What must be disabled when filling the tank to check the high high level alarm?

Disabling the high level shut off valve is essential when filling the tank to check the high high level alarm because this allows for an accurate assessment of the alarm's function without interference. The high level shut off valve is designed to prevent overfilling by shutting off the fuel or water supply when the tank reaches a certain level. If this valve is active while filling the tank, it would automatically stop the flow before the level reaches the threshold required to activate the high high level alarm.

By ensuring this valve is disabled, the system can continue to fill the tank, allowing for a proper test of the high high level alarm functionality. This testing is necessary to verify that the alarm can adequately signal when the tank is approaching its capacity, thus preventing any potential overflow situations. It ensures the alarm system is fully operational as intended, contributing to effective safety and maintenance protocols in water and fuel systems.