When should dual control switches operated by hand be returned to power?

Returning dual control switches to power after at least one unit or car has passed over the switch points is crucial for ensuring the safety and proper function of the switch. This practice helps to confirm that the switch has been correctly aligned and is securely set in the desired position before re-engaging the power. Once a train or vehicle has traversed the switch, it acts as a physical validation that the switch is functioning as intended, hence reducing the risk of misalignments that could lead to accidents.

Other options may suggest earlier actions or contexts that do not ensure the same level of safety or operational reliability. For instance, returning the switch immediately after use or after a signal becomes visible could potentially lead to scenarios where the switch is not yet confirmed as safe for the next movement of trains. Furthermore, relying solely on instructions from a control operator without confirming the physical state of the switch may not adequately ensure the switch's proper function and safety. Thus, waiting until a unit or car has passed provides a better assurance of operational integrity before restoring power to the switch.