A DC Circuit Breaker is a mechanical switch that protects against short circuits and over-currents in circuits supplied with direct current. They are designed to break the current flow in DC electrical systems in the event of a fault.
Figure 1 – DC Circuit Breaker
They apply mechanisms that can limit the current and also extinguish arcs caused by over-current. Through this, the differential timing of the circuit is greatly improved.
DC Circuit Breaker Basics and Fundamentals
DC Circuit Breaker Symbol and Wiring
We use these symbols mainly in electrical circuits.
Figure 2 – DC Circuit Breaker Symbol
However, for a wiring diagram, you will find a 3D image such as this:
Figure 3 – DC Circuit Breakers Wiring
Function in DC Systems
It is clear from their name that we use such electrical breakers to protect systems that use direct current to operate. Such systems have a constant voltage output unlike AC systems.
They use a combination of magnetic and thermal principles to protect the DC Systems. If current goes beyond the rated value, the breaker is tripped using thermal protection.
It is designed to hold out momentarily any current fault that occurs within the circuit. It also serves to rapidly put out any arcs brought about by excess voltage.
Basically, the breaker thermal protection is a fail-safe against any overload current in the system. When dealing with strong fault currents, the rapid magnetic protection trips the breaker.
Having a constant current flowing in the DC Circuits means that the contacts must be fully open beyond their limits. This is to ensure that the flow of the excess current is fully interrupted.
This means that the breakers guard the DC System from any faults or short circuits. Such short circuits tend to be greater than the overload.
Types of DC Circuit Breakers
Solid State DC Circuit Breaker
This breaker is an advanced replacement of the electromechanical breakers. It replaces the moving parts with semiconductors used in power control with rapid current interruption.
With advanced software technology, they can clear faults in seconds after very fast interruptions. They are mostly used in electrical grids with energy storage systems to reduce the effect of downtime due to fault.
Figure 4 – DC Solid-State Circuit Breaker
Arc flashes don’t occur in them during interruptions. This is because zero energy is released.
Thermal-Magnetic DC Circuit Breaker
This breaker applies two mechanisms. Overload protection is achieved by thermal tripping while magnetic tripping prevents short circuits.
We can alternatively call them inverse-time breakers. As the name implies, a higher overload will shorten the opening time for the breaker.
Heat is produced by the excess current in the event of an overload. The bimetallic element picks this up and the breaker trips when its rating is exceeded.
In the event of a short circuit, the electromagnetic sensor detects the fault current. It then responds by disconnecting the circuit.
High Voltage DC Circuit Breaker
The HVDC breaker serves the sole purpose of fault current protection in high voltage DC Circuits. It is worth noting that the current and voltage in DC circuits is never zero.
This means that during contact separation, the current and voltage is usually very high between them. The contacts will end up overheating due to arcing and destroying the breaker.
To counter this, we introduce a low current circuit parallel with this breaker. To break the circuit, it will create an artificial zero current in the circuit.
Figure 5 – High-voltage DC circuit breaker
Since we know the current and voltage level to be directly proportional to the arc strength, we use an external circuit. This will break the circuit just after reducing the fault current to zero.
DC Mini Circuit Breaker (MCB)
The design of the DC MCB is specific circuit breakers using direct current. It protects the electrical equipment from short circuits and over-currents.
It should be noted that its operation and functions are similar to an AC MCB. However, the areas of application differs.
The application of DC MCBs is mainly on systems working with direct current such as solar photovoltaic (PV). The breaker operates within a voltage range of between 12-500V.
The breaker has the positive and negative symbols marked on them. Additionally, we have the current direction indicated on them too.
Figure 6 – DC Mini circuit breaker
DC Molded Case Circuit Breaker (MCCB)
We mostly use the DC MCCB in applications that require storage of energy. They are also the best choice for use in industrial DC Circuits.
Figure 7 – DC Molded Case Circuit Breaker
Arc Suppression Circuit Breaker
When it comes to extinguishing Arc Suppression, DC Arcs are the most difficult. We supply Direct Current continuously meaning it is very stable on a very wide gap.
