Which of the following is NOT a type of initiating device in a fire alarm system?

The correct answer is the photovoltaic sensor, as it is typically not categorized as a type of initiating device in a fire alarm system. Initiating devices are crucial components that either detect a change in environmental conditions indicating a fire or allow users to manually alert the system.

Manual pull stations, thermal sensors, and smoke detectors all serve as initiating devices. Manual pull stations directly allow individuals to initiate an alarm by activating the system during an event. Thermal sensors respond to changes in temperature, often triggered by heat from a fire, and smoke detectors sense the presence of smoke particles in the air, both of which can successfully alert the fire alarm system to the potential danger.

On the other hand, photovoltaic sensors are primarily used for converting light energy into electrical energy and are not associated with fire detection or alarm initiation. Thus, this sensor does not fall within the category of devices that trigger an alarm response in a fire alarm system.