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Unread 12-02-21, 08:24 PM
Frank Benvin's Avatar
Frank Benvin Frank Benvin is offline
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Run Stand/By Switch

1967 Cessna337b

How do you test the run standby switch?? When turned to the run position it should supply power in case of a battery solenoid contactor failure correct. If you disconnect the ground wire from the battery solenoid contactor simulating a failure you should be able to turn the standby switch on and have power. I tried this on the ground yellow light comes on but no power to aircraft Tried with both the master on and master off What am I missing Does the aircraft have too be running for it to work ??? I am guessing you are getting power from the alternators only and not the battery so the engines must be running for it to supply power to the aircraft

Thanks In advance

From the POH: ALTERNATOR RUN/STANDBY SWITCH AND WARNING LIGHT. A rocker type switch (Labelled Run/Stand-by) is provided for emergency use in case of complete loss of the electrical system due to a malfunction of the battery contactor. The alternators are self-sustaining, and, under normal circumstances, will remain in operation even though the battery is isolated from the system by failure of the battery contactor. However, under conditions of extreme electrical load (such as IFR conditions) any additional heavy surge load (such as wing flap operation) may result in a lowering of system voltage to a point where alternator output cannot be maintained; a complete loss of the electrical system will result, as evidenced by loss of all electrical equipment. The "Run/Stand-by" switch should immediately be placed in the "Stand-by" position, thereby restoring electrical power to the system. NOTE: The "Stand-by" system is intended for temporary use, and should not be used except in case of emergency. When ground checking the "Stand-by" system, turn on the master switch as a precaution against damage to electronic equipment by voltage transients.
1967 337B
Flying in Skymasters since I was 16

Last edited by Frank Benvin : 12-02-21 at 08:27 PM.
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Unread 12-06-21, 12:38 PM
JAG JAG is offline
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Hi Frank - you are correct that the engines have to be running to see it power the aircraft; the Run/Standby switch provides power from the battery to the excitation of the alternator (field). This is really just for an inflight failure of the contactor.
I have never tested it beyond what you have done on the ground - turning the switch to 'standby' and verify light comes on. Getting the light indicates 24 volts downstream of the run/standby switch confirming the fuse is good.

If you wanted to verify a higher level of comfort on this, I would leave the master switch off, and hook up a multimeter to the field terminal of the front alternator Voltage Regulator. Flip the run/standby switch to standby, and you should get 24 volts at the field.

The system basically allows the alternators to keep powering the bus with the battery contactor open, but your battery will not be charging.

Hope that helps.
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