Figure 8 – Arc Suppression Circuit Breaker
Magnetic DC Circuit Breaker
This is a form of over current protection device. It is designed in such a way that miniature magnets inside it are used to close and open the contacts.
Figure 9 – Magnetic DC Circuit Breaker
Thermal DC Circuit Breaker
It uses a latch mechanism that contains a bimetallic strip connected to it. The bimetallic strip reacts to heat by expansion of its two different metal components at different rates.
Figure 10 – Thermal DC Circuit Breaker
Hybrid DC Circuit Breaker
This is DC Breaker having three separate branches that are configured in parallel to carry out different breaker tasks. The first branch has a mechanical switch used in conveying nominal current.
Figure 11 – Hybrid DC Circuit Breaker
DC Circuit Breaker Working Principle
The main function of the DC Breaker is to protect the circuit from either fault currents or over-currents. It uses either thermal or magnetic protection mechanisms to achieve this.
When there is an over-current, the DC Breaker is tripped by thermal protection. This means that the electric current went beyond the rated breaker value.
It has bimetallic strips made of two different metals that expand when heated. The difference in their expansion make the bimetallic strip to bend and break contact with the contactor.
The thermal protection mechanism only works for overload currents. This implies that it exceeded the conventional operating current.
Magnetic protection is used when there is a heavy fault current in the circuit. It will trip the DC Circuit Breaker and the action will be rapid and instant.
The circuit breaker can be switched back on using the handle or toggle. This should be done after rectifying the overload or short circuit.
Key Components and Parts
The components of various types of circuit breakers are basically the same.
Let’s have a look at them in detail:
Frame
It is usually very strong and rigid. Its main purpose is to protect the internal components against environmental extremities. It also provides insulation.
Toggle/Handle
Normally used to close or open the DC breaker. For larger breakers, the operators can use a 2-step process for protection.
Contacts
They are responsible for the flow of current once they are connected. In breakers for low voltage, the contacts are located in the chamber housing the arc interruption.
Arc Extinguisher
When the breaker trips due to a fault, it extinguishes the arc generated. Since we can’t prevent arcs from occurring, the best the breaker can do is to control them.
Trip Unit
When the short circuit or overload is long, the operating mechanism is opened by the trip unit. They can either be electronic or operate electro-mechanically.
Figure 12 – DC Circuit Breaker Parts
Applications in Modern Systems
Solar PV Systems
Solar panels are usually assembled in series circuits, which may be several. All the circuits must have a DC Circuit breaker to protect because they are very crucial in the whole circuit.
Battery Energy Storage
The breakers protect battery systems in energy storage applications. They are commonly used between inverters and batteries to provide circuit protection and load isolation capabilities.
DC Motor Protection
The breakers protect DC Electric motors having various applications. Most of them are automated with rapid response times with control circuits using DC. All of these require a DC breaker for protection.
DC Power Transmission
High voltage DC breakers offer protection when power transmission is over long distances. The terminals required when converting AC/DC or DC/AC cost a lot and have to be protected. Fault currents can cause damages to any connected equipment hence a breaker is necessary.
Selection and Sizing Guide
DC Circuit Breaker Sizing
Sizing a DC Circuit Breaker can prove to be a very daunting task. However, it’s never an impossible task.
We are fully aware that the breaker size has to be large enough to accommodate the required load current. Under-sizing a breaker means you run the risk of causing an electric fire.
80% Breaker Rule
The rule basically says that you can only have 80% of the current rated ampacity. Let’s take the example of a 40A breaker.
The safest maximum current you can allow is 32A. This safety measure prevents the breaker from burning.
Calculating Amps from Wattage
All electronic devices you will use have a wattage rating indicated. Let’s take an example of a 2000W toaster.
Since breaker sizing is all about Amps, you need to convert the wattage to Amps. Assuming your supply is 240V, the current will be 2000W/240v giving 8.33A.
