Dead alarm system troubleshooting advice requested ...
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Yesterday, I had been messing with the main breakers, and accidentally
shut the main 200A breaker without turning off the power generator switch;
so the Generac kicked in; and I heard the alarm system in the garage
vehemently complaining with persistent loud beeps.

I immediately reset everything, and I thought it was all ok.

Then, today, the alarm system sounded three loud long beeps, and
went totally dead.
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Troubleshooting with my Fluke after removing the alarm system external
wires shows the external ELK-TRG21640 16.5VAC, 45VA power-supply
transformer plugged into an outlet has an open secondary, and when
plugged into an extension cord, there is no voltage from any of the
three screws on the secondary windings:
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If I thought it was that simple, I'd just replace the transformer;
but I wondered why the backup battery didn't run the alarm system.
Testing the 12V 5AH battery out of the alarm system shows it's 13.5
volts DC (no load open circuit voltage), which, at least seems OK
(I don't know how to load it though):
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So, two things must be wrong:
1. The ELK AC transformer secondary opened up suddenly
2. Maybe the power supply inside the alarm system box went bad suddenly
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Do you think shutting the breaker without turning off the generator
could have blown the alarm system?

Any other troubleshooting suggestions?