Burned Wire at Breaker

[SAS]

I discovered a burned wire at a 30 amp, 220 volt breaker for my electric clothes dryer. Here is the sequence of events so far:

• Dryer (which has been in place with the same wiring for at least the 4 years we've been in the house) stops mid-cycle with a "PF" (power failure) code.
• I check that there is power at the outlet, good connections at the outlet and machine, and restart the dryer. Nothing else is affected, so it doesn't seem as if there was any significant current disruption to the house, however brief.
• 15 minutes later the dryer stops. This time the breaker has tripped.
• I reset the breaker and the dryer continues normally for the remainder of the cycle.
• The next time we use it, the dryer works normally.
• The time after that, the dryer stops mid-cycle with a tripped breaker, but no error code.
• I reset the breaker and it finishes the cycle.
• I buy a new breaker, and when I remove the panel cover I see that one of the wires has burned, i.e. the insulation for about 2.5 inches from the connection has melted. The connection to the breaker is solid.
• I cut off the section with the melted insulation and install the new breaker. I test the dryer for about 15 minutes. It runs normally.
• I test the current draw in both feeds. One is 22.5, the other is 24. Neither wires is warm to the touch.

I would say that I was done and the problem is solved, except that I can't figure out what would cause the wire to burn in the first place. If the dryer were pulling too much current, the breaker should have tripped before the wire burned. The only scenario I can come up with is that the dryer has an intermittent problem that causes it to pull too much current and the breaker failed to trip due to a breaker malfunction. That seems unlikely. Any other ideas?

 

[Kreemoweet]

Seems like a bad (i.e., high resistance) connection at the breaker. Just because it felt "solid" does not mean
there was no problem there: corrosion can cause "solid" connections to overheat. Also, the loose
connection could have been internal to the breaker, and not necessarily at the wire clamp.

Another possibility is a partial/intermittant short in the circuit wiring, receptacle outlet, or power cord itself.

 

[Sponsor]

 

[SAS]

It's interesting that you mention "corrosion". Initially I though that the culprit was a very dumb human error. It's a red wire and it appeared as if whoever installed the breaker forgot to strip the insulation from the wire. Upon closer inspection, however, it was clear that the insulation was removed but the wire was red in color. I do wonder, however, whether the change in color was the cause of the burning or the result. The problem I have with the other possible causes is that I would expect the problem to have manifested itself elsewhere, rather than on the wire just before the connection point.

 

[Reach4]

copper can get a reddish discoloration by being heated. So I think result. Maybe the heat at a loose connection at the breaker helped trip the breaker.

 

[Jadnashua]

A breaker can fail, creating a high resistance contact internally. That would cause the current to go higher until the breaker trips. Most breaker designs can handle an overload for awhile before tripping unless it is closer to a full short (IOW, they have a delay on a high valve, but will trip fast on a short). ANother thing that may be part of it is the actual bus bar or the breaker's connection to it...if the bus bar is corroded, or the lead to it is loose, or the spring contacts of the breaker are not making good contact with the bus bar, any one of those can create resistance and heating.

While probably fine, what gauge wire is being used on that circuit? An undersized wire would get hotter.

 

[WorthFlorida]

Since you not have had the problem again after replacing the breaker there is a little chance that the problem was caused elsewhere.

It's not unusual that a wire burned but you must look at this way, the breaker performed its function and saved your house from burning down. Breakers do trip because of heat. The load might not have been large enough to trip the breaker but because of a bad connection (many reasons why) the wire heated up and transferred the heat to the breaker. I've seen where a breakers' heat with a normal load on it was tripping the breaker above it because of a design defect, not because of an overload or bad connection. In this case the entire panel was replaced.

I had this happened at a church I maintained. A three phase 60 amp breaker tripped. One of the 6 gauge wires was loose and burned back in inch or two. There was enough slack to cut out the burned wire to clean copper and made a tight connection on the breaker. My electrician looked it over and said OK. A few months later the same problem on the same wire. The breaker was replaced and again there was enough slack to cut back to clean copper. It has been working after four years. Either the original wire connection was never tight enough and caused the connection overheat and the screw to tighten the wire lost its tensile strength, or the breaker was bad when the building was built three years earlier.