Ground Leakage through Snow Melting Mat

[ruxton.stanislaw]

One potential fix for this, if the manufacturer supports it, could be to rectify the input current. I expect an isolation transformer would also help.

 

[winnie]

Based on the physical layout described by the OP, feeding the mat from a balanced source should indeed cancel out the capacitive voltage/current on the outside surface of the mat. But the effectiveness of that would be less as the spacing between the rails is increased. OP did not state the spacing.

But these mat are _not_ being fed by a balanced source. They are being fed with 208V and as described this is single phase. The neutral point between the two hot legs is at 60V relative to ground.

As I see it, there are several possibilities.

1) A mechanical defect or installation error that creates an actual hole in the insulating material. You are getting direct conduction through this hole, either electrons flowing through conductive material, or ions moving in the water. This is very unlikely given 2 sets of mats used.

2) A defect in the materials used to construct the mats. Both polyethylene and EPDM are generally insulators, but can be processed or 'doped' to make them slightly conductive. This is sometimes done for things such as static control, or could be caused by an unintentional contaminant, Perhaps an incorrect batch of materials was used by the mat manufacturer, and all the mats have exactly the same leakage current defect.

3) Capacitive coupling. Capacitive coupling is _always_ present when you have AC circuits. @synchro described this quite well. If you have electrodes inside the mat at 60-120V relative to ground, and an electrode outside of the mat connected to ground, you _will_ get some current flow, even if the mat insulation is _perfect_. Think about how the 'non-contact voltage sensing pens' work.

IMHO the manufacturer needs to do some megger testing to determine if there is an actual conductive defect. If there isn't an actual conductive defect, then you will need to do more investigation to figure out why capacitive coupling is evident in this installation but not in all of the other installations.

I have a _guess_: that the other installations are on grounded conductive surfaces, which provides a capacitive path to 'shunt' the capacitive leakage, or the other installations don't have a nearby metal guard rail that someone touched, and so the capacitive leakage is present but has not been noticed.

A useful additional measurement would be a low impedance voltage measurement between mat surface and grounded metal, and if the low impedance measurements shows a low voltage, then the next measurement I would make is current (amps) to ground from a region on the mat (say a 1 square foot area) to ground.

Going back to slightly conductive EPDM; I wonder if a grounded conductive EPDM mat would solve this issue.

-Jon

 

[synchro]

Heaters are 208 vac rated and installed on an EPDM Roof with adhesive.

2-pole 208 vac GFEPD (30 ma GF Trip) Circuit Breaker.

Would the current be below the 30 ma GF trip level? I was on the phone with EC and he said when mats are energized they all eventually trip the GFEPD.

Is there a metallic roof underneath the EPDM membrane? If so, there will be some capacitance between the heating structures (flat bus wires and resistive elements) and the metal roof, which is presumed to be grounded. The common mode voltage present on the 208V lines will produce a common-mode current through this capacitance. Whether that could trip a 30mA GFPE would depend on the capacitance.
The worst case is when only one of the lines to the heaters is switched off, because the common mode voltage would be 120V. With both lines switched on, the common-mode voltage from 2 phases of a 208V system would be 60V as was described earlier.

A suggestion is to disconnect the heaters from both phases, and measure the capacitance from one of the lines to the metal building structure and/or an equipment grounding conductor. The other line to the heaters should be left open, or the two lines can be connected together (it shouldn't matter if only tens of milliamps are flowing). Many handheld meters can measure capacitance.

The amount of capacitance needed to draw 30mA when 120V 60Hz is applied across it is 0.66 μF (660 nF).
60Vac would require twice that capacitance to draw 30mA.

You could also measure the resistance to ground while you're at it, although this will not be applying any voltage stress to the insulation. You may have to wait for the reading to settle down because of the capacitance.

 

[drcampbell]

Is there a metallic roof underneath the EPDM membrane? ... which is presumed to be grounded. ...

