How did this clothes dryer circuit start a fire?

[TAH]

An unlicensed, unpermitted person who does not understand circuitry used a 10-2NM cable to wire a clothes dryer. I have recreated this circuit to study why it caused a fire.

At the circuit breaker panel, the black and white conductors were wired to the 2-30 breaker and the bare was wired to the ground bar.

In the 3 wire with ground dryer receptacle they wired black to black, white to white, and green to green. The fourth terminal, usually for the red conductor, was not connected to anything.

At the dryer terminals the 3 wire with ground dryer cord was wired black to black, white to white, green to the terminal that normally receives the red conductor, and red to nothing. The bonding jumper from the white terminal to the dryer casing was left in place.

Voltage at the dryer shows 240VAC black to white, 120VAC black to green, 120VAC white to green, 240VAC black to dryer casing, 0VAC white to dryer casing, 120VAC green to dryer casing.

Whether the dryer is running or not, you do not get shocked when you touch the dryer casing, but you do get shocked when you touch the dryer vent.

When the dryer is running, an ammeter shows 11 amps on the black, 5.3 amps on the white, and 10 amps on the green. An ammeter also shows 10 amps on the black and white inside the same CT, 11 amps on the white and green inside the same CT, and 5.2 amps on the black and green inside the same CT.

When the dryer is not running an ammeter shows 0 on all conductors.

The dryer motor and heater worked for 3-4 days before it started a fire. It is not known if the dryer was in use when the fire started.

The fire pattern seems to show that the fire started at the circuit breaker panel, not at or near the dryer, but the building was so badly damaged that it remains unknown.

The new flexible foil dryer vent may have been combined with an existing aluminum bath fan vent. The bath fan was properly wired and vented by a master electrician 12 years ago.

Can anyone help me determine why it took 3 to 4 days for this wiring to start a fire? And why it appears that the fire may have started at the circuit breaker panel, not at the dryer?
The fire had a tragic affect on my family. I am a master electrician. Thanks, TAH

 

[K8MHZ]

Were these voltage readings taken after the fire?

 

[augie47]

If I drew it out correctly from your description, the 10/2 white, connected to the breaker, eventually made it to the dryer frame.
If so the only reason the frame did not shocked is you was your not being grounded and/or the enamel coating on the frame.
Purely a guess but if the vent hose from the dryer and fan were connected the hose may have been carrying enough current to cause a fire.

 

[TAH]

I recreated the circuit to get those readings.

 

[TAH]

If I drew it out correctly from your description, the 10/2 white, connected to the breaker, eventually made it to the dryer frame.
If so the only reason the frame did not shocked is you was your not being grounded and/or the enamel coating on the frame.
Purely a guess but if the vent hose from the dryer and fan were connected the hose may have been carrying enough current to cause a fire.

That’s what I thought too, but why 3 to 4 days after he wired the dryer?

 

[Little Bill]

If they went white-white in the receptacle, 120V was sent to the dryer frame when plugged in.

 

[hbiss]

At the circuit breaker panel, the black and white conductors were wired to the 2-30 breaker and the bare was wired to the ground bar.

In the 3 wire with ground dryer receptacle they wired black to black, white to white, and green to green. The fourth terminal, usually for the red conductor, was not connected to anything.

So it was wired to supply 240 volts between the neutral and one hot leg of the dryer that would normally expect to see 120V. Nothing on the other hot leg. It's possible that the dryer heating element that is wired between the black and red for 240V operation is center tapped to the neutral which is why there appeared to be some heating.

So your answer is that 240 volts was supplied to components that expected 120 volts with the expected consequences.

I believe that anything happening in the panel was a consequence of the fire, not the dryer.

-Hal

 

[TAH]

That’s what I thought too, but why 3 to 4 days after he wired the dryer?

Also, my thought is that

If they went white-white in the receptacle, 120V was sent to the dryer frame when plugged in.

That’s correct. I still can’t figure out why it took 3 to 4 days for the fire to start. And I can’t figure out why the fire pattern appears that the fire started at the circuit breaker panel

 

[hbiss]

If they went white-white in the receptacle, 120V was sent to the dryer frame when plugged in.

That would assume the bonding jumper between neutral and ground. I'm assuming that because this was a 4 wire receptacle it was properly removed. If not, it would have created the secondary possibility of the dryer or dryer vent faulting to ground and arcing.

And I can’t figure out why the fire pattern appears that the fire started at the circuit breaker panel

Who said it started at the panel?

Edit: Ok. assume the bond at N/GND bond was intact. That would put 120V on the green ground back to the panel at 30A. A bolted short. Why didn't it trip the breaker? Umm probably because the green wasn't properly connected or tightened down on the ground bar causing it to overheat. That would cause a fire at the panel.

Only problem with that theory is that there would be current draw when the dryer was off.

I think there is a lot here that went up in smoke and not known.

-Hal

 

[wwhitney]

Given everything else that was done wrong, perhaps one of the terminals in the panel was not properly tightened, and the fire was due to simple resistive heating from a bad connection?

Cheers, Wayne

 

[Little Bill]

That would assume the bonding jumper between neutral and ground. I'm assuming that because this was a 4 wire receptacle it was properly removed. If not, it would have created the secondary possibility of the dryer or dryer vent faulting to ground and arcing.




