What is the primary cause of sprinkler system failure?

The primary cause of sprinkler system failure being the system being shut off highlights a critical aspect of fire protection systems. If a sprinkler system is deliberately or accidentally turned off, it cannot function as intended during a fire event. Sprinklers are designed to activate and suppress or control a fire when certain heat levels are reached. If the system is turned off for maintenance, testing, or because of neglect, it poses a significant risk since it will not provide the necessary protection when needed.

The other options, while also relevant, represent conditions that contribute to failures but are not as direct in their impact as the system being turned off. For instance, having no water in the system would certainly render it ineffective, but that's often a result of the system being shut off or a more systemic issue like a supply problem which can usually be remedied. System neglect can lead to various issues over time, but it usually manifests over a longer period and is not as immediate as the potential consequences of having the system off during a fire. Unlawful tampering is certainly serious, but again, it usually comprises a smaller subset of the factors contributing to sprinkler failure compared to the immediate and total loss of function when the system is off.

Thus, identifying the system being shut