Douglas Krantz - Technical Writer - Describing How It Works
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You are right about keeping power and control wires separated. Had a fire alarm system many years ago that had 24 volt smoke detector wires and 208 volt fan shut down wires in the same conduit: 250' run in the dirt under the bottom slab of a building.

Fire alarm inspection is the next day and about midday ground fault shows up in the FACP. Scratched heads for a while and then started disconnecting fan shutdown wiring. Each wire removed made the ground fault indicator light get dimmer. Last wire off and light goes out (at around midnight).

Next day had induction relays installed in each fan starter. They output a nominal 24 volt signal off a bias coil with the main coil 120 or 220 volt, which when shorted picks the relay.

I had enough (just) relays on the shelf at home for another project. Brought them in in early AM and just got them in in time for the test and all was good.

Oh, and don't use THHN wire in a wet location (that was part of the problem). It ships water over time: percolates through the insulation. Use cross linked polyethylene.

Signed, Dan Fast

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 8 gauge wires, and also went through the main switchgear for the building.

Most of the time, the ground fault light on the panel was not lit, but once in a while it showed a ground fault. The installing fire alarm company never found the ground 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

P. S. If you haven't seen it yet, you might be interested in the article on soft ground faults at
Thanks for the soft ground fault article. I liked the Nuts & Volts article on the home made low voltage megger (all right, insulation tester: it is more accurate).

Regards, Dan Fast

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