Do bi-metal strips fatigue?
- Thread starter Cletis
- Start date
- Status
Depends- What are your parameters?
From what I recall, very generally, most breakers are capable of 1000s of mechanical (flipping switch/opening /reclosing) operations, but only a few fault current trips before replacement is recommended.
AFAIK, there is no set EOL and no reason to believe that your avg thermo/mag breaker installed today wont function reliably under normal conditions for at least a half century or so....
Last edited:
JFletcher
Senior Member
- Location
- Williamsburg, VA
Bimetallic strips can fatigue, tho it's rarely the cause of a breaker failure. A 50 year old breaker may have never tripped and only been operated a handful of times over its lifespan. Corrosion, heat, stiction (static friction), mechanical jams, etc. can all cause a breaker to fail.
Some breakers, like the old 2p FPE Stab-Lok, are prone to jamming once they trip. In cases like those, I'd replace a tripped one like a fuse (treat it as a one-time OCPD). Other breakers, unless there is visible damage to the case, handle, rivets, lugs or stabs/bolt on to the buss, it is opening well before its rating, will not stay on, or feels 'weak' when operating, I wouldnt replace a tripped breaker. Since they dont come with a counter, there is no way to know if it has tripped once or five hundred times.
GFCI and AFCI breakers ofc should be replaced if they will not trip with the button, or reset correctly.
In our other house, 5 of the 6 original thermostats for the baseboard heat installed in 1969 are still operating, and within a degree or two of the setpoint vs actual air temperature. Overloads in can lights are also bimetallic strips, and the contact to the bottom of the bulb is likely to fail before they do.
Some breakers, like the old 2p FPE Stab-Lok, are prone to jamming once they trip. In cases like those, I'd replace a tripped one like a fuse (treat it as a one-time OCPD). Other breakers, unless there is visible damage to the case, handle, rivets, lugs or stabs/bolt on to the buss, it is opening well before its rating, will not stay on, or feels 'weak' when operating, I wouldnt replace a tripped breaker. Since they dont come with a counter, there is no way to know if it has tripped once or five hundred times.
GFCI and AFCI breakers ofc should be replaced if they will not trip with the button, or reset correctly.
In our other house, 5 of the 6 original thermostats for the baseboard heat installed in 1969 are still operating, and within a degree or two of the setpoint vs actual air temperature. Overloads in can lights are also bimetallic strips, and the contact to the bottom of the bulb is likely to fail before they do.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I agree with most of what JFletcher said.
Bimetal can fatigue but is probably more likely some other compent will wear out, get debris lodged in some moving part and jam up, etc. before the bimetal element fails.
Bimetal can fatigue but is probably more likely some other compent will wear out, get debris lodged in some moving part and jam up, etc. before the bimetal element fails.
ptonsparky
Tom
- Occupation
- EC - retired
Recently had flies in a QO330. They crawled in for the warmth. 3R panel.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I've had that many times, blow compressed air in the bus jaw opening and they get forced out the arc relief chute, and breaker works again.
I also agree.
I haven't had many failures. Of those that I can recall, some were because of loose terminals causing arcing and subsequent overheating.
I believe most lower amp breakers are rated to be used as a switch. Thus are able to handle thousands of manual on/off operations.
Not so for higher amp breakers....they are not designed for lots of mechanical operations. Check with the breaker manufacturer, they are all different and will have specific replacement guidelines for high amp breakers that experience high on/off use.
This topic was brought up by standby generator users who were constantly throwing their 200amp main panel breaker to test their backup generators. Those breakers are not rated for that kind of switching action over a long period of time.
Not so for higher amp breakers....they are not designed for lots of mechanical operations. Check with the breaker manufacturer, they are all different and will have specific replacement guidelines for high amp breakers that experience high on/off use.
This topic was brought up by standby generator users who were constantly throwing their 200amp main panel breaker to test their backup generators. Those breakers are not rated for that kind of switching action over a long period of time.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Because the bi-metal elements themselves are based on the thermal coefficients of the two metals, unless the metals change into some other metal, they don't change their properties. Alchemists have failed for centuries to get one metal to transmute into another one, so it's a safe bet it isn't going to happen now.
In theory there is a small risk of the bi-metal strip permanently deforming, but that would require more current than what the breaker would allow, given how the bi-metal strips are used. In other words IF your main contacts all welded so the current flow through the breaker went unabated until a much higher upstream device opened, it's possible the thermal elements would be deformed, but by then that's the least of your problems.
What DOES happen however is that the bi-metal strips act against a spring loaded "trip bar" that is holding the main contacts closed against their springs. That trip bar has a little pawl (notch) that holds it in place, the thing that catches when you reset a breaker. That little pawl wears out and after a number of trips, requiring less and less force from the bi-metal strip to make it move and release the latch for the main contacts, which we experience as "nuisance tripping". So it's not really the bi-metal strips that change, it's what they act upon that wears out. And that trip bar is the same one that is activated by the mag trips too, so multiple trips from either sensor wears out that mechanism.
