Where does the vacuum breaker install in the drain line?

A vacuum breaker is usually installed at the top of a vertically-mounted drain pipe leading to a drain.

The water drainage hose from an appliance such as a dishwasher or washing machine should never be plumbed directly into a drain stand pipe without a vacuum breaker.

The vacuum breaker prevents contaminated water from the drain being sucked back into the appliance if it has a fault.