What type of fire detection systems have output signals connected so that both must signal for a fire warning?

The correct answer is based on the concept of redundancy and reliability in fire detection systems. A dual-loop fire detection system is designed with two independent sensing loops that must both detect a fire condition in order to activate the fire warning signal. This means that the system increases the likelihood of accurate detection by requiring agreement between two separate sensors, reducing the potential for false alarms due to a single point of failure.

This type of system is particularly beneficial in environments where safety is critical, as it ensures that a single sensor malfunction does not result in a missed fire alert. By validating a fire signal through two separate loops, the system enhances overall dependability, which is essential in powerplants where potential fire hazards can have serious consequences.

Other types of systems, such as single-loop systems, would issue a fire warning based on a single signal; thus, they lack the redundancy of dual-loop systems. Multi-loop systems typically contain more than one sensing loop as well, but they do not necessarily require both loops to activate simultaneously for a fire warning. Continuous-loop systems are designed to provide ongoing monitoring and can signal an alarm based on various conditions, but again, they do not operate on the same principle of dual verification as described.