Nails/staples - anything which penetrates the insulation on a wire. Could be done during installation, during construction by someone else, or work done to an existing building. (Air conditioning guys seem to cause a lot of problems if you ask me.)
Sharp edges. Wiring run across sharp edged metal.
The worst one for us was a Miniscan FACP I installed many years ago. The roof leaked, allowing rain to enter a smoke detector. Not only was the smoke wrecked. The fire alarm control panel was ruined!
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