Floor Heat Ground Fault


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Old 05-03-21, 10:35 AM
J
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Floor Heat Ground Fault

I am troubleshooting a ground fault error on a set of electrical floor heat mats and I'm getting stumped. The system that is installed is a 120v QuietWarmth Film Heat (4 mats, all model QWARM3X10F120 ) on a programmable thermostat (OJ Microline UTN4-4999). The installation is on a concrete basemen floor, with LVP flooring.

Troubleshooting I've Done:
-Confirmed that the thermostat was rated for the wattage being used.
-Swapped the thermostat with a confirmed functional thermostat. Still received the error.
-Checked proper voltage for the system, voltage was good. Still received the error.
-Disconnected floor sensor and switched to room temperature control. Still received the error.
-Disconnected all the mats from the 3 junction boxes that tie back into the thermostat. With no mats connected, the system will stay active, which leads me to believe it's an issue with the mats.
-Here's where I'm getting baffled. I tried to connect the mats one at a time to find the "bad mat". Individually, and of the mat's will run without a ground fault. With 2 mats connected, the mats will run without a ground fault. If I connect 3 or more mats, there is a ground fault error.

I'm trying figure out why there is only a ground fault when 3 or more mats are connected, here are my current speculations:
-The romex jumpers from the thermostat to the junction boxes where the leads from the mats tie in, when under load, are having a weird interaction that triggers the GFCI.
-The flooring installers who installed the mats and have not been very responsive, overlapped something somewhere.
-There is minor damage to multiple mats that doesn't create enough fault current to trip the GFCI until the load is high enough.
-The vapor barrier was not properly installed and moisture has gotten into the system, but doesn't create enough fault current to trip the GFCI until the load is high enough.

Any thoughts, insights, similar experiences that anyone could share would be very helpful.

Thanks,
-John
 
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Old 05-03-21, 11:56 AM
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Hi, where are you seeing this GF error?
Geo 🇺🇸
 
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Old 05-03-21, 01:59 PM
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Each mat may have a slight leak. You'll need to use an ohmmeter set to a very high scale.... 100k or higher to check each met. Technically there should be no continuity from any mat wire to ground.
 
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Old 05-03-21, 02:10 PM
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Thanks for the responses, I really appreciate the help here guys.

Geochurchi, the thermostats for the floor heat system have built in GFCI protection and when 3 or more mats are connected, a Ground fault detected message flashes on the LCD screen of the thermostat and the red fault light on the side of the unit blinks.

PJmax, my multimeter is showing very small reading to ground on the leads for all four mats, I believe it cycles down to fractions of milli-ohms before it it displays anything. I thought it unlikely that all four mats could have been damaged, so that there may have been something going on with my meter.
 
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Old 05-03-21, 03:03 PM
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It's not milli ohms you're looking for.
You're looking for very high leakage resistances.... in the order of 20k ohms or better from white or black to ground.

Are your four mats wired separately ?
It would be extremely rare for more than one mat to have a problem.
 
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Old 05-03-21, 03:49 PM
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The mat's are wired together (I have 2 separate junction boxes that each have 2 mats into a new romex wire, that then runs to the thermostat location, because the mat installers chose not to run the leads back to a single point) but I separated all the leads from one another to check the them all individually. I did get a similar reading to ground on each of the 4 mats though, and registered no continuity to ground to each of the conductors on the romex jumpers when I had them isolated. I may be getting my "m" and "M" mixed up for the resistance reading though and that's quite a difference.
 
 

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