Intermittent Ground Faults

Could someone explain to me what would be possible causes of intermittent ground faults that float in and out? Water? What else?

I’ve seen motor controllers back feed voltage into monitor modules for fan status. The AC voltage (24VAC or 120VAC) on the monitor side of the module can cause the SLC to ground intermittently.

I’ve also seen vibration or partially crushed cables cause it. The vibration factor comes into play when it slowly eats away at the insulation. Pinholes in the cable can also cause it.

Then there was the time a rectifier was used to convert 24VAC to DC on the output side of a panel’s transformer. The rectifier 24VDC side was used to power 24VDC remote ice cube relays for fan shutdowns. The building looked like it was a retrofit from an old conventional panel to addressable (EST3) since the wire was appeared to be the original THHN that was put in when the building went up (blue + & white - ). An AC tech saw the white wire on the 24VDC ice cube relay in his controller and assumed it was AC ground. That caused 120 to travel through the negative side with no return path so the ground would come & go every few seconds until we found that neutral connected to the 24VDC negative. Once we removed that neutral everything was fine.

Ground faults that float in and out can be a pain. On some occasions I found a cracked battery that had leaked just enough for the acid to eat through the paint on the bottom of the box to the metal. For some reason it always happened on the back side of the battery out of sight. Then there is a ground through the battery that can come and go.

You mentioned water. The most common place I found water to be a problem was the wiring to a PIV. The device is outside and the conduit is in the ground. Sometimes the conduit fills with water and the wire insulation has a nick or just deteriorates over time. If properly installed a PIV should have a suppressor where the wire enters the building that could be compromised by nearby lightning. The conduit going up to the device can also be hit and broken by lawn mowers.

I always put my voltmeter between chassis ground and negative power supply. Then moved the batteries around and disconnected the PIV.

On simplex panels, ground faults can be caused by tight tye wraps to the red iron of the building. cut them loose.
More typical problems are at outdoor h/s. , door holders, damp locations ect.

If the grd is locked in. then start removing wires, leave them off.
once the ground clears, start hooking them back up.
Leave off any circuits that create thee ground. Keep in mind that you may have multiple grounds.

Once you isolate the circuit, go to the middle of it and split it . Meter the circuit to ground. the ground will be from the feed or towards the field. keep splitting the circuit while you troubleshoot.
if the ground clears, then it “MAY” be fixed. Leave a note on what you found (the ground may of just cleared and was not fixed).
Some ground faults only show themselves under power. They will clear after the circuit power is removed and will come back later.

If possible use an old analog meter a digital meter that has megaohm scale. On a “swinger” (intermittent) ground you might want to use a continuity light or buzzer. they are a much faster reaction on a fast intermittent ground.

Ground fault finding is the skill that defines the true techs from the wannabees.
Good-luck
JMO,
Telephone_Jim

[quote=newtech post_id=79441 time=1520536439 user_id=4492]

Could someone explain to me what would be possible causes of intermittent ground faults that float in and out? Water? What else?

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If the wiring is run free air, it can get caught in “pinch points” in the red iron, uni-strut or any other metal up there. Vibration from HVAC equitiment, wind, Trains, planes or anything thing else that causes vibration to wiring run can create ground faults.

When an intermittent ground first starts. Check to see if any work was done in the ceilings, If there was, do a visual check of any of your wires. If needed adjust the run so they wire isn’t under heavy tension or catching in pinch points.