Final answer:
Pool circulation systems must have vacuum relief systems to prevent entrapment hazards by reducing suction pressure in the event of a blockage.
Step-by-step explanation:
When referring to pool circulation systems, they are generally required to be equipped with vacuum relief systems. These safety mechanisms are crucial for preventing potential entrapment hazards that can occur when limbs or other parts of the body become stuck against drains due to high suction forces. Vacuum relief systems are designed to immediately reduce suction pressure when a blockage is detected, thereby allowing for the quick release of the person or object trapped.