181011-2232 EDT
Still too little information.
Apparently there are three conductors in the MC cable. One is an insulated green wire for the EGC. We assume this cable only supplies the alarm box.
I would do the following:
1. Open the EGC wire at the alarm box.
2. Make a long test lead from the main panel EGC bus to this alarm box location. Can use extension cords, or roll out some wire for this lead. The wire size of the test lead could be almost anything, but #20 or larger would be mechanically good.
3. Using a high impedance meter, 10 megohm input impedance, measure the voltage between the test lead and the MC EGC wire. Should be in the millivolt or less range. Keep the long test lead close to the MC cable if possible. This reduces voltages induced into the one turn loop from stray magnetic fields.
4. Keeping the meter connected as in 3 reconnect the EGC to the alarm box. There should be no appreciable change in the voltage reading. If there is it is caused by the alarm box. What is the current in the EGC?
5. If no voltage change, then there is a problem in the alarm box or its circuit.
Provide information from the above tests.
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