Circuit breakers

The idea behind this topic is to help you

  • Familiarize with circuit breakers in general.
  • Determine the benefits and drawbacks of the different circuit breaker types.
  • Recognize and avoid some common mistakes when specifying a circuit breaker.

Circuit breaker - A circuit breaker is an automatically-operated electrical switch designed to protect a circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, “immediately” discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset to resume normal operation.

Circuit Breaker vs Fuse

  • A circuit breaker can be quickly and easily reset after it trips and the circuit fault is corrected. If a fuse blows, a replacement fuse may not be readily available.
  • A breaker helps avoid operator error. In a fuse application an incorrect value fuse could be inserted. Which could either blow to early or allow the fault to damage the circuit.
  • Circuit breakers will have a higher price to start off but depending on the number of faults and the equipment they are one, they can provide a large cost savings as time goes on.
  • Some breakers can perform functions other than circuit protection such as switching and indication. Example – Some breakers can be used as the power switch as well as the circuit protector. Where as fused circuits would always require a separate power switch
    See Also: Circuit Protection Differences

Types of Circuit breakers

  • Thermal
  • Magnetic
  • Magnetic with Hydraulic Delay
  • Thermal magnetic


  • Thermal circuit breakers incorporate a heat responsive bimetal strip or disk.
  • Since the part has to heat up in order to trip there will be a predictable delay.
  • This type has a relatively slow characteristic curve that discriminates between safe temporary surges and prolonged overloads.
  • Thermal breakers are appropriate for machinery or vehicles where high current in rushes accompany the start of electric motors, transformers, and solenoids.

Thermal Breaker Operation

  • In the ON state current will flow through the bimetallic plate. If the current level is too high the bimetal will heat up and flex, opening the contacts, and thereby interrupting the current flow.
  • When the contacts open, an insulator slides into position between the contacts to hold them open (so it will not reset itself as it cools). It will remain in that state until the breaker is manually reset.
  • Looking at how it operates, it is easy to see that ambient temperature will lower or raise the rated trip current. This could be considered good or bad depending on the application.
    Bimetalic Element


  • Magnetic circuit breakers operate via a solenoid. They use electro-magnetism to move an internal armature which opens the contacts.
  • Magnetic breakers trip instantly as soon as the threshold current has been reached.
  • This type is appropriate for printed circuit board applications and impulse disconnection in control applications.

Magnetic with hydraulic delay

  • Often, a magnetic circuit breaker is combined with a hydraulic delay to make it tolerant of current surges.
  • Preferably, hydraulic delay circuit breakers should be mounted in a horizontal position to prevent gravity from influencing the movement of the solenoid. If mounted in a non-horizontal position, derating may be needed.

Magnetic Breaker Operation

  • Current flowing through the coil causes the piston/core to move up or down. When anything above the rated current is reached the core will move beyond a certain point. Once the core is within a certain distance of the “Pole Piece”, the electro-magnetic force will be strong enough to move the armature which in turn opens the contacts.
  • Magnetic breakers with hydraulic delay utilize the exact same design but they have one key difference. In a simple magnetic breaker the core will be sitting in air making for little resistance for core movement.
  • Hydraulic delay parts will have some sort of fluid surrounding the core. The heavier the viscosity the longer the delay will be.

Thermal Magnetic

  • Thermal-magnetic circuit breakers combine the benefits of a thermal and magnetic circuit breaker: a delay that avoids nuisance tripping caused by normal inrush current, and fast response with high currents impulses.
  • High over currents cause the solenoid to trigger the release mechanism rapidly, while the thermal mechanism responds to prolonged low value overloads.
  • They have a characteristic two-step trip profile that provides fast short-circuit protection of expensive electrical systems while minimizing the risk of disrupted system operation.

