Avionics Master Switch Question

[Beniama]

I was recently asked this question and am stuck on what the correct procedure would be. The question is "External power is turned on. The pilot turns on the avionics master switch and nothing happens. You cycle the switch and still nothing. Using the diagram below you have to figure out what is happening."

Anything would be helpful thankyou!

 

[Let'sgoflying!]

my wag:
energizing the NC avionics contactor breaks that circuit, so the radios are not powered during ground power use.
probably to prevent surges to the avionics during GPU use.

 

[Capt. Geoffrey Thorpe]

NC = normally closed. When you put power to the avionics contactor it opens - if you follow the diodes (current flows + to - in the direction of the arrow) the avionics connector opens when the ignition switch is in "start" or there is power on the bottom pin of the external power connector (which you need to close the normally open external power solenoid (relay))

 

[3393RP]

To expand a bit further on LGF's answer, remember schematics typically show components in a deenergized state. Note the external power solenoid (EPS) contact is shown normally open, while the avionics contactor (AC) contact is shown normally closed.

When external power is applied, the EPS coil is energized, and that closes the EPS contact. This action provides power to the coil of the AC. That opens the AC contact, preventing the avionics bus from receiving power. The AC contact also opens when the start position of the ignition switch is engaged.

 

[Silvaire]

It's kind of a trick diagram. The line for the avionics contactor relay should be drawn below the contact points.