prevent accidental contact during transportation and handling, but once the device is ready for installation, removing them allows for the necessary metal-to-metal contact that completes the circuit. This is crucial because electrical connections need to be stable and secure to reduce the risk of shorts or poor conductivity.

The other options do not relate to the proper installation process in the same way. Covering tape is typically used for protection and does not hinder installation. Mounting screws are necessary for securing devices and should remain in place. Safety caps are used as protective covers for terminals in devices not yet connected, but once installation begins, they too would typically be removed to allow for the actual wiring to take place. Hence, removal of the insulation pieces is vital for ensuring a safe and functional installation in metallic boxes.