Anybody had a breaker get “weak” as in tripping well below its rating? This happened to me. I put clothes in the dryer one night and went to bed. When I got up the next morning, the clothes were still wet. The circuit breaker feeding the dryer had tripped during the night. I reset it and turned the dryer back on. It ran all the way through the cycle with no problems. The way the house is wired is there is a small subpanel in the laundry room which is fed from the main panel with a 40 amp 2-pole breaker. There is a 30 amp breaker in the subpanel for the dryer and a few single pole breakers for the lights and washing machine. There was another 30 amp 2-pole in the subpanel for the water heater which is no longer used (we changed the water heater to gas). The breaker that tripped was the 40 amp feeder breaker in the main panel. The dryer draws 26 amps at 120/240 volts. There are no faults in the dryer I’m aware of. A 26 amp load should not trip a 40 amp breaker.

I once had an issue where the 50a breaker would randomly trip when multiple burners and/or the oven were on, eventually it got to the point where even one burner on started tripping it. Couldn't see anything obviously wrong with the stove, so I went down to watch the meter (see if anything odd happened like fast spinning that would indicate a short) & had my mother turn on the stove... I also opened the breakerbox.

With the stove on, I could hear a slight buzzing from that particular breaker (& sure enough after a couple minutes it tripped), so I suspected something was wrong with it. I took off the inner panel on the breakerbox just to look around, and felt the wires for the stove..one of them was hot. My suspicions then turned right to the connection on that wire, I went to the shed & got a screwdriver to sheck if the breaker's terminal screw that holds the wire was actually tight. And sure enough it wasn't. tightening it solved the problem - a very easy fix. (while there with the box open, I went through one at a time checking every breaker)
 

So first thing check to make sure the screws that hold the wires on your breakers are still properly tightened down!

No arcing damage from the loose connections? I did feel the breaker once after the dryer had been running for a while and it was warm to the touch. Maybe it’s time to open up the main panel and check for loose connections.

Is this just a simple overcurrent protection breaker, or combined with GFCI or AFCI ?



Simple overcurrent protection breakers never trip for no reason. The reason may be something that actually happened in the circuit, or internal fault of the breaker. Some examples for things that might happen once and leave no detectable trace :

 - One-off short circuit in the appliance, for example dirt that accumulated somewhere on terminals or bare heating element, arced over, and burnt away as result of the arc so now everything seems ok again

 - Arc over in a switch contact, for example a phase to phase arc in a switch contact that changes the motor direction

 - Surge protection device such as an MOV (in an electronic control module) reacting to a power line transient

 - Normal load of the heating element, that was drawing higher current at night because the line voltage was a bit higher, since big loads everywhere are switched off (would be possible if it is marginal for the breaker rating anyway)

 - Bad contact in the breaker, heating it's bimetal strip directly. For example, breaker was switched on while internally contaminated with dust, that caused poor connection between the contacts. After tripping the contact surface cleared by arcing, the dust burnt off etc, so maybe now the contact is better than it was that time

 - If the breaker's calibration is off, it would trip to the same lower load every time.... (except if the value is marginal)

 - If the breaker is ok but is marginally overloaded, it may sometimes trip and sometimes not, as result of : temperature around the breaker (were the nearby other breakers in the panel loaded at the time ?), slightly higher or lower currents every time (for example, as result of night/day line voltage changes), shorter or longer run time (depending on what exactly you loaded into the dryer and how wet it was), etc



GFCIs can trip for leaks from Phase to Earth

 - Dirt on bare conductors somewhere getting moist and passing through leakage current. As result of the current (or of heat, if it is on the heating element) it will dry up and stop conducting until next time, or if it escalated into a breakdown with an arc, the dirt might have burnt off from the arc and is no longer there

 - A short between Neutral and Earth in the appliance, will cause some load current sharing between the 2 conductors when load is present, and the sharing depends ultimately on the resistance in the point of the short (especially so, since there is no high voltage between N and E, so it is unlikely for that spot to arc and become a "good" short, so if the contact is weak it will remain high resistance). Such contact resistance can change anytime and is not predictable. The higher the load is, the higher the chance that the "leakage" current is sufficient to trip the GFCI

 - Many GFCIs use a mechanism in which a permanent magnet holds the breaker "up", and an electromagnet, feeding from the secondary of the current transformer, cancels it out when there is leakage detected (in one half cycle of the AC). Over time and number of trips, the permanent magnet can wear out, making the GFCI react to lower leakage currets than rated. Since leakages in general are sometimes intermittent, there is no rule when exactly such GFCI will falsely trip



AFCIs trip for what they consider (using digital signal processing) as a current waveform of "something bad happening". Some of those "something bad happening" are not particularly well defined or put into code (i mean, into the code that runs in their processors, not into NEC), so as a rule, AFCI breakers can be expected to falsely trip for a variety of things they see in a current waveform, or fail to trip for actual arcing, for no easily traceable reason. It is just that they work good enough most of the time, that such events are uncommon

Nothing special. Just a standard 2-pole 40 amp breaker. I have noticed it does get warm when the dryer is running.

No arcing damage from the loose connections? I did feel the breaker once after the dryer had been running for a while and it was warm to the touch. Maybe it’s time to open up the main panel and check for loose connections.

