Troubleshooting Call

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Michael15956

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NE Ohio
Called to a problem with a Carrier Furnace tripping a gfci receptacle on a Ryobi portable genny. This furnace had a pigtail so that it could be connected to the line side by a receptacle or the genny with an extension cord. Line side, (house current) is not gfci protected. This is the first time the furnace was connected to the genny.

Thought there might be a neutral to ground fault and disconnect the internal grounding wires. At this point the furnace operated just fine off the genny.

Customer said that funny because other household appliances run off the genny. Though for sure it was a neutral to ground fault in the furnace but just for a test I connected the grounding wires back inside the furnace and connected it to a line side gfci receptacle. Furnace ran fine off this line side gfci receptacle!!

Have to admit that left me scratching my ass.

Anybody else ever run into this kind of problem before?

Does it matter if this furnace had a converter to provide dc current for the blower motor? This was the point where the gfci tripped on the genny, when the blower motor started, but ran fine on a household gfci receptacle.
 
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a lot of GFCI and AFCI receptacles seem to have issues with motors that use some kind of drive. have you tried replacing the GFCI on the generator with a higher quality unit? many modern furnaces use adjustable speed drives of some sort to run their motors.
 
I talked with Michael and suggested he take his honda generator over there and see if he has the same issue. My feeling is that there is dirty power from the generator that is affected by the furnace. It must be the generator or the gfci in the genny (as Petersonra stated) since the furnace worked well when plugged into a gfci circuit from the house. The house gfci was tested and tripped when the test button was used.
 
FWIW, not necessarily relevant in this case, some spark igniters and some current-based flame sensors create a stray current path to furnace ground/EGC that can trip either an AFCI or a GFCI.
It is possible that either the two GFCI units had a significantly different actual threshold or the two GFCIs differed in their response to high frequency ground current compared to the trip point for 50/60Hz.
 
All the Ryobi portables I'm aware of have a bonded neutral (i.e., ground bonded to neutral in the generator). If the furnace has a ground path back to the main panel when the Ryobi is plugged in, then some of the generator neutral current will split at the neutral pigtail, travel along household neutral to the panel, jump to the ground via the service panel's neutral/ground connection, travel back along this ground path to the ground pigtail, travel to the generator along the generator's ground, and get back to the generator coils via the generator's neutral/ground bond. As some of the return current is using this alternate path, the generator GFCI trips.

Disconnecting the ground at the furnace breaks the alternate ground path, ensuring all gen source current returns on the gen neutral.

The furnace works fine connected to line power because the neutral ground bond in the panel is line side of the GFCI.

Other appliances work because they don't have the ground path back to the panel when plugged into the generator.
 
JPinVA said:
All the Ryobi portables I'm aware of have a bonded neutral (i.e., ground bonded to neutral in the generator). If the furnace has a ground path back to the main panel when the Ryobi is plugged in, then some of the generator neutral current will split at the neutral pigtail, travel along household neutral to the panel, jump to the ground via the service panel's neutral/ground connection, travel back along this ground path to the ground pigtail, travel to the generator along the generator's ground, and get back to the generator coils via the generator's neutral/ground bond. As some of the return current is using this alternate path, the generator GFCI trips.

Disconnecting the ground at the furnace breaks the alternate ground path, ensuring all gen source current returns on the gen neutral.

The furnace works fine connected to line power because the neutral ground bond in the panel is line side of the GFCI.

Other appliances work because they don't have the ground path back to the panel when plugged into the generator.
Click to expand...

So the possible furnace's ground path might be the metal duct work?
 
Michael15956 said:
So the possible furnace's ground path might be the metal duct work?
Click to expand...

the issue in that case would be that you have the neutral connected at two points. one at the service equipment and one at the generator. that allows some neutral current to flow on the egc so the neutral and hot current are not the same causing the GFCI to trip. one would expect that to happen as soon as the furnace turned on though.
 
petersonra said:
the issue in that case would be that you have the neutral connected at two points. one at the service equipment and one at the generator. that allows some neutral current to flow on the egc so the neutral and hot current are not the same causing the GFCI to trip. one would expect that to happen as soon as the furnace turned on though.
Click to expand...

got it finally
 
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petersonra said:
the issue in that case would be that you have the neutral connected at two points. one at the service equipment and one at the generator. that allows some neutral current to flow on the egc so the neutral and hot current are not the same causing the GFCI to trip. one would expect that to happen as soon as the furnace turned on though.
Click to expand...
Even if the neutral to the furnace is not interconnected to the furnace sheet metal, the potential problem is that current from either the flame sensor or the spark ignition almost certainly does not flow between its source and an insulated terminal connected to neutral but rather from its source to grounded metal in the furnace burner assembly.
The remaining question is whether the source of the offending current is or is not set up as an SDS. If it is isolated, then current to earth from it should not trip the GFCI.
 
