The newer EEsensors em01b has a contact module for monitoring normally closed circuits. My HVAC has a common alarm circuit that can close if any of the alarms (such as the high water switch or low pressure switch) go off, but it’s normally open.

The logic behind normally closed circuits is that you can detect a wiring fault. I’m not sure why eesensors went with NC instead of NO. They’re generally pretty helpful at answering questions via email though.

The em01b normally returns ‘N’ in the contact section when open and a ‘W’ when closed. Normally you’re in ‘W’ and if it trips to ‘N’ you need to send a reset command to get it back to ‘W’ (/index.html?eL) after you fix the problem. So if you’re querying an attached circuit that is normally open, you need to send a reset every time you query, and keeping in mind that the result that’s returned from the reset command is not a valid query.

I modified the C version of their nagios script to support this with a ‘D’ option instead of a ‘C’ option. You can find the source in git at check_em01.c.