Thermal-Magnetic Operation

  • Thermal-magnetic breakers combine the two different technologies.
  • If operated at or below of the rated current the bimetallic element will not move. And the core will not pull in far enough to trip the breaker (fig A).
  • However like the thermal breakers a large ambient temperature increase and/or slight current increase can trip the breaker (fig B).
  • Also, like the magnetic breaker if the current rapidly increases enough the breaker will trip as well with no delay (fig C).
  • These factors make the thermal-magnet circuit breaker a good choice when an instantaneous trip is needed and when high ambient temperature can negatively affect a product.

Trip Profile
Shown below is a basic snapshot of how each circuit breaker type would trip.

In most cases thermal circuit breakers are going to be far and away the lowest cost. Next on the list would be magnetic circuit breakers. The highest priced would be the magnetic with hydraulic delay and thermal magnetic. However, specifications like sealing, contact rating, material make up, physical size, etc are all going to alter the price.

Common Mistakes When Specifying a Circuit Breaker

Specifying the wrong circuit breaker type for the application.

  • The number one mistake design engineers make is specifying the wrong circuit breaker technology for the application. Designers need be aware of the trip profile and the environment that breaker will be in.

Specifying too high a rating in an effort to avoid nuisance tripping caused by in-rush or transient currents.

  • Engineers are used to over sizing fuses as a way to prevent nuisance tripping. However, there is no need to oversize a circuit breaker.
  • Unlike a fuse rating, a circuit breaker rating tells you the maximum current that the circuit breaker will consistently maintain in ambient room temperature. Thus, a 10A circuit breaker will maintain a 10A current without nuisance tripping. In fact, a typical 4A circuit breaker with a slow trip profile will tolerate a temporary 10A current surge without nuisance tripping.
  • Often times, nuisance tripping is caused by in-rush currents associated with certain electrical components – primarily motors, transformers, solenoids, and big capacitors. In such cases, the designer needs to specify a circuit breaker that has a delay.

Failure to provide spacing in design

  • It is important to maintain recommended minimum spacing requirements between non-temperature compensated thermal circuit breakers.
  • Often, a mere 1 mm spacing between breakers is all that is required. Without this tiny thermal gap, the circuit breakers can heat up and increase the sensitivity of the bimetal trip mechanism.
  • If the breakers must touch each other, mfr’s recommend derating them to 80% of their normal amperage rating (mfr datasheets should have specific derating information).

Failure to Derate

  • As a rule of thumb, the circuit breaker should be rated for 100 percent of the load. However, some applications require a circuit breaker to operate continuously in either high or low temperatures.
  • In these cases, follow the manufacturer’s guidelines for derating. For example, an application calling for 10A protection requires a 12A rated thermal circuit breaker when it is operated at 50 degrees C.

Derating when it is not necessary

  • The performance of a thermal circuit breaker is sensitive to fluctuations in ambient temperature. It will trip at higher amperage in a cold environment, and it will trip at lower amperage in a hot environment.
  • One common mistake is to assume that derating is necessary for thermal circuit breakers in environments that experience rises in ambient temperature.
  • Actually, the performance of a thermal circuit breaker tracks the performance needs of the system, assuming it is exposed to the same heat source. For example, motor windings need more protection from overheating at 90 degrees C than the same windings need at 20 degrees C. A cold motor requires more in-rush current to get started, and therefore a longer delay is advantageous on a cold day.

Specifying the wrong type of circuit breaker for a high vibration environment.

  • Typically, the trigger of a magnetic circuit breaker is a hinged metal armature that closes in response to the movement of a magnetic coil. This design makes magnetic circuit breakers (and magnetic-hydraulic circuit breakers) particularly vulnerable to vibration.
  • In contrast, a typical thermal circuit breaker is comprised of a thermal actuator and a mechanical latch. Thermal circuit breakers are therefore highly tolerant of shock and vibration. If a magnetic circuit breaker is the best type for the application, its vibration resistance can be improved by using a push-pull style actuator. This type of actuator has a latching design.