Metal piping systems must be bonded mandatory to which device?

Bonding metal piping systems is a crucial requirement to ensure safety and prevent electrical shock hazards. According to the National Electrical Code (NEC), metal piping systems need to be bonded to the grounding conductor. This is essential because the grounding conductor provides a safe path for fault current, which helps facilitate the tripping of overcurrent protective devices in the event of a fault.

When metal piping is properly bonded to the grounding conductor, it ensures that all conductive parts of the system are at the same electric potential. This significantly reduces the risk of electric shock to individuals who may come into contact with the piping, especially in wet environments where electrical systems are more susceptible to ground faults.

The other choices, while they relate to electrical safety components, do not serve the same purpose as the grounding conductor when it comes to bonding metal piping systems. Static control devices, overcurrent devices, and transformers have different functions and do not provide the protective grounding and bonding needed to maintain system safety. Therefore, the correct answer is that metal piping systems must be bonded to the grounding conductor.