Using the breaker as a switch by the way does not involve that mechanism in most designs now, hence the "SWD" rating on breakers going away. It's not the test criteria that changed, it's the fact that virtually all breakers went to a "knee" movement mechanism that does not involve using the trip mechanism to open the main contacts manually. That was not always the case, but has been for decades. But occasionally I still see an old spec calling for breakers to have an "SWD" rating. It's safe to ignore that now because as far as I know, all breakers have that so most mfrs stopped showing in on the label.
In theory there is a small risk of the bi-metal strip permanently deforming, but that would require more current than what the breaker would allow, given how the bi-metal strips are used. In other words IF your main contacts all welded so the current flow through the breaker went unabated until a much higher upstream device opened, it's possible the thermal elements would be deformed, but by then that's the least of your problems.
What DOES happen however is that the bi-metal strips act against a spring loaded "trip bar" that is holding the main contacts closed against their springs. That trip bar has a little pawl (notch) that holds it in place, the thing that catches when you reset a breaker. That little pawl wears out and after a number of trips, requiring less and less force from the bi-metal strip to make it move and release the latch for the main contacts, which we experience as "nuisance tripping". So it's not really the bi-metal strips that change, it's what they act upon that wears out. And that trip bar is the same one that is activated by the mag trips too, so multiple trips from either sensor wears out that mechanism.
Using the breaker as a switch by the way does not involve that mechanism in most designs now, hence the "SWD" rating on breakers going away. It's not the test criteria that changed, it's the fact that virtually all breakers went to a "knee" movement mechanism that does not involve using the trip mechanism to open the main contacts manually. That was not always the case, but has been for decades. But occasionally I still see an old spec calling for breakers to have an "SWD" rating. It's safe to ignore that now because as far as I know, all breakers have that so most mfrs stopped showing in on the label.
Last edited:
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
161204-2309 EST
The information I am providing is not important to your question because the components are only designed for a very limited number of cycles. However, the the following information might be useful to you in some other application, such as torque or load cells.
See https://en.wikipedia.org/wiki/Fatigue_limit
If you make a loadcell from steel and design for a microstrain level below some limit, then you can expect infinite fatigue life. If the material is an aluminum alloy, then below a 1000 microstrain for full scale will probably provide at least a 10,000,000 cycle life.
One old time way to test fatigue life of automobile wheel studs was to put a car in its maximum tight turn position and drive it in the circle until a wheel falls off.
Also never lubricate wheel studs and tighten to the torque spec. This alone may pop the stud with no further loading.
If you have a differential that uses a collapsible spacer, tighten the nut to get correct drag torque, and the nut pops off sometime later (this might not occur for several days or longer), then the pinion probably had improper heat treatment. The collapsible spacer defines the maximum tension in the pinion shank. This is usually in the 25,000 to 35,000# range.
.
The information I am providing is not important to your question because the components are only designed for a very limited number of cycles. However, the the following information might be useful to you in some other application, such as torque or load cells.
See https://en.wikipedia.org/wiki/Fatigue_limit
If you make a loadcell from steel and design for a microstrain level below some limit, then you can expect infinite fatigue life. If the material is an aluminum alloy, then below a 1000 microstrain for full scale will probably provide at least a 10,000,000 cycle life.
One old time way to test fatigue life of automobile wheel studs was to put a car in its maximum tight turn position and drive it in the circle until a wheel falls off.
Also never lubricate wheel studs and tighten to the torque spec. This alone may pop the stud with no further loading.
If you have a differential that uses a collapsible spacer, tighten the nut to get correct drag torque, and the nut pops off sometime later (this might not occur for several days or longer), then the pinion probably had improper heat treatment. The collapsible spacer defines the maximum tension in the pinion shank. This is usually in the 25,000 to 35,000# range.
.
FionaZuppa
Senior Member
- Location
- AZ
- Occupation
- Part Time Electrician (semi retired, old) - EE retired.
the question as posed is different than post title.
day-1 the breaker has measurable trip curve.
after years of heating and cooling but not tripping, fatigue will change that day-1 curve.
day-1 the breaker has measurable trip curve.
after years of heating and cooling but not tripping, fatigue will change that day-1 curve.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
But if applied within some prescribed conditions should be able to maintain a certain performance level as well.
Ten year old breaker that has not been abused, probably still within reasonable performance parameters. Twenty year old breaker may be starting to get more questionable.
FionaZuppa
Senior Member
- Location
- AZ
- Occupation
- Part Time Electrician (semi retired, old) - EE retired.
agreed, but the Q as posed was "does it fatigue", the A is yes.
will it fatigue to failure or fatigue to an unacceptable level, probably no.
David Goodman
Senior Member
- Location
- Pahrump, NV, USA
The place that I have seen bi-metal strips fatigue was in thermostats on electric base board heaters that were 30 plus years old. College students would want their rooms warmer. So, they would turn the t-stat dial all the way up (essentially to a constant on position). The bi-metal switch would over-heat and become weak. When the occupant turned the dial down, the bi-metal switch would not open.
We would turn the power off. Replace the t-stat with with another old one from the PO's salvage supply, and tell the tenant not to turn the dial past 3/4's. No problems for the rest of the season with the same tenant. The salvage t-stats had all been rotated through someone else's over heated unit. They ran fine as long as they weren't turned all the way up.
This is unrelated to breakers, but specifically about bi-metal strips.
- Status