Circuit Breakers

By Pat Sagsveen

Contributed By Digi-Key Electronics

Circuit protection is a large concern in the electronics industry. While a simple fuse is an inexpensive way to protect a device, it will need to be replaced if it is blown. Some applications, while requiring circuit protection, would be very hindered by having a fuse go out. Imagine the control board to the main line at a manufacturing assembly. If a fuse on this blows open it will literally be a “line down” situation. If an improper fuse is used and the board is damaged it will be an even longer “line down” situation. Having a resettable circuit breaker would be the perfect compromise between having a fuse and no circuit protection at all. If a fuse is used, it is possible that an incorrect fuse could be used as a replacement. A circuit breaker is a guarantee that the proper circuit protection will be in place when the breaker is reset. The control board will be protected, and if the breaker is tripped it is easily reset. This article will go over some different kinds of circuit breakers, the vernacular used to describe them, and potential applications.

A circuit breaker is a device that automatically opens a circuit in the case of an overload or short circuit. There will be a “current rating” which is the maximum current the circuit breaker can operate at without opening. The current rating is a searchable parameter on the Digi-Key website. The upper right hand part of Figure 1 illustrates where the current rating can be found on the “Circuit Breakers” page. By highlighting the desired current rating and selecting the red “Apply Filters” button in the lower left hand corner, all of the applicable options will remain while others are filtered out.

Image of current rating on Digi-Key’s parametric search

Figure 1: Current rating on Digi-Key’s parametric search.

Another important characteristic for a circuit breaker is its voltage rating. It is acceptable to use a circuit breaker that is rated at a higher voltage than will be present on the circuit. If a circuit is using 120 VAC, it is fine to use a circuit breaker that is rated at 250 VAC. It is not acceptable to go the other way and use a circuit breaker that is rated for 120 VAC on a 250 VAC line. Digi-Key has two selectable parameters that help in narrowing down the options that have the proper voltage rating. There is an AC parameter and a DC parameter. These parameters are operated the same way that the previous example was. Simply highlight the desired voltage rating, and then click the red “Apply Filters” button to remove options that are not on that list.

Types of Breakers

Magnetic breaker

There are different kinds of circuit breakers that are better suited for given applications. A magnetic breaker with a hydraulic delay is a good way to protect against short circuits. They work by using a solenoid that is hooked up in series with the input and output. As current enters the circuit breaker, a magnetic field is created around the solenoid. As more current enters the circuit breaker, the magnetic field gets stronger. At a certain point, there is enough current to cause the magnetic field to become strong enough to overpower an internal spring and release the latch on the circuit breaker causing an open. The core of the coil is inside a sealed cylinder which has a piston and a spring in hydraulic fluid. The viscosity of the fluid determines the amount of time it takes the breaker to trip. This is useful for applications that have high inrush currents; it allows the large current right away without having a nuisance trip. Magnetic breakers are not temperature sensitive, making them ideal for environments that experience dynamic climates. They are voltage sensitive, so an AC unit cannot be substituted for a DC unit. Figure 2 depicts the N11-B0-24-610-121-D3, a magnetic circuit breaker from Carling Technologies, while Figure 3 shows the internal workings.

Image of Carling Technologies’ N11-B0-24-610-121-D3 magnetic breaker

Figure 2: Carling Technologies’ N11-B0-24-610-121-D3 magnetic breaker.

Image of internal workings of Carling Technologies N-Series breakers

Figure 3: Internal workings of Carling Technologies N-Series breakers. (Image source: Carling Technologies)

Thermal Breaker

A thermal breaker works a little bit differently than a magnetic breaker. This will involve some kind of bimetallic element which will have one of the contacts attached to it. When the current reaches a certain value, the bimetal strip will heat up to the point of bending and opening the contacts to break the circuit. Typically there will be an insulator that slides into place to keep the contacts open until a manual reset is done. One of the difficulties with this type of breaker is that it is very temperature dependent. At higher operating temperatures, these will be able to handle less current. The opposite is true for lower temperatures, a breaker that is operating at less than 25˚C will require more current than advertised to trip. Typically there will be documentation for ambient temperature compensation. Figure 4 is an example of an ambient temperature compensation table from the W57 series from TE Connectivity Potter & Brumfield Relays. If one amp was being used as the baseline, this shows what the current rating would be for a thermal breaker at different temperatures and models. There is no standard for this, so always check the manufacturer’s documentation.

Image of ambient compensation table of TE Connectivity’s W57 series breakers

Figure 4: Ambient compensation table of TE Connectivity’s W57 series breakers. (Image source: TE Connectivity)

It typically takes a little bit longer to trip a thermal breaker than a magnetic breaker. Looking back to the W57 series documentation, there is also a “Time vs. Current Trip Curve” that gives a detailed graph of trip times for different percentages of overload currents depicted in Figure 5.

Graph of TE Connectivity’s W57 series time vs. current trip curve

Figure 5: TE Connectivity’s W57 series time vs. current trip curve at 25˚C. (Image source: TE Connectivity)

Thermal breakers are typically smaller than magnetic breakers and cheaper. They are not voltage sensitive so the same unit can be used for AC or DC operation. An example of a 15 amp thermal breaker is the W57-XB7A4A10-15 from TE. Figure 6 is an illustration of the W57-XB7A4A10-15.

Image of W57-XB7A4A10-15 from TE Connectivity

Figure 6: The W57-XB7A4A10-15 from TE Connectivity.

Figure 7 is a comparison table for some of the differences between magnetic and thermal circuit breakers.

