Joe, under normal operating conditions both fans are in "auto". There is a small mechanical room located behind the isolation room where the HOA controls are mounted side-by-side on a wall. On occasion there will be a need to run the exhaust fan in "hand" without the supply fan running. This is why I think a simple disconnect to prevent the supply fan from running would suffice. With both fan controls switched to "off" open the disconnect to break the circuit on the load side to the supply fan and then switch both controls to "hand". Once they're done running the exhaust fan switch both controls back to "off", close the disconnect, and then switch back to "auto".
A malfunction at the junction