Circuit Breakers


Circuit Breakers. Most everyone has heard the term. Any residence built after 1960 has them, power plants have them, commercial buildings have them. But what are they, how do they work, and why do we need them?

Wikipedia defines a circuit breaker as "an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current from an overload or short circuit." To put it more simply, a circuit breaker "breaks" the flow of current in an electrical circuit if it is too high. Hence the term circuit breaker.

Circuit breakers come in many types and voltage ratings. High voltage circuit breakers (69,000 volts and above) tend to be oil-filled or SF6 type. These circuit breakers use mineral oil or sulferhexafloride to serve as an insulation between the contacts in the circuit breaker. Medium voltage circuit breakers (between 600 volts and 69,000 volts) tend to be vacuum or air type. These circuit breakers use vacuum or air to serve as an insulation between the contacts. Low voltage circuit breakers (600 volts and below) tend to be air or molded case circuit breakers. For this article, we will focus on Molded Case circuit breakers (MCCB) such as those you have in your residence.

Circuit Breakers

Figure 1 shows a typical 20 amp molded case circuit breaker, and Figure 2 shows a dual 20 amp molded case circuit breaker. These are the types of MCCBs you would see in your electrical panel. The reason they are called molded case circuit breakers is because the circuit breaker's enclosure, or casing, is molded from a fiberglass-polyester material or a thermoset composite resin. This material is very strong, fire-resistant, and is a great electrical insulator.

When a MCCB breaks the flow of current, which is called a trip or tripping, it can do it one of two ways. The first way is with a magnetic sensing device internal to the MCCB. As current flows down a wire, there is a corresponding magnetic field around the wire. The higher the current, the stronger the magnetic field. The magnetic sensing device senses this magnetic field from the current, and if the magnetic field becomes too high, it breaks that current flow, or trips, the MCCB. The actual level of magnetic field required to trip the MCCB is based on the actual rated size, in amps, of the MCCB.

The second way the MCCB can trip is through a thermal overload device. Current flowing in a wire creates heat. The higher the current, the more heat the current generates. The thermal overload device is also internal to the circuit breaker, and at a certain setpoint, also based on the rated size of the MCCB, it will trip the MCCB.

With respect to the magnetic sensing device, this is very effective under short-circuit conditions. A short-circuit condition takes place when the insulation on a current-carrying wire breaks down and shorts to ground or another conductor. When a short-circuit takes place the resulting current can be very high, it can create a large arc (which can also start a fire, which we will get to in a bit) and the MCCB needs to react very quickly to trip the breaker and stop the short-circuit. Depending on the level of short-circuit current, the magnetic sensing device can trip a MCCB as quick as one cycle, which is .017 seconds.

With respect to the thermal overload device, this is used for lower, but longer time overcurrents. The thermal overload device is usually two strips of different metals bonded together, called a bimetal strip, each having different thermal expansion rates. The heat as a result of the current through the MCCB will cause the bimetal strip to expand. When the bimetal expands enough it results in the MCCB tripping. The bimetal strips are selected based on the size of the MCCB and are calibrated at the factory. The higher the overload current the quicker the MCCB trips. The time it takes for a MCCB using the thermal overload device can range from a few seconds to up to three hours.

A ground fault circuit interrupter (GFCI) is a specialized MCCB that can detect small ground fault currents. They are designed to prevent an electrical shock in the event the electrical device comes into contact with water or the circuit develops a small leakage current. Therefore, they are required in all wet areas of a residence or facility, such as bathrooms and kitchens. These breakers have an additional wire that gets connected to the neutral bus in the main electrical panel. A GFCI MCCB will trip on a very small amount of fault current, usually less than the MCCB's rating.

So why do you need MCCBs in your residence? The first answer is the easiest, and that is because the National Electrical Code (NEC) requires every electrical circuit in your residence to be protected by an overcurrent device, which is an MCCB. It should be noted that the NEC's purpose is to safeguard persons and property from hazards arising from the use of electricity. That implies that the reason MCCBs are needed is so you can live safely in your residence, which is the second answer. So let's look at a couple of examples where a circuit breaker was needed.

Example 1 - Let's say you live in a very cold climate. In the winter you use electric space heaters for supplemental heating. So you plug in a 1,500 watt heater into a receptacle in your kitchen. That same receptacle has your countertop microwave. The receptacle is on a 20 amp circuit. In looking at the current draw, one can see the 1,500 watt heater will require almost 13 amps continuous. The microwave is also rated at 13 amps. And you decide to cook something in the microwave that takes over 30 minutes. So now, that 20 amp circuit is called on to handle 26 amps, 30% over its rating. Why is that a problem? Recall that current flowing through a wire generates heat. Too much current would then result in too much heat. If a wire gets hot enough it can melt its insulation and start a fire. Luckily, the MCCB sees the higher than allowed current, and trips the MCCB to eliminate the overheating hazard. And you need to go find a different receptacle to plug in your space heater.

Example 2 - You just bought a new refrigerator. It was installed and seems to be working great. But unknown to you, the installer pushed it too far back, and it put a kink in the electrical plug and cord, splitting the insulation open. The exposed cord is now making slight contact with the frame of the refrigerator. But lucky for you the refrigerator is on a GFCI, and it saw the small current caused by the exposed cord touching the frame. It tripped. You reset the GFCI but it tripped again. Pulling the refrigerator out you discover the problem. If the GFCI had not tripped, you could have been shocked or even electrocuted when you attempted to open one of the doors.

In both these examples, disaster was averted because of an MCCB or a GFCI. As you can see, they do indeed serve a safety function, and are necessary in every residence or facility that has electrical power.

For more information on circuit breakers or other things electrical contact any of our electrical engineers.  Their contact information can be found on our website:


About the Author

Albert M. Rose, P.E., B.S.E.E. is a consulting engineer with our Orlando Office. Mr. Rose provides consulting services in electrical power transmission, distribution and control, including electrical root cause failure analysis and fire origin and cause investigation. This work often involves damage assessment and repair/replacement analysis. You may contact Al for your forensic engineering needs at or (407)865-9900.

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