Yep, no real damage I think there mighta been a little bit of that blackening stuff - like what  builds up on contacts that was easily cleaned off...but I don't remember for sure (this was a number of years ago)

Slightly warm is normal, but if its fairly warm that may be signs of an issue...though you really need to feel the wire itself (near the breaker). Obviously goes without saying, gotta say it anyway...caution is required when inside the breakerbox with all those exposed live connections!

Oh and those breakers of yours look exactly like the ones I have.

Slow oxidation of the terminals over the years is not generally a problem, since the contact spot between the wire and terminal is sealed and not affected. However, it will be a problem once the terminal is released, as it is not possible to re-insert a new wire to be exactly on the same spot that was protected by the old wire

But when things are "blackening" - even without melting the plastic, it means they get hot - The contact is not good as is, and it will only degrade with more cycles. Also, even if the plastic (or bakelite in your pictures ?) does not actually deform, it becomes brittle if it gets exposed to high enough heat repeatedly or continuously

Also, i can't imagine 40A double pole breaker (available power = 40A * 240V) being loaded anywhere near full capacity by a dryer. Our dryers over here (230V) plug into ordinary 16A receptacles on 16A breakers. 40A (1 Phase supply) is the rating of a main breaker in most houses built in the 90s

The breaker tripped again. I was there to witness it. I heard the dryer shut off before the cycle had finished. So I turned on the light/ceiling fan in the laundry room, nothing. So I went outside to the main panel, where the feeder breaker was located. It was tripped. I felt it. Both poles were hot. Not burning, but very warm. I opened up the main panel and felt the wires coming from the breaker. They were hot close to the breaker. About 1 1/2 inches from where the wires connect to the breaker, they were cool. The screws on the breaker were loose, not enough to pull the wires out, but loose enough to make a high resistance connection. The high resistance connection was heating up the bimetal strip in the breaker and causing it to trip, even though the load (26 amps) was well below the breaker’s rating (40 amps). I pulled the breaker out and saw no evidence of arcing between it and the busbars. There was also no evidence of arcing where the wires attach to the breaker. I tightened up the screws, reset the breaker, and turned the dryer back on. Hopefully this fixes the problem.

I had problem with fuse for air conditioner.When I turn on air conditioner into cooling or heating mode it tripepd after 10 minutes.I replaced the fuse and everything works like a charm.Voltage in my country is 230 Volts for single phase outlets and in 3 pbase outlets voltage between phase one and phase two is 400 Volts.

The breaker tripped again. I was there to witness it. I heard the dryer shut off before the cycle had finished. So I turned on the light/ceiling fan in the laundry room, nothing. So I went outside to the main panel, where the feeder breaker was located. It was tripped. I felt it. Both poles were hot. Not burning, but very warm. I opened up the main panel and felt the wires coming from the breaker. They were hot close to the breaker. About 1 1/2 inches from where the wires connect to the breaker, they were cool. The screws on the breaker were loose, not enough to pull the wires out, but loose enough to make a high resistance connection. The high resistance connection was heating up the bimetal strip in the breaker and causing it to trip, even though the load (26 amps) was well below the breaker’s rating (40 amps). I pulled the breaker out and saw no evidence of arcing between it and the busbars. There was also no evidence of arcing where the wires attach to the breaker. I tightened up the screws, reset the breaker, and turned the dryer back on. Hopefully this fixes the problem.

Similiar thing happend to my grandfather few months ago but problem was bad outlet which blew the fuse(my grandfather is using old style fuses not circuit breakers).When outlet was replaced it didn't happen again.

@Bamaslamma1003:
Sounds exactly like what I had happen.
My guess is the screws work loose over years & years of the little bit of heat created by power flowing...

When looking to the construction of multiple types of circuit breakers, it looks like the bimetal trigger is intentionally arranged so it gets heat from the output side terminal (the one where is expected connection of just a wire and not the bus bar).
So when the wire gets loose, the breaker trips sooner and so to some extend prevents from really severe overheating there.

I think so, because all breakers I have seen open had the thermal trigger thermally coupled to the terminal (riveted or welded to the metal piece forming the wire terminal), even when there were three incompatible styles of the breakers (few of the DIN strip mounted style now popular in Europe, the US style and the old black OEZ ones used here before the DIN stripe style became popular here).

That indeed seems to be the case, as long as the busbar is inserted from the bottom terminal. But DIN breakers are designed to accept it from either side

That indeed seems to be the case, as long as the busbar is inserted from the bottom terminal. But DIN breakers are designed to accept it from either side

As far as I remember, the bimetal was at the bottom (so expects the less reliable connection there),
E.g. see the bimetal on the left side, which is normally the lower one (assuming the lever is up when on).
But I have quite troubles to believe the busbar to be more reliable in connections than the wires, I would expect them to be the same. So then the effect is not that big, when it does not respond to the other terminal as well.
So I dont know, if it is really an intentional feature, or rather a coincidence of a convenient place for the bimetal.
Definitely even when the real benefit could be marginal, it is clearly for free...

I recalled the bimetal being on the top in ABB

Now checked - it is on the top in ABB, on the bottom in Moeller (Eaton), Merlin Gerin (Schneider, Chinese clones like the one in your link)