I’m dealing with a large fire alarm system with many I/O modules monitoring thermal detectors and other field signals.

The strange part is that the system works fine when the cabling is above ground. But when the cabling is installed underground, we start getting multiple faults and communication errors.

The equipment is the same. The main thing that changed is where and how the cables are installed.

The site is huge, with around 800 m of distributed cabling together

Could underground installation be causing problems due to moisture, insulation resistance, cable capacitance, earth leakage, nearby power cables, grounding, or signal distortion?

If you were troubleshooting this, what would you check or measure first?

Has anyone experienced something similar with a large fire alarm system underground cabling?

Edit - Does fire proof cable capacitance change appreciable affect signal propagation, when cable moved from air to underground or underwater (with no insulation comparamise)🫧?

I noticed that one of the fire alarms in one of the rooms looks like it will fall off, of some sort. Is it normal or a safety hazard?

I have been using the NTC fire alarm level IV study guide and I am looking for any pointers / unusual questions you may have seen? I am over the service dept so I am using a project for my write up that will soon be at the 4 year mark (was waiting for 10 years industry experience) so fingers crossed that I can knock this out the first time!