Ground Leakage through Snow Melting Mat

synchro

synchro

Senior Member
Location
Chicago, IL
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EE
RickLosi said:
Did not measure current readings from mat to ground, too low as the GFEPD Circuit Breaker would trip in excess of 30 ma.
Click to expand...

If you have a clamp meter that is sensitive enough to measure currents less than 30 mA, you can measure the leakage current on the circuit by putting the clamp around both of the line conductors feeding the mat(s). That will give you the difference in the currents flowing through the two conductors, which will be the leakage current that's flowing to ground or through another path back to the neutral/EGC bond.
If you have a standard clamp meter, you may be able to resolve milliamp current levels by putting the clamp around 10 or more turns of a twisted pair of wires which carries the line currents, like in this post I made a couple years back:
https://forums.mikeholt.com/threads/gfci-randomly-tripping.2559909/post-2676229
 
synchro

synchro

Senior Member
Location
Chicago, IL
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EE
RickLosi said:
Manufacturer recommended controls that open both legs of the 208 vac circuit when not calling for heat, and also switching the GFEPD CB's to standard CB's, since voltage to ground when energized is less than 50 v.

I need to check NEC code for this application to see if GFEPD is required for portable snow mats.

I think I am leaning toward some sort of capacitance occuring. If this is the case, is there any way to eliminate this phenomenon?
Click to expand...
ptonsparky said:
I would feel real nervous about switching out the GF protection for a standard breaker.
I think that is pretty much a code requirement 426.28
Click to expand...

I agree with ptonsparky that using a standard breaker would not really be acceptable.
Does the GFEP breaker have to feed multiple mats for tripping to happen, or will it trip with just one of them?

Consider the following as an alternative implementation using a GFEP breaker that might reduce or eliminate tripping, or at least verify that capacitive leakage is causing the tripping:

Each capacitance between the two 208V lines and ground has 120V across it, and these voltages are at 120° from each other. As an example, if each capacitance is 0.33μF then each will draw 15mA of current at 60Hz. Because these currents will be at 120° from each other, a net common-mode current of 15mA will flow to ground. This "missing" current from the 208V circuit will be detected by the CT inside the GFEP breaker.

An assumption is that such capacitive current in a new mat is not an actual ground fault caused by a defect or damage, but is inherent to the physical construction and this capacitive current will persist and be relatively constant over time. If this assumption is valid, this capacitive current might be "trimmed" out to recenter the GFEP breaker's response so that it will trip when there is any additional leakage that exceeds 30mA. One way to do this is to have a standard 2-pole breaker feed two capacitors, the other terminals of which are connected to the load neutral terminal of the GFPE breaker. This will then supply current to the load neutral which will duplicate and therefore compensate the missing current on the line conductors caused by capacitive leakage between the mat to ground. As a result, the compensation current will cancel the magnetic field from the mat's capacitive leakage current that would otherwise be detected by the CT within the GFEP breaker.

If the common-mode capacitance of the mat to ground is enough to cause the 30mA GFEP breaker to eventually trip , but the value of this capacitance is not known, compensating capacitors of 0.33μF as in the example above might be used. Compensating capacitors of this value should not trip the GFEP breaker by themselves when the mat is disconnected, but when the mat is connected the capacitors will effectively subtract 15mA from the inherent capacitive leakage current of the mat. If more compensation is wanted you'd probably have to disconnect the compensating capacitors when the mats are disconnected to prevent the capacitors from tripping the GFEP breaker.
An example of such capacitors:
https://www.mouser.com/ProductDetai...MKP1839433124HQ?qs=5W%2BGeepUzoeB5Ob4x0s5Jg==

The above seems to be a form of multiwire branch circuit using the load neutral, where one of the branch circuits is only drawing 10's of milliamps. Even if the above arrangement is not considered suitable for a permanent installation, it could be used to verify that capacitive leakage from the mats is the culprit causing tripping of the GFEP breakers.
 
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RickLosi

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Connecticut
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Sales
ptonsparky said:
I would feel real nervous about switching out the GF protection for a standard breaker.
I think that is pretty much a code requirement 426.28
Click to expand...
Both manufacturers are telling me that they do not need GFEPD since their equipment is not "fixed", it is "portable". This is a gray area that needs to be defined.
synchro said:
Do you know if both the top and bottom of the heating elements are covered by a grounded conductor?
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Grounded only on one side. We took readings from both sides of mat. Same result
 
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winnie

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Location
Springfield, MA, USA
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Electric motor research
RickLosi said:
Both manufacturers are telling me that they do not need GFEPD since their equipment is not "fixed", it is "portable". This is a gray area that needs to be defined.

Grounded only on one side. We took readings from both sides of mat. Same result
Click to expand...

What do you mean 'grounded one side only'? Do the mats include a grounded conductive layer?
 
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RickLosi

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Connecticut
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Sales
Portable in the sense they are free standing and can be rolled up and put away during the summer. The adhesive is a double sided tape that holds the corners down so wind wont blow them around. There are no mechanical fasteners.
Similar to those seen online
 
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RickLosi

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Location
Connecticut
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Sales
winnie said:
What do you mean 'grounded one side only'? Do the mats include a grounded conductive layer?
Click to expand...
The manufacturer bonds a ground shield onto one side of the heater film. This ground shield is then connected to a ground wire that is part of the power cord.
 
W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
RickLosi said:
The manufacturer bonds a ground shield onto one side of the heater film. This ground shield is then connected to a ground wire that is part of the power cord.
Click to expand...

If that ground shield is intact and actually grounded/bonded, then any capacitive coupling from the power circuits should be shunted through the ground shield. If you energize the heating elements with your imbalanced 208V, you would expect current flowing on the ground wire. But any measurements made from outside on the grounded side of the heater should not see any significant voltage, because the capacitive voltage gets masked by the grounded shield.

-Jon
 
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RickLosi

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Location
Connecticut
Occupation
Sales
After speaking with both manufacturers, they both indicated that this was the first time that they have heard of this problem. One mfg has sold over 200 units in last month for commercial applications in this voltage range. Has not received any calls with this problem.

I was leaning toward a problem with the building's electric service, but since the mat was tested off site and exhibited the same problem, I don't believe this is the problem.

I will proceed with dedicated controller for each CB and hopefully this works.

I'll post an update next week

Thanks all
 
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