What type of guards must be used to protect sprinklers subject to mechanical injury?

The correct answer emphasizes that guards used to protect sprinklers must be both listed and approved. This specification is crucial because "listed" indicates that the guard has been tested and meets certain established standards by an independent testing laboratory. Being "approved" highlights that the guard is suitable for use in a real-world application and has been accepted by the authority having jurisdiction (AHJ).

Using guards that are merely approved or recommended may not ensure that they have passed rigorous testing processes, potentially leading to inadequate protection for the sprinkler system against mechanical injury. Therefore, the requirement for guards to be both listed and approved ensures that they are not only compliant with safety regulations but also effective in protecting the sprinkler heads in various installations. This dual requirement is a critical factor in maintaining the integrity and functionality of the sprinkler system, particularly in environments where they may be at risk of damage.