I would not presume that. A metal roof structure on masonry walls will be pretty effectively insulated from ground after the concrete & mortar are fully cured. New construction? What kind of construction?

Bring a full roll of green wire so you can make voltage measurements all over the building with reference to the system ground. (at the service entrance) After assuring that the service itself is properly grounded.

Some polymers (rubbers & plastics) are conductive. Tires are, deliberately so, to drain away any static charge that might be on the vehicle body when you pull into a filling station. Raw EPDM might be an insulator, but this particular compounding might not be; it might be blended with carbon black for color.

 

[synchro]

I would not presume that. A metal roof structure on masonry walls will be pretty effectively insulated from ground after the concrete & mortar are fully cured. New construction? What kind of construction?

It still may have a low enough resistance to earth that 30mA would produce a relatively small voltage drop compared to 120V. I was presuming it to be grounded as a worse case condition to calculate a maximum allowable capacitance before a GFCI could trip. But I agree that detailed measurements need to be done to determine what is really happening.

 

[TwoBlocked]

...

The EC did say that they were using the T-stat to break two hot legs from two Circuit breakers. I advised him not to do this.

...

2-pole 208 vac GFEPD (30 ma GF Trip) Circuit Breaker.

...

EC said that there was no leakage prior to energizing the CB's. ...

These statements don't add up to me. Find a different Electrical Contractor?

 

[synchro]

When a meter is used, it is measuring 110 vac to ground on the outside of the mat (a t-stat is breaking one hot leg of the circuit when not calling for heat). When the T-stat calls for heat, the voltage to ground drops by 50% to 55 Vac.

Is there a metallic roof underneath the EPDM membrane? If so, there will be some capacitance between the heating structures (flat bus wires and resistive elements) and the metal roof... The common mode voltage present on the 208V lines will produce a common-mode current through this capacitance. Whether that could trip a 30mA GFPE would depend on the capacitance.
The worst case is when only one of the lines to the heaters is switched off, because the common mode voltage would be 120V. With both lines switched on, the common-mode voltage from 2 phases of a 208V system would be 60V as was described earlier.

2-pole 208 vac GFEPD (30 ma GF Trip) Circuit Breaker.
Would the current be below the 30 ma GF trip level? I was on the phone with EC and he said when mats are energized they all eventually trip the GFEPD.

I think a first priority would be to use a contactor or other means to break both legs of the 208V. When only one leg is broken to turn the heaters off, the common mode voltage to the mats (and therefore the common mode current from capacitance assuming it's symmetrical on both legs) will be 2X what it'll be when the heaters are turned ON. After a contactor is in place you can see if the GFEP breaker will then hold.

 

[ptonsparky]

I think a first priority would be to use a contactor or other means to break both legs of the 208V. When only one leg is broken to turn the heaters off, the common mode voltage to the mats (and therefore the common mode current from capacitance assuming it's symmetrical on both legs) will be 2X what it'll be when the heaters are turned ON. After a contactor is in place you can see if the GFEP breaker will then hold.

Is the breaker tripping now?...I missed that part.

 

[synchro]

The EC did say that they were using the T-stat to break two hot legs from two Circuit breakers. I advised him not to do this.

It's not clear what you mean by this. Does this refer to breaking one leg on each of two different 2-pole GFEP breakers?

 

[synchro]

Is the breaker tripping now?...I missed that part.

From the OP's post #5:

Mats are on GFEPD CB's (30 ma trip). No tripping.

From post #16:

I was on the phone with EC and he said when mats are energized they all eventually trip the GFEPD.

 

[GoldDigger]

But these mat are _not_ being fed by a balanced source. They are being fed with 208V and as described this is single phase. The neutral point between the two hot legs is at 60V relative to ground.

...

...

...

Going back to slightly conductive EPDM; I wonder if a grounded conductive EPDM mat would solve this issue.

-Jon

Exactly. But the topic under consideration at the time I posted my comment was whether adding a transformer to provided a balanced voltage would be a possible fix for the OP's problem.

