Control Wiring Exercises
If an HOA is in a run condition during a power failure, it can unexpectedly start back up when the power comes back on.
One big difference between a START STOP and HOA control circuit is what happens during a power cycle.
A START STOP relies on the seal in circuit from the relay/contactor to stay energized, so when the power is cycled, it will not start back up.
A HOA Hand Off Auto only uses mechanical switches to run, so if they are in a run condition when the power fails, they will be in a run condition when the power is restored.
The HOA scenario may not always be desirable since there may be checks that need to be done before starting the system back up plus this could cause an unsafe condition since you don’t know when the power will come back on.
Using the circuit in this video, we can combine the power up behavior of the start stop with the functionality of the HOA Hand Off Auto.