Which device is used to allow normal operation of a system even when it is offline?

The device that allows normal operation of a system even when it is offline is a bypass installation. A bypass installation is designed to enable the continued functioning of essential services or processes while maintaining the integrity and functionality of the overall system. This is especially crucial in water treatment and filtration systems, where maintenance or repair might render the primary system temporarily inoperative. By providing an alternate pathway for water flow, a bypass ensures that there are no interruptions to service, thus keeping operations smooth and reducing downtime.

The other options, while relevant in different contexts, do not serve the same purpose. Startup procedures focus on the proper method to initiate system operations, which does not address offline functionality. Leak detectors are used to identify the presence of leaks, playing a critical role in system integrity but not contributing to operation during offline conditions. Pre-filtration refers to the initial phase of filtering out large particles before water enters a more refined treatment process, but it does not facilitate continued operation in the event of system shutdown.