You're describing one situation I worked on, although that was with an intermittent ground fault problem that had been in the fire alarm system for 10 years. We were taking over the system from another company, and I was sent to add devices to an addressable panel in a school.
The original fire alarm system was a 110 VAC system, so many years ago when the system was first installed, putting some wires in the same conduit as other power equipment wires was no problem.
The new addressable system reused the old wiring, so the Signaling Line Circuit (SLC) going to the waterflow and tampers for the sprinkling system used THHN wire in the same conduit as air handling unit 8 gauge wires, and also went through the main switchgear for the building. 8 gauge wire means lots of current, lots of current means lots of magnetism created by the current, lots of magnetism means lots of power transferred through transformer action from the 8 gauge wire to the fire alarm wire.
Most of the time, the ground fault light on the panel was not lit, but once in a while it showed a ground fault. That's when the air handling equipmet turned on. The installing fire alarm company never found the ground fault because the ground fault was an induced type of ground fault, not a shorted to ground type of fault.
An ohmmeter didn't detect a ground fault on the SLC. An insulation tester (a 36 volt ohmmeter) couldn't detect a ground fault. However, the AC voltmeter showed .6 volts of AC coming into the panel on the SLC. When I disconnected the SLC from the panel, the SLC showed 12 VAC to ground. That was inductive coupling of the current from the 8 gauge wires into the SLC in the same conduit.
The fix was installing new SLC wiring in 10 feet of flex conduit to get around the 40 feet where the SLC shared crosstalk with the 8 gauge wires.
Easy fix for a 10 year old problem, and the school district facilities superintendent considers me a hero.
Douglas Krantz
