T- Stat Caught Fire

[kwired]

I just spoke with the homeowner.

Apparently the problem was at the compressor. According to the HVAC guys the compressor went bad and the high voltage wire came in contact with the low voltage wire. At least this is there diagnosis- I have not seen it so that's all I got. Now how did the low voltage cable come into contact with the HV cable is another question and why 46V and not 120v or more.

Maybe someone can answer this-- if one leg of the 240V circuit touched one wire on the thermostat would it be possible that there was 120 to ground but 46V across the T-stat conductors?

I don't see compressor failure as too likely of problem, but maybe carbon deposits on compressor contactor after a flash event at the contactor is likely. Both power and control circuit are in close proximity at the contactor.

 

[SEO]

Did 240 volts get applied to the furnace? That could explain the 46 v reading at the thermostat. (Open grounded conductor ?)

 

[kwired]

I don't see compressor failure as too likely of problem, but maybe carbon deposits on compressor contactor after a flash event at the contactor is likely. Both power and control circuit are in close proximity at the contactor.

What condition is the EGC to the compressor? A ground fault in compressor with poor or no EGC and a grounded 24 volt control conductor (intentionally grounded or not) would give a not so good path for the fault current, if the thermostat was part of the path this could possibly happen. I would suspect there would also be good chance of damage to the thermostat cable though.

 

[Gibbs]

thermostat fire

thermostat fire

I've seen this before. A thermostat in a children's bed room probably means it is zoned
with dampers. Triacs in the thermostat run the dampers. They can over heat and start a fire.

 

[MAK]

I don't suppose it could have been a Chinese counterfeit thermostat could it?
Maybe cheaper components more likely to catch on fire.

 

[Dennis Alwon]

Did 240 volts get applied to the furnace? That could explain the 46 v reading at the thermostat. (Open grounded conductor ?)

The heat pump is 240V, so there is no grounded conductor

I've seen this before. A thermostat in a children's bed room probably means it is zoned with dampers. Triacs in the thermostat run the dampers. They can over heat and start a fire.

The thermostat was moved from the hall that was open to the 1st floor. No zone involved.

I don't suppose it could have been a Chinese counterfeit thermostat could it?
Maybe cheaper components more likely to catch on fire.

It was a Honeywell-- that's all I know.

 

[hurk27]

Dennis, If you ever get to the bottom of this, please do tell

and just an FYI, a transformer with shorted turns on the secondary will increase the turn ratio and result in lower voltage out, a transformer with shorted turns on the primary will lower the turn ratio and result in a higher voltage out. Almost all class 2 transformers manufactured in the last 20 years or so have a thermo fuse embedded in the windings that should open if the transformer gets hot, and it opens the primary.

The above is what make this post such a mystery

 

[Mgraw]

and just an FYI, a transformer with shorted turns on the secondary will increase the turn ratio and result in lower voltage out, a transformer with shorted turns on the primary will lower the turn ratio and result in a higher voltage out.

Correct, unless the secondary shorts to the primary.

Almost all class 2 transformers manufactured in the last 20 years or so have a thermo fuse embedded in the windings that should open if the transformer gets hot, and it opens the primary.

Common in wall-warts but I have not come across one in a HVAC unit. Generally they are fused externally.

 

[Dennis Alwon]

Again, the final info I got, I probably will get nothing else, is that the contactor in the compressor unit somehow shorted the HV to the LV. My guess is a loose connection that melted the contactor and then shorted the wires.

 

[Gibbs]

thermostat fire.

thermostat fire.

The heat pump is 240V, so there is no grounded conductor


The thermostat was moved from the hall that was open to the 1st floor. No zone involved.



It was a Honeywell-- that's all I know.

The zoned thermostat fire I am talking about, the damper motors don't have limit switches. The power to the damper motor is shut off when the damper blade hits the stop, and the thermostat control senses the motor current spike.

 

[hbiss]

...the contactor in the compressor unit somehow shorted the HV to the LV. My guess is a loose connection that melted the contactor and then shorted the wires.

Ok, that makes the most sense. It wasn't the compressor itself but the outdoor condensing unit that they are talking about. I can see the LV coil wiring in contact with the line voltage wiring around the contactor. When it heated up the insulation melted on both.

Kinda makes you appreciate the restrictions on LV wiring in the same enclosure as line voltage.

-Hal