Circuit Breaker Dissection

[jakeru]

Anyone ever wonder how a circuit breaker works? I dissected some and thought it was interesting to see how they work, and thought I'd share the pics and what I learned.

These ones that I dissected are an older style that is called "Zinsco" brand. First, the view of the breaker from the outside with it put together:

(I drilled out those rivets, by the way, to take it apart.)

Now, the inside views...
Switched on:


Switched off:


"Tripped" off (from too much current):


Here is some detail of the electrical contacts, which "make or break" the electrical circuit. The contacts themselves are little "pucks" of some alloy of silver (and possibly including tungsten.) Those silver alloy pucks are soldered (using a solder that has cadmium in it, I learned), to moving and stationary copper parts.

The motion of the moving parts of the circuit breaker, when it trips, is apparently designed to pull air into the circuit breaker, which gets directed through the nearby plastic "air baffles" and assists with "blowing out" any arc that is still happening between the contacts.



You can see views of breakers that have different amperages here (notice the color of the handle is different and indicates the breaker's amperage rating)


The only functional part that makes a breaker trip at a particular amperage (15 amp, 20 amp, 30 amp, etc.) is a bimetallic strip shown as the bent "L" shaped part here (sorry, it's out of focus, but it is welded to the multi-stranded braided copper wire, and also to the copper piece on the right side:

There were numbers stamped into the bimetallic that were different with the different circuit breaker amperage ratings. It has a nickel alloy that doesn't expand/contract with heat as much as the other side of the strip (which is a more conventional alloy), so the strip bends when it gets hot and trips the spring-loaded mechanism that opens the circuit.

It's a pretty complicated mechanism with multiple springs and moving parts. What's remarkable is how it manages to reset itself when the handle of the breaker is pressed back into the "on" position.

I believe there also may be some mechanism for a sudden current to trip the breaker with magnetic force, and I think that may be what the U-shaped piece of metal in the bottom of the above picture is for, but honestly I haven't got that completely figured out yet. But supposedly, these breakers can trip either from excessive sudden current (with likely some kind of electromagnet involved), or from excessive prolonged mild current (which would be from the heating up and the bending of the bimetallic strip.)

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By the way, these Zinsco breakers have a bad rap (in the home inspection industry at least) for some of them using aluminum in the bus bars and the metal parts that grab the busbars, instead of copper. Although most of mine were copper, I did have a couple breakers (I don't think they were original breakers they were probably replacements) that had aluminum parts that grab the bus bars. The bus bars inside my breaker box were copper (tin plated), but I've heard some of them have aluminum bus bars.

There was one spot where the bus bar in mine had evidence of arcing (sooting and and pitting.)

[charger891]

Thanks for the informative thread, always wondered what was in those breakers, just never had the time to disect one like you did.

[jakeru]

Thanks. I thought it might appeal so some of the folks on this forum, given that we sometimes discuss which circuit breakers are appropriate to use for our welding machines.

One more thing I wanted to add (because I thought it was kind of interesting...) notice the small screw in the lower right corner of each of the overall, opened-up circuit breaker pictures? I noticed that the different breakers I opened had these screws set differently.

That screw is used during manufacturing I believe, for fine-tuning how quickly the circuit breaker trips. Adjusting that screw would rotate the bi-metallic strip part and change how much the bimetallic strip was pre-loaded against the tripping mechanism (so that a greater or lesser warming of the strip would be required before the breaker would trip.)

It looked like the factory melted the plastic housing over this adjustment screw hole, to prevent tampering of this adjustment screw.

[kenwhite]

Great information and it is 100% percent on the money.

There are 2 basic types of circuit breaker technologies. One type uses the magnetic field generated by the current flowing through it to trip - these are fast acting types and are used for shorted/fault conditions. The other uses thermal characteristics of the metal to break the circuit - slow acting types and are used for a normal overload condition. There is also a 3rd type, which consists of both thermal and magnetic technologies in one package.

There are also 2 special types of circuit breakers:
GFCI - Cuts power when tripped by an overload of current, a short circuit or a line-to-ground fault.
AFCI – Protects against fires by tripping when a quick surge of heat (arc) is detected.

Great work jakeru...

[sw2x]

Wow, you just Disected a $100 breaker. Zinsco's are getting hard to find now, so people who need them have to pay more for the breaker cause they are in such low supply.

Interesting information tho

[geezer]

Something we take for granted,,,it must sit there for years in the panel box and it must work everytime when the need arises,,,,it's one thing to be smart about electricity,,,but the guy who built that had to be a mechanical genius as well....great post,,,

[Brian Ski]

I believe there also may be some mechanism for a sudden current to trip the breaker with magnetic force, and I think that may be what the U-shaped piece of metal in the bottom of the above picture is for, but honestly I haven't got that completely figured out yet.

I think you have that correct. The bimetalic strip under a large current spike looks like it would attract the metal piece and trip the breaker.

I always like giving different stuff to the kinds to tear apart. Makes us all learn. Next chance you get take apart and old Ground fault outlet. They use several coils to balance out the incoming and outgoing current. So if there is a imbalance it will almost instantly trip. Amazed how they jam all that stuff into the little outlet.

BTW Leviton is now making thin GFCI outlets. They are a lot smaller.