Wiring reference to ground...

I understand this is Access Control forum, but my question really pertains to wiring - and hopefully is okay to ask here. Find a good fire alarm forum is tough! And I expect due to life-safety risks...

Anyway, thought I'd bounce this off you guys as a sanity check.

I have an alarm system with two 2-wire fire loops. I have a loop that covers the basement (4 detectors), and one that is a main living area smoke loop and also covers the garage and attic (heat only). This was run with a 4-conductor fire-rated cable (2 spare).

The basement loop is fine; no shorts, reads the EOL resistor ok and has no issues.

The second loop keeps going in to trouble intermittently. I cleaned the heads per the manual, and after several hours/days it goes back in to trouble. Reading the loop, there are no shorts between conductors and the EOL resistor at the last device reads fine from the panel end. The odd thing with this loop though, is that each and all four conductors show a 20meg reading to house ground. The other loop does not.

The second loop is longer, and goes through walls, parallel to a short bit of romex power circuits. Is this just inductance I'm reading? The odd thing is that it's been fine for 5+ years and just started going in to trouble this winter. I do not know how the cable read with reference to ground when I installed the system, so I have no initial reference. I would think if the cable was compromised, I'd see an issue between conductors.

Thoughts?

Maybe a rat has chewed the wire?

So these are 2 wire smokes?

That sounds like quite a load on the panel. Have you calculated the total load? Checked voltage? Power supplies get weak.

If there was rodent damage, I'd expect shorts between conductors - or one, maybe 2 conductors to electrical ground somewhere. Good thought though. The fact that I read (albeit a very high resistance) to ground - 20meg on all 4 conductors makes me think I am simply reading conductance between the fire cable and a parallel AC wire somewhere?

This is a commercial Bosch panel (D9412G), with a D125B Dual Class B Initiating Module I'm using for the fire loops. This module allows for detector current @ 12 VDC of 3 mA maximum. My entire system consists of 4 smoke detectors and 3 heat detectors - so I'm good on current capacity!

Also - I thought of cable capacitance as another potential issue to look at, but the D125B supports a max wiring impedance of 50 ohms. My longest run (the run with the issue) is maybe 60 feet.

Did you check the voltage?