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  1. #27
    Join Date
    Nov 2004
    Location
    Anchorage Alaska / Seattle WA
    Posts
    205
    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.

  2. #28
    Join Date
    Nov 2011
    Location
    St. Louis
    Posts
    5,247
    Quote Originally Posted by genman View Post
    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...

  3. #29
    Join Date
    Nov 2004
    Location
    Anchorage Alaska / Seattle WA
    Posts
    205
    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...

  4. #30
    Join Date
    Nov 2011
    Location
    St. Louis
    Posts
    5,247
    Thanks Genman...I've tucked away your response for the next time I forget.

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