What is a drain wire and where does it go on the Alarm.com controller or expansion module?

Some readers contain a drain wire. These drain wires are used in cables in conjunction with a metallic shield to ensure there is effective grounding. The drain wire completes an electrical circuit from the shield and carries unwanted electrical noise to ground away from the wiring circuit. 

Alarm.com door controller's and expansions do not have a terminal designated for the drain wire. However, attaching it to a mounting screw in the enclosure should suffice. When using a reader with a drain wire, the drain wire can be affixed to one of the standoff screws used to hold the mounting plates to ground the wire as pictured below with the ADC-ACC1-17-PSE kit.