 

[RickLosi]

But these mat are _not_ being fed by a balanced source. They are being fed with 208V and as described this is single phase. The neutral point between the two hot legs is at 60V relative to ground.

As I see it, there are several possibilities.

1) A mechanical defect or installation error that creates an actual hole in the insulating material. You are getting direct conduction through this hole, either electrons flowing through conductive material, or ions moving in the water. This is very unlikely given 2 sets of mats used.

2) A defect in the materials used to construct the mats. Both polyethylene and EPDM are generally insulators, but can be processed or 'doped' to make them slightly conductive. This is sometimes done for things such as static control, or could be caused by an unintentional contaminant, Perhaps an incorrect batch of materials was used by the mat manufacturer, and all the mats have exactly the same leakage current defect.

3) Capacitive coupling. Capacitive coupling is _always_ present when you have AC circuits. @synchro described this quite well. If you have electrodes inside the mat at 60-120V relative to ground, and an electrode outside of the mat connected to ground, you _will_ get some current flow, even if the mat insulation is _perfect_. Think about how the 'non-contact voltage sensing pens' work.

IMHO the manufacturer needs to do some megger testing to determine if there is an actual conductive defect. If there isn't an actual conductive defect, then you will need to do more investigation to figure out why capacitive coupling is evident in this installation but not in all of the other installations.

I have a _guess_: that the other installations are on grounded conductive surfaces, which provides a capacitive path to 'shunt' the capacitive leakage, or the other installations don't have a nearby metal guard rail that someone touched, and so the capacitive leakage is present but has not been noticed.

A useful additional measurement would be a low impedance voltage measurement between mat surface and grounded metal, and if the low impedance measurements shows a low voltage, then the next measurement I would make is current (amps) to ground from a region on the mat (say a 1 square foot area) to ground.

Going back to slightly conductive EPDM; I wonder if a grounded conductive EPDM mat would solve this issue.

-Jon

I purchased a 2' x 3' mat (same construction) from a different manufacturer. Brought it to the site, wired it to one of the circuits (all of the three remaining CB's were turned off). I am getting the same result;

1. Powered at 208 vac, the dry mat is tested to ground and voltage is 2v.
2. When water is poured onto the mat, depending on the depth of the water, I was getting 50 v to ground when powered, and 110 v when I disconnected one leg of the power.

I meggared the mat (at 1000v) from the bus-wires to building ground: Result 2.2G ohms
I meggared the mat (at 1000v) from the mat ground wire to building ground: Result 2.2 G ohms.
These results indicate that the insulation is intact.

I tested the mat in three location:
1. laying on the surface of the roof (EPDM)
2. We held the mat in the air (not touching any part of the building).
3. On ground Floor of Building outside on concrete
4. Inside on a wooden workbench at my EC's shop

The results for each area are the same

Did not measure current readings from mat to ground, too low as the GFEPD Circuit Breaker would trip in excess of 30 ma.

I am doubtful that the original set of mats, the replacement set of mats and the new mat from an entirely different manufacturer are defective.

Both the replacement mats and the new test mat were manufactured with a ground covering the heating element.

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?

Thanks for the input

 

[RickLosi]

Is there a metallic roof underneath the EPDM membrane? If so, there will be some capacitance between the heating structures (flat bus wires and resistive elements) and the metal roof, which is presumed to be grounded. The common mode voltage present on the 208V lines will produce a common-mode current through this capacitance. Whether that could trip a 30mA GFPE would depend on the capacitance.
The worst case is when only one of the lines to the heaters is switched off, because the common mode voltage would be 120V. With both lines switched on, the common-mode voltage from 2 phases of a 208V system would be 60V as was described earlier.

A suggestion is to disconnect the heaters from both phases, and measure the capacitance from one of the lines to the metal building structure and/or an equipment grounding conductor. The other line to the heaters should be left open, or the two lines can be connected together (it shouldn't matter if only tens of milliamps are flowing). Many handheld meters can measure capacitance.