Practical Sizing Example
Let’s take the 2000W toaster drawing 8.33A. Taking the 80% breaker rule, that gives us the 8.33A.
To arrive at the circuit breaker size, we take a 1.25 factor and multiply it by the amp drawn. This puts the minimum breaker size at 8.33A × 1.25 = 10.42 Amps.
Since the breaker ampacity should be minimum of 10.42, we can as well use a breaker size of 15 Amps. In summary, we will require a 15A breaker for the 2000W toaster supplied by 240V.
How to Choose Circuit Breaker for the DC Grid
You will find various DC Circuit Breakers available in the market. With such options, it becomes easier for you to make a choice.
However, ask yourself some of these questions before settling for the most appropriate:
- What is the current rating of your intended device?
- How many poles do you require for your breaker?
- What is the voltage requirement for your device?
- What is your circuits total current?
- What are your atypical operating conditions?
If you are still unclear, please contact LETOP technical team, we are happy to help you choose.
Frequently Asked Questions
1. What is a DC Circuit Breaker and how does it work?
A DC circuit breaker is a protective device designed to interrupt direct current flow during overloads, short circuits, or faults. It works using thermal and magnetic protection mechanisms – thermal protection for overloads and magnetic protection for short circuits.
2. Can AC Circuit Breakers be used in DC Systems?
No, AC circuit breakers should not be used in DC systems. DC breakers have a constant value of current with no frequency, while AC systems have amplitude fluctuations. Using AC breakers in DC circuits can cause contact melting and system failure due to different arc extinction requirements.
3. What are the main types of DC circuit breakers?
The main types include: Solid-state DC circuit breakers (semiconductor-based), thermal-magnetic breakers (dual protection), high voltage DC breakers (HVDC applications), DC MCBs (miniature), and DC MCCBs (molded case) for different voltage and current ratings.
4. Why is arc extinction more difficult in DC circuit breakers?
DC arcs are more difficult to extinguish because direct current flows continuously without natural zero-crossing points, unlike AC. The constant voltage and current create stable arcs that require special arc suppression mechanisms and rapid contact separation to interrupt effectively.
5. What applications require DC circuit breakers?
DC circuit breakers are essential in solar PV systems, battery energy storage, DC motor protection, electric vehicle charging, data centers with DC power, HVDC transmission, and marine electrical systems where DC power is predominant.
6. How do I size a DC circuit breaker correctly?
Follow the 80% breaker rule – use only 80% of rated ampacity. Calculate load current (Watts/Voltage), then multiply by 1.25 safety factor. For example, a 2000W load at 240V = 8.33A × 1.25 = 10.42A minimum breaker size, so choose a 15A breaker.
7. What is the difference between DC MCB and DC MCCB?
DC MCBs (Mini Circuit Breakers) handle currents less than 100A with fixed trip characteristics, mainly for low-voltage residential applications. DC MCCBs (Molded Case Circuit Breakers) handle 10-2500A with adjustable trip settings, used in industrial and high-current applications.
8. Are DC circuit breakers directional?
Yes, DC circuit breakers are directional because DC current flows in only one direction. They are designed to interrupt current flow in the specified direction and have polarity markings. Reversing polarity can damage electrical devices and create safety hazards.
9. What are solid-state DC circuit breakers?
Solid-state DC circuit breakers use semiconductors instead of mechanical contacts for current interruption. They offer faster response times (microseconds), no arc formation, longer lifespan, and are ideal for applications requiring frequent switching or high-speed protection.
10. How do I choose between 2-pole and 4-pole DC circuit breakers?
Choose 2-pole DC breakers for single-phase applications and energy storage systems between inverters and batteries. Select 4-pole breakers for three-phase systems requiring neutral connection control, or applications needing complete circuit isolation in multi-phase installations.
Need Expert Guidance on DC Circuit Breaker Selection?
Our technical team at LETOP has over 20 years of experience in DC circuit protection. Get professional advice on selecting the right DC circuit breaker for your solar, battery, or industrial DC applications.