Image of thermal and magnetic breakers comparison

Figure 7: Comparing thermal and magnetic breakers.

Thermal-Magnetic Breaker

Thermal-magnetic breakers are hybrids of thermal and magnetic breakers. They contain an electromagnet and a bimetal strip. The thermal side of the breaker protects against low level overloads while the magnetic side will give an instant trip at higher current faults like a short circuit. An example of a thermal-magnetic breaker is the UTE100E-FTU-100-3P-LL-UL from American Electrical Inc. shown in Figure 8.

Image of American Electrical UTE100E-FTU-100-3P-LL-UL thermal-magnetic breaker

Figure 8: UTE100E-FTU-100-3P-LL-UL thermal-magnetic breaker.

Solid-State Breakers

The solid-state relay is an emerging technology that is working to replace magnetic and thermal breakers. These replace the bimetallic strip of thermal breakers and the solenoid operation of a magnetic breaker with a digital microprocessor or microcontroller and current sensor. The waveform will be sampled, converted to a digital signal, and evaluated with a microprocessor or microcontroller. These will be much more accurate than the magnetic or thermal breakers, but typically more expensive. An example of a solid state breaker is the 3000763 from Phoenix Contact depicted in Figure 9.

Image of Phoenix Contact’s 3000763 solid-state breaker

Figure 9: Phoenix Contact’s 3000763 solid-state breaker.

Mounting Options

Panel Mount

Circuit breakers are used for many different applications. As a result, manufacturers have strived to make many different sizes and mounting types to suit several different needs. Panel mounted circuit breakers are very popular. These are great for several applications. If there is an application which has a panel that can be cut through, a panel mount option is a great place to start. An example of a panel mounted circuit breaker is the 1658-G21-01-P10-10A from E-T-A. The actuator is round making it extremely easy to drill out a mounting hole. Panel mounted options will usually come with a nut to hold them into place. The 1658-G21-01-P10-10A has an accessory cap available which can be viewed at the bottom of the product page in the “Associated Product” area on the Digi-Key website. The accessory cap is the X20128501. Figures 10 and 11 show the 1658-G21-01-P10-10A and the X20128501 respectively.

Image of 1658-G21-01-P10-10A panel mount breaker from E-T-A

Figure 10: The 1658-G21-01-P10-10A panel mount breaker from E-T-A.

Image of E-T-A’s X20128501 accessory cap

Figure 11: E-T-A’s X20128501 accessory cap for the 1658-G21-01-P10-10A panel mount breaker.

DIN Rail

Industrial environments often require many breakers and their floor space can be quite precious. Often circuit breakers can be hung on a DIN rail inside of an equipment rack to save floor space. A DIN rail is a standard sized metal rail which these breakers will fit onto. These will usually be 35 mm wide and products that are made to mount onto these will be designed to fit snugly on the rail. An example of a DIN rail mounted circuit breaker is the 2907573 from Phoenix Contact, pictured in Figure 12. The bottom of the image shows the DIN rail and how the circuit breaker attaches to it.

Image of Phoenix Contact’s 2907573 DIN rail mounted breaker

Figure 12: Phoenix Contact’s 2907573 DIN rail mounted breaker.

Holder Mounted

Circuit breakers often utilize holders as well. This makes replacing an old circuit breaker very easy. Pictured in Figure 13, the 1610-21-15A from E-T-A, mounts into a holder which can then be mounted onto a DIN rail. The holders can be found on the bottom of the product page for the 1610-21-15A on the Digi-Key website in the “Associated Product” area as shown in Figure 14.

Image of 1610-21-15A from E-T-A

Figure 13: The 1610-21-15A from E-T-A.

Image of “Associated Product” for the 1610-21-15A

Figure 14: “Associated Product” for the 1610-21-15A.


Circuit breakers are not the most sophisticated pieces of equipment, but they play a very crucial role in the electronics industry. There are different kinds of circuit breakers that are better suited for different applications. Magnetic breakers are good for environments that have changing climates because they are not temperature sensitive. They are voltage sensitive so it is not possible to use a magnetic AC breaker on a DC circuit. Magnetic breakers rely on the viscosity of the fluid inside of the breaker to determine their trip times. This is helpful for applications that have an inrush current by allowing the breaker a little bit of time to let the current subside without having a nuisance trip.

Thermal breakers rely on a bimetal strip to interrupt current flow. When the bimetal strip gets warm enough, it will bend and release contact inside of the breaker opening the circuit. These are very temperature dependent, but also cheaper than magnetic breakers. As temperature goes up, the amount of current that a thermal breaker can handle decreases and a higher current rating will be needed. As temperature goes down, the amount of current that a thermal breaker can handle increases and a lower current rating will be needed.

Thermal-magnetic breakers are hybrids of the aforementioned styles. These will use the thermal element to break the circuit if an overload occurs, while using the magnetic characteristic for short circuits to quickly open the circuit.

Solid-state relays use current sensing and microprocessors or microcontrollers to sense an overload or short and open a circuit. These are much more reliable and faster than magnetic and thermal breakers, but also much more expensive than both.

There are several mounting styles for breakers to help the end user in just about any application.

There is more that can be said about circuit breakers, but hopefully this is a good start to understanding what they are and some of the key differences between the main types.


  1. Understanding Overcurrent Circuit Protectors.” Retrieved Sept. 27, 2017
  2. Thermal Magnetic Circuit Breakers.” Feb. 4, 2014
  3. Molded Case Circuit Breaker Basics.” Retrieved Sept. 27, 2017
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