Two contactors are used in a changeover system to prevent mains and generator supplies from being connected simultaneously. A pushbutton start circuit with normally closed auxiliary contacts provides electrical interlock, while a mechanical interlock prevents both contactors from closing at once.
Read the decisions behind it.
Use normally closed auxiliary contacts of each contactor in series with the opposite contactor coil for fail-safe electrical interlock.
Implement separate pushbutton start controls for mains and generator modes.
Label contactor symbols clearly with KM1 and KM2 reference designators.
Reuse this pattern for any dual-supply transfer system where back-feeding or source paralleling must be avoided, such as standby generator panels, UPS bypass, or solar/grid changeover.