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Molded Case Breakers are not so much special as they are Universal. They are the most commonly used breaker. They are found everywhere. In homes, office buildings, and manufacturing plants. The small black breakers in the main electrical panel of homes are molded case circuit breakers. They can range from one amp to 1000's of amps; A few ounces to several hundred pounds. They are recognized by their simple rectangular phenolic, or plastic, case.

Yes, Molded Case Breakers provide overload protection and fault current protection just like air circuit breakers, power breakers, and insulated case breakers. They can also provide many other selective or special protective functions.

The basic design purpose of molded case circuit breakers is to protect the conductors connected to the output of the breakers from damaged due to overloading or due to high fault currents during a short circuit.

Yes, molded case circuit breakers, just like power breakers and air circuit breakers, have a "Voltage Rating" and an "Interrupting Current" rating.

In a three phase system, when a circuit breaker, whether molded case breaker, insulated case breaker, power circuit breaker, or air circuit breaker operates, it opens all three phases. A fuse might blow and only open one or two of the three phases. This could cause serious damage to motors due to “single-phasing.” Also, molded case breakers, insulated case breakers, power breakers, and air circuit breakers are much more tolerant of in-rush currents than fuses. But, the decision to use fuses or circuit breakers should be based on good engineering analyses and not on simple rules-of-thumb. Especially now, with important concern for arc flash hazards and arc flash hazard analysis.

A circuit breaker is a switch that automatically interrupts the flow of electrical current when the rated current is exceeded due to an electrical overload or fault.

The single best and most inexpensive service on a molded case circuit breaker is to exercise the breaker when you have the opportunity. Open and close the circuit breaker carefully five times. Does the circuit breaker latch open and close properly? And when you exercise the circuit breaker, listen to the circuit breaker operation. Does the circuit breaker open with a single snapping sound or does the circuit breaker sound like one or more of the three poles opened slower than the first pole to open. When the circuit breaker is closed, do all three poles of the circuit breaker close simultaneously. The circuit breaker should open and close with a single snapping sound. Opening and closing the circuit breaker exercises the circuit breaker operating mechanism. It also “wipes” the contacts of the circuit breaker, lowering the contact resistance of the circuit breaker. When the circuit breaker contacts close, they will rub, ie wipe, slightly together, cleaning the contact surfaces of each pole of the circuit breaker. And while you’re doing this, visually inspect the circuit breaker. If the power is on while you’re exercising the circuit breaker, be careful! Be safety conscious! Use your right hand to operate the circuit breaker. Don’t touch anything else when operating the circuit breaker. Keep your body and face as far away from the circuit breaker as possible. Turn your face away from the circuit breaker when you are exercising it. There should not be any exposed energized circuit breaker parts, bus or cable terminations. If there is, use extreme caution and use the proper personal protective equipment (PPE), per NFPA 70E. Actually, if there are exposed energized parts, you must be able to justify being near them. You must justify not turning the power off to exercise the circuit breakers. This also relates to NFPA 70E. One last caution. If the handle of the molded case circuit breaker is broken, be very careful because the metal stud exposed by the broken handle may actually be energized at full voltage. On some circuit breakers, the metal stud is electrically connected to the center pole and therefore part of the electrical circuit. This can be deadly. So be careful.

Molded case circuit breakers are not intended to be maintained or repaired in the same manner as air circuit breakers or power circuit breakers. Molded case circuit breakers are considered by most people as disposable. And most small molded case circuit breakers should be thrown out if they have been damaged or become wet or been submerged. But the larger molded case circuit breakers can be maintained and repaired after they have become wet. The question is whether the cost to repair a damage circuit breaker is worth it or would you be better off purchasing a replacement breaker. When we repair water, mud, or dust damaged molded case circuit breakers, we put the breakers through a thorough disassembly, maintenance, cleaning, repair, re-assembly, and overcurrent testing procedure. It is particularly difficult to repair the “trip element” of a molded case circuit breaker. You really need to know what you are doing and have the specialized test equipment to proof test the circuit breaker after the repairs are complete. But even if the trip element is not repairable, there can be a huge savings by replacing just the “trip element” of the circuit breaker. Again, these repairs are usually only worthwhile for larger circuit breakers, such as 400 amps and larger.

We presume you are referring to molded case circuit breakers. All standard molded case circuit breakers have a “rating” that tells the purchaser what size wire they can connect to the breaker. The electrical current rating of the breaker must protect the wire from carrying too much current, melting, and burning down your Taj-Mahal. But all circuit breakers sales are not necessarily the same for circuit breakers with the same rating. So your electrician is correct. The reason is because even standard molded case circuit breakers have other ratings and functions than just the basic current rating. These other circuit breaker functions can vary dramatically. These functions are (1) Instantaneous, ie fault or short circuit, pickup setting, (2) Overload time delay, ie how long will it take the breaker to trip if the current exceeds the rating, and (3) Circuit breaker current interrupting rating, ie how much current can the breaker interrupt without blowing up itself. More complex molded case circuit breakers, insulated case circuit breakers, power circuit breakers, and air circuit breakers have many more functions and many settings within each function. But those are for engineers to dream about. In your world, if you are replacing a molded case circuit breaker, that’s those black plastic looking breakers, replace them with “identical” breakers. Identical means same breaker manufacturer, same rating, same catalog number, or a direct replacement as determined by the breaker manufacturer. And be sure to have a qualified person replace the breaker and that person should make sure the power is off before changing the breaker.