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g/fire_alarm • Posted by u/smoke_detector_steve •

Ground fault on a Class B NAC that only shows up when it rains — took four visits Panel Work

Notifier NFS2-3030, retail strip center. Intermittent ground fault on NAC 3. Clears itself. Three previous visits by two different techs found nothing.

Pattern I finally noticed going back through the panel history log: every fault event correlates with rain within the previous six hours.

Megger on the circuit reads over 500 megohms dry. Waiting for rain to test is not a business model.

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u/admin_jake ADMIN •
✓ Accepted Solution

Both correct on where to look, and here is how to find it without waiting for weather.

Do not wait for rain. Make rain. Take a garden hose, isolate NAC 3, put a megger on it with someone watching the reading, and wet one device at a time starting from the far end of the circuit. Thirty seconds per device, wait a minute, watch the meter. When the insulation resistance drops you have found it, and you found it on a Tuesday instead of whenever the weather cooperates.

This is standard practice on the utility side for locating moisture faults and it transfers directly. The only discipline required is doing one device at a time and being patient enough to wait between them, because if you wet three at once you have to start over.

Bring a helper and a radio. Two people, two hours, done.

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✓ Solution
u/smoke_detector_steve OP •

Did this yesterday. Fourth device from the end, a weatherproof strobe under the canopy at the far end of the center. Gasket was hard and cracked and the conduit above it had no drain.

Resistance went from 500 megohms to 40 kilohms in about ninety seconds of hose. Replaced the device, added a drain fitting, and it has held through last night's storm.

Four visits over eight months and it was a two hour test.

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u/fire_code_fred •

The reason nobody did it in the first three visits is that intermittent faults get diagnosed by whoever is standing there when it is not happening. The technique of forcing the condition is the whole skill and it barely gets taught.

Add the hose test to your standard intermittent-ground procedure and you will use it a dozen times a year.

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u/fire_code_fred •

Water is getting into a junction or a device backbox somewhere on that circuit. The megger reading is meaningless because you are measuring the dry condition, which is the condition where there is no fault.

Walk the entire NAC 3 run and look for anything exterior or anything on an exterior wall. Weatherproof horn/strobes with failed gaskets are the usual answer.

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u/suppression_sue •

Also check anything in a soffit or under a canopy. Strip centers are full of devices mounted where water runs off a roof edge and the backbox fills from above through the conduit rather than through the device.

If the conduit comes down from the roof with no drain fitting, the box is a bucket.

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u/panel_programming_paul •

Also worth saying — the panel history log had the answer the whole time. Three techs did not correlate the event timestamps with anything. Pulling the log and looking for a pattern before you open a single box should be step one on any intermittent.

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