-Hal

How can you assume the bonding strap was removed when NONE of the wiring was correct?

 

[TAH]

So it was wired to supply 240 volts between the neutral and one hot leg of the dryer that would normally expect to see 120V. Nothing on the other hot leg. It's possible that the dryer heating element that is wired between the black and red for 240V operation is center tapped to the neutral which is why there appeared to be some heating.

So your answer is that 240 volts was supplied to components that expected 120 volts with the expected consequences.

I believe that anything happening in the panel was a consequence of the fire, not the dryer.

-Hal

I think in the dryer the heater was 240 V and the motor was 120 V. So the heater was getting to 40 V and the motor was looking for the completion of the 120 V circuit. Possibly got that through the vent

 

[TAH]

How can you assume the bonding strap was removed when NONE of the wiring was correct?

The bonding strap was not removed

 

[TAH]

I am the original poster. What do you all think of this theory? The 120 VAC dryer motor circuit needs to be completed. It finds a path to ground through the dryer vent, which is connected to the bath fan vent. The bath fan circuit was 14 gauge. So now he’s overwhelmed the ampacity rating on the ground wire of the 14 gauge circuit?

 

[TAH]

Though in that scenario, I think the circuit breaker trips.

 

[TAH]

Here are pictures of the circuit I recreated based on the evidence I could see after the fire.

 

[James L]

Edit: Ok. assume the bond at N/GND bond was intact. That would put 120V on the green ground back to the panel at 30A. A bolted short. Why didn't it trip the breaker? Umm probably because the green wasn't properly connected or tightened down on the ground bar causing it to overheat. That would cause a fire at the panel.

I think there is a lot here that went up in smoke and not known.

-Hal

This right here.

If the bonding strap was intact, why didn't the breaker trip? That's problem #1 imho

 

[Hv&Lv]

At the circuit breaker panel, the black and white conductors were wired to the 2-30 breaker and the bare was wired to the ground bar.

In the 3 wire with ground dryer receptacle they wired black to black, white to white, and green to green. The fourth terminal, usually for the red conductor, was not connected to anything.

Im going to assume you meant green to bare wire?

At the dryer terminals the 3 wire with ground dryer cord was wired black to black, white to white, green to the terminal that normally receives the red conductor, and red to nothing. The bonding jumper from the white terminal to the dryer casing was left in place.

OK, so white with 120V is now landed on the dryer neutral circuit going to the control panel..

Voltage at the dryer shows 240VAC black to white, 120VAC black to green, 120VAC white to green, 240VAC black to dryer casing, 0VAC white to dryer casing, 120VAC green to dryer casing.

240 black to white, check
120 black to green, check since it’s tied to the ground bar
120 white to green, same as black..
240 black to dryer casing, casing is at 120V since the bond is in place.
0 white to casing, check.. since your checking the same potential.
120 green to dryer casing, check. Since your checking ground bar to energized casing.

Whether the dryer is running or not, you do not get shocked when you touch the dryer casing, but you do get shocked when you touch the dryer vent.

is this while leaning over the dryer with a hand on the dryer?

When the dryer is running, an ammeter shows 11 amps on the black, 5.3 amps on the white, and 10 amps on the green. An ammeter also shows 10 amps on the black and white inside the same CT, 11 amps on the white and green inside the same CT, and 5.2 amps on the black and green inside the same CT.

Your adding vectors here with two wires in the same CT.

When the dryer is not running an ammeter shows 0 on all conductors.

The dryer motor and heater worked for 3-4 days before it started a fire. It is not known if the dryer was in use when the fire started.

The fire pattern seems to show that the fire started at the circuit breaker panel, not at or near the dryer, but the building was so badly damaged that it remains unknown.

How close was the dryer to the panel?

The new flexible foil dryer vent may have been combined with an existing aluminum bath fan vent. The bath fan was properly wired and vented by a master electrician 12 years ago.

Can anyone help me determine why it took 3 to 4 days for this wiring to start a fire? And why it appears that the fire may have started at the circuit breaker panel, not at the dryer?
The fire had a tragic affect on my family. I am a master electrician. Thanks, TAH

But your not sure yet if it started at the panel or the dryer yet since the building was so badly damaged..

just trying to make sure I have all the facts straight first.

 

[hbiss]

The 120V motor circuit had 240 on it only when the dryer was turned on. How often was it used during those 3-4 days?

OK, so white with 120V is now landed on the dryer neutral circuit going to the control panel..

The white was landed on the breaker along with the black. So it's 240V between one phase and neutral on the dryer terminal block.

How can you assume the bonding strap was removed when NONE of the wiring was correct?

There would have been a BIG arc when someone plugged in the dryer plug- unless the breaker was off.

The 120 VAC dryer motor [and controls] circuit needs to be completed.

It was. With 240V on it.


-Hal

 

[TAH]

This right here.

If the bonding strap was intact, why didn't the breaker trip? That's problem #1 imho

The ground was not intact back to the panel. At the dryer, he put the green wire where the red wire was supposed to go.
I took that green wire off the red terminal, and the motor won’t spin at all.