GoldDigger said:
Even if the neutral to the furnace is not interconnected to the furnace sheet metal, the potential problem is that current from either the flame sensor or the spark ignition almost certainly does not flow between its source and an insulated terminal connected to neutral but rather from its source to grounded metal in the furnace burner assembly.
The remaining question is whether the source of the offending current is or is not set up as an SDS. If it is isolated, then current to earth from it should not trip the GFCI.
Click to expand...

most times it is a transformer because the voltage needs to be increased a lot.
 
petersonra said:
most times it is a transformer because the voltage needs to be increased a lot.
Click to expand...
Yes, but as we know a transformer can be wired either as an SDS or a non-SDS, and it is not obvious to me that furnace designers are aware of the impact of that decision on interoperability with GFCIs.
 
JPinVA said:
All the Ryobi portables I'm aware of have a bonded neutral (i.e., ground bonded to neutral in the generator). If the furnace has a ground path back to the main panel when the Ryobi is plugged in, then some of the generator neutral current will split at the neutral pigtail, travel along household neutral to the panel, jump to the ground via the service panel's neutral/ground connection, travel back along this ground path to the ground pigtail, travel to the generator along the generator's ground, and get back to the generator coils via the generator's neutral/ground bond. As some of the return current is using this alternate path, the generator GFCI trips.

Disconnecting the ground at the furnace breaks the alternate ground path, ensuring all gen source current returns on the gen neutral.

The furnace works fine connected to line power because the neutral ground bond in the panel is line side of the GFCI.

Other appliances work because they don't have the ground path back to the panel when plugged into the generator.
Click to expand...

There's not a path from the furnace neutral to the house wiring since the furnace is wired with a pigtail. In order for current to flow, the path has to be available to being with. Unplugging the cord removes the house wiring neutral connection.
 
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GoldDigger said:
Yes, but as we know a transformer can be wired either as an SDS or a non-SDS, and it is not obvious to me that furnace designers are aware of the impact of that decision on interoperability with GFCIs.
Click to expand...
Sometimes the ignition spark is supplied by an autotransformer, with both low-voltage and high-voltage current returned through the sheet metal.
 
Michael15956 said:
....... This furnace had a pigtail so that it could be connected to the line side by a receptacle or the genny with an extension cord. Line side, (house current) is not gfci protected. This is the first time the furnace was connected to the genny..........
Click to expand...

Important information for those who are looking for the leaking current flow.
 
JPinVA said:
All the Ryobi portables I'm aware of have a bonded neutral (i.e., ground bonded to neutral in the generator). If the furnace has a ground path back to the main panel when the Ryobi is plugged in, then some of the generator neutral current will split at the neutral pigtail, travel along household neutral to the panel, jump to the ground via the service panel's neutral/ground connection, travel back along this ground path to the ground pigtail, travel to the generator along the generator's ground, and get back to the generator coils via the generator's neutral/ground bond. As some of the return current is using this alternate path, the generator GFCI trips.

Disconnecting the ground at the furnace breaks the alternate ground path, ensuring all gen source current returns on the gen neutral.

The furnace works fine connected to line power because the neutral ground bond in the panel is line side of the GFCI.

Other appliances work because they don't have the ground path back to the panel when plugged into the generator.
Click to expand...

petersonra said:
the issue in that case would be that you have the neutral connected at two points. one at the service equipment and one at the generator. that allows some neutral current to flow on the egc so the neutral and hot current are not the same causing the GFCI to trip. one would expect that to happen as soon as the furnace turned on though.
Click to expand...

480sparky said:
There's not a path from the furnace neutral to the house wiring since the furnace is wired with a pigtail. In order for current to flow, the path has to be available to being with. Unplugging the cord removes the house wiring neutral connection.
Click to expand...
exactly, if there were a second neutral to ground connection it would trip the "house GFCI" also when trying to operate from it. This also presuming the furnace is the only thing connected to the generator. Other loads connected could introduce troubles.

drcampbell said:
Sometimes the ignition spark is supplied by an autotransformer, with both low-voltage and high-voltage current returned through the sheet metal.
Click to expand...
120 volt low voltage returned in the appliance frame (bonded to EGC) is a problem - that is your GFCI protected circuit, that too should cause tripping of the "house GFCI" when running from it though. If 24V low voltage it should be isolated by the 120x24 volt transformer and not be a GFCI issue on the 120 volt supply circuit.
 
I'm am lost also.

It might help to remember that the furnace runs fine when connected to a house gfci receptacle with the furnace grounds connected, which would put the furnace on the load side of gfci protection.

Plug the furnace into the genny, it trips the gfci genny receptacle. Disconnect the internal furnace grounds, the furnace will ran fine.
 
I thought about a bad GFCI as well.

Another option: If there's a 120/240 receptacle, like a 14-30 twistlock, get a matching male cord end and pick up one of the hot wires along with the ground and neutral. Feed an ordinary GFCI with that and see if it trips.
 
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