Typically this switch is wired directly to the HVAC air handler's shutdown circuit, so that when the pump is overflowing, the air conditioning system will be told to shut down to prevent further condensation from dripping.
Now, I want to use this switch to also alert me via an Elk zone. So I will incorporate a relay (DPDT because I have 2 air handlers draining to this pump) to signal the shut down circuits. I plan to energize the relay by taking Vaux +12VDC and wiring in the overflow switch between the Vaux COM and the relay. Then I plan to wire a zone input to the N.O. side of the overflow switch (program the zone to be N.O.), so that when the switch throws an overflow condition, the zone essentially is shorted.
Please see the attached wiring diagram.
The question here is, if I take aux power from a remote power supply such as Elk-P212S to energize the relay, do I need to splice an additional COM going back to the zone's COM (dotted line in diagram)? Or is the one COM going back to the aux power supply terminal block sufficient to make the zone (1-wire) input work?