AFCI Troubleshooting air handler in crawlspace

[Bluetec]

Greetings,

I have a subpanel (Siemens E0408ML1125SU 4 Space, 8 Circuit, 125-Amp, Main Lug, Surface Mount, Indoor Load Center) fed from a 40amp double pole breaker in a main panel. The neutrals are not bonded to the grounds. I have replaced the breakers in the old Square D box with three with Siemens Q115DF and one Q120DF AFCI/GFCI. One of the 15amp circuits is for an air handler for a TRANE furnace/AC in a crawlspace.

Now, when the furnace starts, the ignitors light and both the vent fan and squirrel-cage fan start. About a minute later, the 15-amp Q115DF trips. After resetting, two LEDS are lit which is supposed to mean "Arc Fault to Ground". I have traced the circuit to a single pole switch and 15amp outlet at the furnace. I disconnected the outlet, replaced the switch, opened up the furnace, and see that all grounds are connected to the chassis and the supplied ground from the circuit feeder is connected to the chassis. There is continuity between all grounds that I have located. I have checked all power leads to ground and neutral leads to ground (de-energized) and there is no connectivity. The other breakers (2 Q115DF and 1 Q120DF) do not trip.

I moved the circuit breaker to another leg on the panel and it trips after about a minute. I replaced the Q115DF with a Q120DF and it trips after the same amount of time. The air handle has a dual-stage fan. It starts low and then speeds up after a set amount of time. The fan seems to have already reached the higher speed when the breaker trips. I replaced the Q115DF with a regular 15-amp circuit breaker and everything works fine.

Has anyone encountered this problem? Does anyone have a clue what I should be looking for that it tripping the AFCI/GFCI breaker?

Thanks

 

[suemarkp]

Is an AFCI required in a crawl space? If not, try a GFCI only breaker. That will at least eliminate the arc fault circuitry as thinking the furnace is arcing. I would not want a furnace on an AFCI circuit if I had a choice.

If the GFCI breaker trips, maybe the furnace is defective. That probably won't go over well...

 

[Bluetec]

The furnace was working fine with a normal breaker and the switch at the air handler on the load side of a GFCI receptacle. I think you're right about needing AFCI protection in crawl spaces. I was trying to be overly cautious about arc-fault requirements. It would be comforting to know why the air handler is tripping the arc-to-ground feature of the Q115DF.

 

[tom baker]

AFCI is not required (yet) in a crawl space. Why is it tripping? That is a question for another topic.
However, AFCIs like GFCIs will trip on a neutral to case connection

 

[kwired]

GFCI not required on the furnace either, unless it would happen to be cord and plug connected - which in most cases it should be required to be "hard wired" anyway, unless maybe it is some sort of portable unit.

should you have a switch for the circuit to said furnace located in a room mentioned in 210.12 - that would introduce AFCI requirement for the entire circuit.

 

[g-and-h_electric]

To the OP:

Let me understand better what is going on........ You have a TRANE gas furnace, There is a "call for heat", the furnace starts, and begins delivering heat and a minute or so AFTER THAT the AFCI trips?

If this is the correct sequence, then I believe the tripping is being caused by the ECM style blower motor. An ECM motor is not a simple multi tap capacitor run motor, it has a bunch of DC switch mode electronics to control the speed ( think built in VFD). This CAN AND WILL throw all kinds of noise back along the powerline. This noise will drive AFCI's bonkers.

As I am the staff electrician for an electrical /HVAC firm that is a Trane house, if you could let me and the group know the model serial, and age, I will do my best to find out if there is an ECM motor in your furnace, and if TRANE has any RFI suppression kits, of who you may have to talk about your situation.

Howard

 

[Bluetec]

Thank you, Howard. The unit is a TRANE XV95 TUH2D120A9V5VAB with serial #9144PFX7G. It was installed in 2009.

 

[tesla six]

I would also make contact with Siemens and start a dialogue. They have been very valuable to me in the past diagnosing the cause of acute nuisance trips, 1 time spending a couple hours on the phone as we worked through it to ID two LED fixtures causing two kitchen dedicated circuit issues.

 

[Bluetec]

Thanks, everyone, for the input. The switch was put inline, at the furnace location, to give the servicing tech a means of disconnecting and reconnecting power to the unit, for troubleshooting. The GFCI receptacle was for a light or tool but according to 210.63(A) it cannot be on the same circuit. I think I will replace the AFCI/GFCI breaker with a GFCI breaker, remove the switch, and put the receptacle on the crawlspace lighting circuit which is protected by a AFCI/GFCI breaker. Kwired mentioned that furnaces should be hardwired. It seems that if a disconnect has to be installed at a condenser/heat pump, outside, there should be some type of disconnect required for a furnace in a crawlspace, even though it is a 15amp single pole circuit.

 

[Dennis Alwon]

Take the afci off the furnace. It doesn't need gfci either if it is direct wired.

 

[kwired]

Thanks, everyone, for the input. The switch was put inline, at the furnace location, to give the servicing tech a means of disconnecting and reconnecting power to the unit, for troubleshooting. The GFCI receptacle was for a light or tool but according to 210.63(A) it cannot be on the same circuit. I think I will replace the AFCI/GFCI breaker with a GFCI breaker, remove the switch, and put the receptacle on the crawlspace lighting circuit which is protected by a AFCI/GFCI breaker. Kwired mentioned that furnaces should be hardwired. It seems that if a disconnect has to be installed at a condenser/heat pump, outside, there should be some type of disconnect required for a furnace in a crawlspace, even though it is a 15amp single pole circuit.

Outside unit is an art 440 application, it does need a disconnect per 440.

Inside unit is not a 440 application. It is usually a 422 motor operated appliance, if there is electric heat within might be a 424 application. You need to see those sections for requirements. 422.31(A) does not require local disconnect for appliances not over 300 VA or 1/8 HP.

Cord and plug can be considered a disconnect - presuming cord and plug is allowed to be used to supply the appliance. Having a receptacle can cause GFCI to become required where hardwiring may not. Depending on which code year applies some appliances can require GFCI whether hard wired or not. Dishwashers has been one of them more recently, 2020 has added outdoor units at dwellings - think it can be about anything, most common thing will be AC's/heat pumps though. Art 680 related items have been pretty likely to need GFCI for a very long time now whether cord and plug or hard wired.