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 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)🫧?

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