The amount of capacitance needed to draw 30mA when 120V 60Hz is applied across it is 0.66 μF (660 nF).
60Vac would require twice that capacitance to draw 30mA.

You could also measure the resistance to ground while you're at it, although this will not be applying any voltage stress to the insulation. You may have to wait for the reading to settle down because of the capacitance.

I purchased a 2' x 3' mat (same construction) from a different manufacturer. Brought it to the site, wired it to one of the circuits (all of the three remaining CB's were turned off). I am getting the same result;

1. Powered at 208 vac, the dry mat is tested to ground and voltage is 2v.
2. When water is poured onto the mat, depending on the depth of the water, I was getting 50 v to ground when powered, and 110 v when I disconnected one leg of the power.

I meggared the mat (at 1000v) from the bus-wires to building ground: Result 2.2G ohms
I meggared the mat (at 1000v) from the mat ground wire to building ground: Result 2.2 G ohms.
These results indicate that the insulation is intact.

I tested the mat in three location:
1. laying on the surface of the roof (EPDM)
2. We held the mat in the air (not touching any part of the building).
3. On ground Floor of Building outside on concrete
4. Inside on a wooden workbench at my EC's shop

The results for each area are the same

Did not measure current readings from mat to ground, too low as the GFEPD Circuit Breaker would trip in excess of 30 ma.

I am doubtful that the original set of mats, the replacement set of mats and the new mat from an entirely different manufacturer are defective.

Both the replacement mats and the new test mat were manufactured with a ground covering the heating element.

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?

Thanks for the input

 

[RickLosi]

I think a first priority would be to use a contactor or other means to break both legs of the 208V. When only one leg is broken to turn the heaters off, the common mode voltage to the mats (and therefore the common mode current from capacitance assuming it's symmetrical on both legs) will be 2X what it'll be when the heaters are turned ON. After a contactor is in place you can see if the GFEP breaker will then hold.

Building roof is EPDM

 

[RickLosi]

These statements don't add up to me. Find a different Electrical Contractor?

The T-stat is a snow control switch, DS-9, mfg by ASE Colorado, the instruction manual clearly states that it can be used to Control two Circuits fed from two CB's. The contactor has two isolated poles that would be used to "break" one hot leg from two different CB's.

Wed are sending out two more units so that each of the four circuits has its own control and ability to "break" or open both legs of the circuit.

 

[RickLosi]

I purchased a 2' x 3' mat (same construction) from a different manufacturer. Brought it to the site, wired it to one of the circuits (all of the three remaining CB's were turned off). I am getting the same result;

1. Powered at 208 vac, the dry mat is tested to ground and voltage is 2v.
2. When water is poured onto the mat, depending on the depth of the water, I was getting 50 v to ground when powered, and 110 v when I disconnected one leg of the power.

I meggared the mat (at 1000v) from the bus-wires to building ground: Result 2.2G ohms
I meggared the mat (at 1000v) from the mat ground wire to building ground: Result 2.2 G ohms.
These results indicate that the insulation is intact.

I tested the mat in three location:
1. laying on the surface of the roof (EPDM)
2. We held the mat in the air (not touching any part of the building).
3. On ground Floor of Building outside on concrete
4. Inside on a wooden workbench at my EC's shop

The results for each area are the same

Did not measure current readings from mat to ground, too low as the GFEPD Circuit Breaker would trip in excess of 30 ma.

I am doubtful that the original set of mats, the replacement set of mats and the new mat from an entirely different manufacturer are defective.

Both the replacement mats and the new test mat were manufactured with a ground covering the heating element.

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 occurring. If this is the case, is there any way to eliminate this phenomenon?

Thanks for the input

 

[ptonsparky]

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

 

[synchro]

1. Powered at 208 vac, the dry mat is tested to ground and voltage is 2v.
2. When water is poured onto the mat, depending on the depth of the water, I was getting 50 v to ground when powered, and 110 v when I disconnected one leg of the power.

