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Thanks Genman...I've tucked away your response for the next time I forget.

Yes they do get weaker, especially breakers that are used in residential panels. If you ever want to see an interesting video just look at a slow motion video of a breaker tripping... its a pretty eventful video. There are two elements involved in a circuit breaker. The thermal element and the magnetic element. Their titles are pretty much self explanatory. The thermal element trips the breaker with heat. The current actually runs through an element that bends with heat. The bending, straightening out depending on the design will cause the trip pawl to release the breaker. The thermal element will trip the breaker on overload. It is an inverse proportional device... the more the overload the faster it will trip. The more times it is used the easier it will work. This reliance of this feature is brand specific. So that is why you see some brands of breakers that will not trip and some that are "weak" as i called it. The next tripping element in the breaker is the magnetic element. In the breaker the current carrying wire is actually wrapped around a magnetic plunger. In the event that enough current is passed through the breaker the coil will magnetize the plunger, just like a solenoid, and activate the trip mechanism - The amount of instantaneous current required to trip the magnetic trip is about 10 times the rating of the breaker. Breakers used in residential panels are rated for 80% of their rating continuously. So, as you can see, if the load on a circuit is pretty high, and has been for a while, it wont take much to put it over the edge. I have seen lots of times circuits designed with loads that exceed the 80% and sure as the world in about 3 or 4 hours the circuit will trip. Next time you come across an old residential breaker break it open and see. When you get up into the commercial equipment you can get breakers that are rated for 100% of the nameplate - also you can get some fancy breakers where you can adjust all the trip parameters that you might need. In commercial plants you have to have what is referred to coordination in your system of clearing faults. Otherwise you see exactly what happened in this example. The fault or overload jumps over an intermediate breaker or two.... I purposely created a fault in a 277 volt lighting circuit one time so I wouldn't have to go trace down the breaker. It was a new school - the fault jumped over the 20 amp circuit I wanted to trip... jumped over the 200 amp main in that panel - jumped over a 600 amp breaker in a distribution panel and took out a 2000 amp main for the entire building. Nobody was happy.... however that is how instantaneous fault current works. It all depends on the available fault current in the system and available from the utility... which is another story...

Originally Posted by genman

odds are it had tripped 2 dozen times already and was weak... .

Do all breakers in fact get weaker after repeated trips? What's the mechanics/science behind this? What, specifically, wears out?

Are there different types of mechanisms that force a trip...current v. heat. It occurs to me I might need a refresher course in breakers. I know TB posted pretty thoroughly on it a couple years ago before there was an electrical section here, but I forget what subject brought that on and there's no way I'm searching through every circuit breaker reference in the subject body of 3k posts...

Well you more than likely hit the nail on the head with your last comment. As I am sure you know household circuit breakers are only rated at 80% of their nameplate on a continuous basis. Continuous is figured at 3 hours so if she was running the furnace, cooking with the oven and whatever else might have been happening the feeder breaker was probably already getting the thermal on the breaker all nice and warm. As you know when a solenoid operates it has a pretty good initial spike in order to initially energize the magnet and move the valve. The inrush caused by the solenoid will look like a fault to the circuit for a couple of cycles - that fault will be passed through the transformer in the proper mathematical relation. If we take Dan's number for a bolted fault at 2.6 amps it is safe to surmise that the initial inrush was even more than that. So you take an over loaded feeder breaker that is on the verge of tripping anyway and odds are it had tripped 2 dozen times already and was weak... and your little inrush was enough to push it over the edge. And I bet the electrician will be back in 6 months replacing that breaker because of the basic overload on that feeder. I am sure you are familiar with inrush associated with motors... Solenoids, transformers and any other devices that depend on magnetism to operate is subject to inrush currents up to about 10 times their full load amps. FYI - the highest inrush is found in incandescent light bulbs... about 12 times the wattage.

The other thing to look at too is that on most of our equipment, the wires supplying the transformer are too small to carry enough current to trip the breaker.

wow is that some incredible bad luck you had nothing to do with the breaker tripping.

The primary winding of a standard 24v transformer is a MUCH finer wire than the secondary. The primary wire is the one that almost always goes bad in the event of a transformer overload. The wire is a small enough that a 20A breaker ain't gonna protect it, it will smoke before the 20A breaker trips. Some transformers (especially newer ones) have internal fuses on the primary that are designed to blow before a total transformer meltdown.

Originally Posted by timebuilder

Most panels have more circuit breaker capacity than the main breaker can handle. This is mainly due to the use of demand factors, which is a perfectly legal design criteria, based on the idea that most devices and circuits will never be asked to perform at their full capacity.

