When does an overfill alarm activate to inform operators of an impending overfill situation?

An overfill alarm is designed to activate at a predetermined level to alert operators of an impending overfill situation and to prevent spills or overflow during tank filling operations. In the context of underground storage tanks (USTs), the alarm typically activates at 90% full, which provides sufficient time for the operators to take action to halt the filling process before reaching the tank's full capacity. This threshold is set to not only protect the environment by preventing potential leaks or spills but also to ensure compliance with safety regulations and standards.

Setting the alarm at 90% allows for additional space in the tank, accounting for the expansion of the liquid due to temperature changes or other factors, thereby enhancing safety. The option that indicates 90% as the activation point reflects industry best practices designed to mitigate overfill risks.