It would be interesting to see if the voltage to ground when powered up stays fairly close to 50V when you probe it at different points from one edge to the other, first across the width and then across the length. This is to address the issue GoldDigger brought up in post #15 that there may be a voltage gradient across the mat depending on how much the out-of-phase L-G components from the 208V source cancel at various points between the rails. The 50-60V in-phase components of course would not be cancelled.
If the measured voltage does not vary much across the mat, then there could be a good chance that supplying mats with 208V power that has L1-G and L2-G at 180° from each other would significantly lower the measured voltage to ground below the present 50V. This could be achieved with a transformer from the existing source (which has L1-G and L2-G at 120° from each other) to obtain 208V with L1-G and L2-G at 180°. A commonly available 240-120/240V transformer could be used with a 13% derating on the kVA.

If you had a similar mat that could run off of 240V, then you could do your tests from a split-phase 120/240V source such as a residential service. If that significantly lowers your measured voltages to ground when the mat is wet, it could provide some justification for a transformer.

You mentioned in post #16 that the mats eventually trip the 30mA GFEP breakers. Do you know if that is only happening when only one of the lines is switched open? If it's also tripping when both lines are powered, then that could be another justification for a transformer because it could provide better cancellation of the L1-G and L2-G leakages than with the 120/208V source.

 

[synchro]

Both the replacement mats and the new test mat were manufactured with a ground covering the heating element.

Do you know if both the top and bottom of the heating elements are covered by a grounded conductor?

 

[winnie]

I purchased a 2' x 3' mat (same construction) from a different manufacturer. Brought it to the site, wired it to one of the circuits (all of the three remaining CB's were turned off). I am getting the same result;

1. Powered at 208 vac, the dry mat is tested to ground and voltage is 2v.
2. When water is poured onto the mat, depending on the depth of the water, I was getting 50 v to ground when powered, and 110 v when I disconnected one leg of the power.

I meggared the mat (at 1000v) from the bus-wires to building ground: Result 2.2G ohms
I meggared the mat (at 1000v) from the mat ground wire to building ground: Result 2.2 G ohms.
These results indicate that the insulation is intact.

I tested the mat in three location:
1. laying on the surface of the roof (EPDM)
2. We held the mat in the air (not touching any part of the building).
3. On ground Floor of Building outside on concrete
4. Inside on a wooden workbench at my EC's shop

The results for each area are the same

Did not measure current readings from mat to ground, too low as the GFEPD Circuit Breaker would trip in excess of 30 ma.

I am doubtful that the original set of mats, the replacement set of mats and the new mat from an entirely different manufacturer are defective.

Both the replacement mats and the new test mat were manufactured with a ground covering the heating element.

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?

Thanks for the input

I think you are doing a great job with your testing.

Questions:
What sort of meter are you using to measure the 'voltage to ground'? I presume you are using a standard 'high impedance' multimeter.

When you measure the dry mats, I presume you are measuring a very small point (the area of the probe tip). When you measure the wet mat, the effective area of your probe tip gets larger because of the conductivity of the water. Have you tried anything to get a larger measurement area for the dry mat, for example using a square of aluminum foil?

When you meggared the mats, were they on a conductive surface?

Did you try an insulation resistance test on a wet mat?

Regarding eliminating capacitive coupling, the suggestion that others have made to use a _balanced_ supply (voltage balanced relative to ground) will help. This doesn't eliminate leakage, but balances it so the resulting voltage is lower. The only other way to deal with capacitive coupling is to increase spacing (make the insulating layer thicker and the capacitive conduction goes down) or add shielding. Shielding in this case is a grounded metal sheet over the mats; any capacitive current is shunted to ground reducing the exposed contact voltage. Shielding actually makes the capacitive coupling current worse, but shunts it so that the _touch voltage_ is lowered.

-Jon