This is true and and reference specifically in the NEC. I ran across that recently while studying to take the Tennessee limited electrical license test.

Originally Posted by timebuilder

Most panels have more circuit breaker capacity than the main breaker can handle. This is mainly due to the use of demand factors, which is a perfectly legal design criteria, based on the idea that most devices and circuits will never be asked to perform at their full capacity.

Makes sense.

Most panels have more circuit breaker capacity than the main breaker can handle. This is mainly due to the use of demand factors, which is a perfectly legal design criteria, based on the idea that most devices and circuits will never be asked to perform at their full capacity.

Yeah...ran into the boss days later and he said I just got caught at the wrong place, wrong time. Still bothersome to me that 120+ worth of breakers are being fed by a 100. Oven and resistive heater being the chief consumers...but that's the way it's been for years.

I never tested which side burnt up, but I've never had a transformer trip the primary that I am aware of. Granted most transformers I deal with are 24V and 100VA or less.
100VA at 24V = 100VA/24V = 4.167 amps continuous amps available on the secondary, thus many times these are 5A circuit breakers on the transformers with circuit breakers.

The conductors in the windings are so small that they will eventually be your fuse if you don't have one, assuming your smallest CB in the sub-panel was 15A.

I think it was coincidence, but something else likely happened at the same time, the homeowner plugged something they shouldn't have in while you were messing with their humidifier or a bird/string/branch/gopher/mouse crossed the wires in the main.

In residential I would think this less likely, but it can be a real issue in commercial (Think Superbowl) a Ground Fault Circuit Interrupter (GFCI) is designed to be very touchy - even moisture can cause enough leakage current (translates into load imbalance) to get it to trip.

Codes might be requiring GFCI even on an equipment circuit, if it goes outside, I dunno.

A GFCI will save your life though - I checked again this summer!

Jim A.

I can't think of any bad transformer that I have found where the secondary side failed. It has always been the primary even though the short was on the secondary side. I have never encountered one where the short caused enough load on the primary side to cause a line voltage breaker to trip. The failure has always been in the windings of the primary. Has anyone else had a different type of failure?

My advice:

Don't let the smoke out

From your description, someone needs to go there and have a look at their electrical distribution....

My guess it was doing it long before you got there.

The motivation for this thread was a simple humidifier call last week. I read power across the humidistat even in test mode. While there I wanted to make sure that was the only issue and went to verify water flow. The xfmr was one of those little guys that sits right on the humidifier power supply outlet box with the secondary taps exposed. So I broke loose the water line on the humidifier side and ran leads right to the solenoid from the xmfr, got my water flow when the solenoid opened and buttoned that piece of it back up. Then I went to verify my initial findings again and...nothing. Not across the humidistat, xmfr 2ndary, primary, disconnect, breaker nor lugs on the sub panel.

I'm now hopelessly stuck in a classic "but it was working when you got here" mess. And it was. Somehow the breaker feeding the subpanel tripped. Not the breaker on the subpanel for the humidifier circuit I was concerned with, but the breaker feeding the subpanel. I just could not understand how jumping a low voltage solenoid could cause an event way back at the feeder panel. The examples above were extreme - at no time did I cause a short...just opened and closed a solenoid as the circuit ultimately does anyway.

Homeowner was bent out of shape and kind of freaky anyway under the best of circumstances. Fretting away loudly about her plants in the green room (not even related to this part of the house and panel). Probably had a half dozen cats locked up in the back rooms somewhere. But she is the good doctor's wife and had the owner of my company on the phone within minutes. Ultimately I was sent away, puzzled and shamed and wondering what the hell could have happened. But before I left I did note that they had a hundred amp breaker in the feeder panel supplying a sub panel with a 60 amp for the electric furnace, a 60 for the oven, a 15 for the humidifier and maybe one more fifteen for something else. The tech dispatched as a second set of eyes quickly called an electrician in (feeder breaker kept tripping.

Just wondering if my little 24v, miniscule amp circuit could have been the proverbial straw and why didn't it act up before, when the humidifier was operational? Or was it coincidence? All I know is I was the last one to touch it

Originally Posted by hurtinhvac

Awesome! On demand documented lab results!

So how about a 660/220 residential street transformer? If I shorted out the incoming lugs in my panel for a max of 5 seconds or so...

j/k I have a good idea of what woud happen...

The primary on your residential street transformer is most likely around 7500 volts. I hope you like yours extra crispy

It's made by Elk Products, I use them for powering up small ddc panels, you wouldn't normally find this one on a refrigeraton system (especially since it's a 120V primary).