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.
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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.