Choosing the Right Circuit Breaker

Numerous factors must be taken into consideration when choosing a circuit breaker.

First are the basic electrical characteristics of the equipment involved, such as the following:  

• Whether it uses an AC or DC current.
• The voltage, whose determining factor is the highest voltage that can be applied across any two conductors in the circuit.
• The short circuit current level that will trigger the device to protect it.

Other factors include the type of environment in which the equipment will operate, including the ambient temperature. This will impact the type of box or protection needed to protect your circuit breaker from factors such as humidity, temperature, and dust.

There are multiple types of circuit breakers and one of the main differentiators is the dielectric material – or non-material in the case of a vacuum –  they will utilize to quench any electric discharges they encounter. These can include air, a vacuum, oil, or the gas sulfur hexafluoride SF6. Each is suited for particular applications:

• Air for low-voltage applications 
• Vacuum for higher voltages 
• Oil for medium to high voltages
• SF6, which is the most versatile and widely used for most medium to high-voltage applications because of its
  • high dielectric strength
  • thermal stability and conductivity 
  • high density (some five times that of air) 
  • inertness 
  • non-toxicity
  • ability to recombine very rapidly after the source of sparking ceases

Steps to selecting a circuit breaker include determining the load type. The key factor is whether the load will be static or dynamic:

• If the load is static, it will never draw more than the rated current even when at full power.
• If the load is dynamic, the device may draw a higher current than rated when starting.

Devices in the former category might typically be heaters and in the latter motors or transformers.

Given the extreme danger that may result from the use of faulty or otherwise inadequate devices, another important consideration in circuit breaker selection is to ensure that they are genuinely manufactured by reputable suppliers and are not counterfeits. Visual clues that signify a device is non-compliant in this respect might include poor surface finishes such as imperfections in the outer casing, corrosion of metal parts, and illegible markings. In Europe, all compliant devices must without fail carry a CE mark. Other factors to consider might also include the product’s weight (ensure consistency with previously purchased compliant products), and the provision of a Type Test certificate from a recognized laboratory or authority to prove compliance.

In terms of their physical construction, there are several types of circuit breakers including differential, thermal, and electronic devices. Each has its own performance characteristics and suitable area of application (see the following sections for more detailed information).

EATON Differential Circuit Breaker

Differential devices measure current flowing in a loop and are therefore tripped if they detect any discrepancy between an incoming and outgoing current.

As such, a differential circuit breaker will be activated by discrepancies that might be close to zero and which might typically be caused by high impedance faults such as inefficient insulation that might not be detected by other types of devices. 

They are commonly used for protecting generators and transformers.

Doepke Thermal Circuit Breaker

Thermal devices utilize the heat generated by an excess current to increase the temperature of a bimetallic strip wired in series with the circuit. 

The heat causes the strip to deform which in turn triggers the breaker. 

The drawback of these types of devices is that they are relatively slow to operate, however, they do have the added benefit that they can be used as the main on/off switch for the equipment they are protecting.

Electronic devices incorporate an automatically operated switch that acts in response to feedback from the load. 

The switch will be triggered by a series resistor that will detect any excess current that in turn will cause a corresponding decrease in voltage across the resistor. 

The advantage offered by such devices is that they react almost instantaneously because their operation is based on current sensing rather than much slower thermal expansion.

The obvious alternative to a circuit breaker is a fuse, which seemingly offers the advantages of relatively low cost and simplicity. Technical standards will often determine whether you need to use a circuit breaker or a fuse, and although circuit breakers are more expensive and complex, they are advantageous for several reasons. These include: 

• The condition of a fuse is difficult to estimate and it may degrade over time whereas a circuit breaker can be regularly tested to ensure it is always in optimal working order.
• A circuit breaker can react to an overload condition much faster than a fuse and is therefore inherently more effective in protecting equipment that would suffer damage from any prolonged exposure to excess current.
• Circuit breakers are easier and safer to install than fuses since their connection points to the circuit will be found on the back of the device. This means that the personnel handling them will be protected from any possibility of direct contact with the circuit itself.
• The low cost of a single fuse does not necessarily translate into the lower cost of a complete protection system. To start, a three-phase electrical system will require three fuses. When one fuse blows, all three will have to be replaced, meaning that a stock of spare fuses will need to be kept available. Fuses also require large enclosures to allow for heat dissipation. Finally, fuses cannot double up as switches as circuit breakers can, therefore, the circuit will require additional switching devices to be purchased and installed.
• Circuit breakers can do more than just break circuits when acting as safety devices or switches. They can also perform additional functions such as providing ground fault protection and can include features such as remote control, status indication, power measurements, and network communication.