What is a Circuit Breaker and how does it work?

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The circuit breaker's basic function is to prevent current flow after a fault is recognized. A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current from a  short circuit or overload.

DC circuit breakers are designed for use in general DC circuits such as, solar photovoltaic systems, transportation applications and ungrounded battery supply circuits. A standard and necessary item to protect your equipment from direct current DC from overloading and short circuit. Resettable by hand after the circuit breaker trip.

A fuse would be the simplest Over Current Protection device (OCPD) in electrical circuits, a circuit breaker which is referred to as a CB by professionals, a much evolved, “higher being”. The leading difference would be that it is resettable. The fuse sacrifices itself while protecting an asset, the circuit breaker has a long life expectancy extending itself over thousands of operations. As you can imagine it is a more expensive class of OCPD. CBs are used for relatively higher currents, this would be another difference.

Circuit Breaker Construction


The circuit breaker protection system will usually consist of up to three parts. A relay that conveys the information to the switching arrangement, a sensing system that detects an overload situation, and the combination of contacts and the contact actuator. This arrangement will be generally found in power transmission and distribution networks dealing with very high currents, voltages and powers. Lower power circuit breakers have the sensing and actuation mechanisms collocated, or even built into a single unit circuit breaker.

Types of Breaker Sensing Systems


There are three types of sensing systems: electronic, magnetic and thermal. However, a combination may also be used.

• Electronic- The current level is monitored through a calibrated resistance, and operates an or electronic switch or a FET. Power-sensing ICs are designed to protect the circuit from faults. The power-sensing circuit breaker is useful for protecting batteries from overpower faults as well as short circuits and blocks the current to the load when the breaker detects an overpower fault.

• Magnetic- They are the most commonly used circuit breakers throughout the USA. The current to be sensed, or a known portion of it is passed through a coil whose magnetic effect can be used to activate the contacts and open them. Magnetic circuit breakers use a solenoid (electromagnet) whose force increases with the current. Different designs apply to electromagnetic forces in addition to those of the solenoid. When the current in the solenoid escalates beyond the rating of the circuit breaker, the solenoid's pull releases the latch and allows the contacts to open by spring action.

• Thermal- A circuit protection apparatus includes a circuit breaker coupled in series with a circuit to be protected. A thermal trip element is coupled in series with the circuit to be protected. The circuit breaker is responsive to a trip signal at the trip input for changing from a closed circuit situation to an open circuit situation removing the electrical current from the circuit to be protected. It is responsive to a calculated current/time characteristic of current flow in the circuit. It delivers the trip signal through the thermal trip element to the trip input, the current passes through a heater which deforms a bimetallic strip and releases the spring-loaded actuating mechanism.

Circuit Breaker Ratings


Circuit breakers are specified by the current and voltage they can interrupt. The speed of operation is important in special situations, ie, how many milliseconds does it stop the specified current. The time frame could be critical where flow of heavy current for an sixable time can damage the protected equipment.

Low-voltage circuit breakers have short-time current ratings and interrupting capacity  ratings that an engineer uses for their application. Interrupting capacity and short-time current ratings determine the different circuit breaker performance characteristics. Short-time rating is used to determine the ability of the circuit breaker to protect itself, other devices and to coordinate with other circuit breakers so the system will trip selectively. While the interrupting of capacity rating levels of circuit breakers are generally consistent in the electrical industry, short-time current rating levels are often inconsistent. In order to apply it properly it is important to understand the performance characteristics of the specific device.

Arc Extinction


An important issue concerning over current protection devices like circuit breakers and fuses being used at high power levels is arc extinction. When trying to interrupt the current flowing in an inductive circuit a voltage is produced that attempts to sustain the current. While the circuit breaker contacts start to distance from each other, the space between can get ionized and an electric arc can be caused. This is important to prevent, or extinguish this arc at the earliest. This is what Power breakers are designed for, and the maximum time of arc extinction at the specified voltage and current levels is an important parameter. Interrupting a direct current (DC) arc is more complex because alternating current (AC) will automatically reduce to a zero-current level two times per cycle, whereas direct current (DC) does not have to experience this.

Circuit Breakers in PV Systems

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The recommendations for the use of protection of devices including circuit breakers in PV modules against over current conditions are available in IEC 60269-6 (volume 6 of IEC 60269-6) and also UL 248-19. For CBs also, IEC has a recommendation of a 20% and 25% margin in overload current and voltage. The UL recommendations would be almost the same. These ratings are stated by the manufacturers. Where the temperatures are exceeding this value, derating has to be applied in accordance to the manufacturer’s specifications. Calculations follow the same lines as for fuses.

100% and 80% Rating

For circuit breakers, the 25% current margin is applicable which are also UL listed for 100% rating. For those listed as an 80% rating, another 25% margin should be allowed bring it to a total of a 56% margin. Here is the maths.

(1.25 x 1.25 = 1.56)

How do you size a breaker for solar panels?


Below is two standard PV breaker examples:

A maximum output current of 16A then multiplied by a 125% safety factor equals 20A. This is a standard breaker size. A maximum output current of 22A multiplied by a 125% safety factor equals to 27.5A. The next standard breaker size would be 30A.

How do I know what size circuit breaker I need?


While calculating the breaker size, you only have to divide the adjusted wattage by 240 volts to find the amperage you need for your subpanel. If the result is not a common circuit breaker size, you round up to the next higher size. It's that simple.

What FUSE do you use for solar panels?


If the  panels are in parallel, a 30-amp fuse will be required for each panel. If your panels are smaller than 50 watts, only use 12 gauge wires, and also 20 amp fuses are required.

How do you size a circuit breaker branch?


When sizing the branch-circuit conductors, the minimum branch-circuit conductor size shall have an allowable ampacity not less than 125% of the continuous load. When sizing overcurrent protection, the overcurrent device rating shall not be less than 125% of the continuous load.

How do I know what size breaker for my battery?


The battery circuit breaker sizing current=1.25 x charging current = 1.25 x 533A = 666A. The standard rating of a direct current (DC) circuit breaker is 700A. The battery short-circuit current, per published data for the battery = 14,750A. So the recommended circuit breaker in this case would be = 700A, 65VDC, 15,000 AIC.

The price to install a new electrical breaker box can vary from $500 to $2,000. The exact cost would depend on the amperage  and the number of circuits of the unit. If the main box is full, installing a subpanel would allow you to add more circuits.

How many outlets can be on a 15 amp breaker?


Say 10 outlets. In general homes have eight to 10 outlets on a 15 amp breaker but not most aren't used at the same time. Lights and outlets will normally be wired on separate branch circuits, which means that the lights will be protected by a separate circuit breaker to the one which protects the outlets.

Can I replace a 15 amp breaker with a 20 amp breaker?


It's possible, but it wouldn’t be recommended without an electrician evaluating the situation. You shouldn’t just upgrade from a 15-amp breaker to a 20-amp just because the current one is tripping. You may burn your house down via electrical fire and we don’t want that do we?

Do circuit breakers work with DC?


The direct current (DC) circuit breaker is used for the protection of electrical devices that operate with direct current. The main difference between direct current (DC) and alternating current (AC) is that in DC the voltage output will be constant but in AC it will cycle several times per second.

Since the protection mechanism is virtually the same for direct current (DC) and alternating current (AC), some models of circuit breakers will work with both types of current. Remember, it is always very important to verify that the current type of the electric supply and the circuit breaker are the same.