Circuit Breaker - Working, Types, and Standard Ratings

A circuit breaker is a switchgear device used in electrical system to interrupt the flow of electric current in the circuit. It is used for either switching or protection purposes.

In this article, we will learn about circuit breakers, their types, characteristics, standard ratings, and applications. So, let’s start with the basics of circuit breaker.

What is a Circuit Breaker?

A circuit breaker is an automatic electrical switching device used in power system to interrupt the current in case of faults or abnormal conditions.

A circuit breaker is similar to a normal electrical switch but it can also perform switching in case of fault to provide protection to the system.

A typical circuit breaker performs the following two important functions :

• Switching of electric power i.e., turn on and off the power.
• Protect the circuit from faults and abnormal conditions like overcurrent or short circuit.

Construction of Circuit Breaker

The construction of a typical circuit breaker is shown in the following figure.

It consists of two main parts namely,

• Fixed and moving contacts
• Extinction medium

The fixed and moving contacts are enclosed within an interrupter. An operating mechanism is provided to open or close the fixed and moving contacts. When these two contacts separate from each other, an arc is produced between them. We need to extinguish this arc to protect the contacts from being damaged. For this, a suitable arc extinction medium is used.

Circuit breakers are classified into various types depending on the medium used for arc extinction.

Operation of Circuit Breaker

In this section, let's focus on the operation of a circuit breaker under normal operating conditions and fault conditions.

• During normal operating conditions, the circuit breaker can be opened or closed manually for switching and maintenance purposes.
• During fault or abnormal conditions, a protective relay detects the fault and trigger the trip circuit, which opens the circuit breaker.

Therefore, a circuit breaker can have any one of the two operating positions namely, open and closed.

When the circuit breaker starts the opening operation, the fixed and moving contacts open and an arc is produced between them. This arc is quenched by means of a suitable medium like air. The open operation completes when the arc is completely extinguished and the circuit current becomes zero.

Fault Clearing Process in Circuit Breaker

Let us now understand how a circuit breaker clears a fault. Here is the step-by-step explanation of the fault clearing process :

Step (1) - When a fault occurs, the circuit impedance becomes low and the circuit current increases. This increased current actuates the relay and the relay contacts are closed.

Step (2) - The closed relay contacts complete the trip circuit of the circuit breaker and energize its trip coil.

Step (3) - The operating mechanism of the circuit breaker starts operating for the opening operation and separates contacts of the circuit breaker.

Step (4) - An arc is produced between the contacts of the breaker. This arc is then quenched by a suitable medium like air or oil.

Step (5) - The process of fault clearing completes when the arc is extinguished and the circuit current reaches to a zero value.

Standard Ratings of Circuit Breakers

The standard ratings of a circuit breaker are determined using its characteristics. The following some important standard ratings of circuit breakers -

Rated characteristics that are to be given for all circuit breakers -

Rated voltage - The maximum value of voltage at which the circuit breaker can operate safely.

Rated current - The maximum value of current that the circuit breaker can conduct continuously without tripping.

Rated insulation level - It is the maximum value of electric stress that the insulation can handle without breakdown.

Rated frequency - It is the value of frequency of the alternating current supply that the circuit breaker is designed for.

Rated short-circuit breaking capacity - It is the maximum value of short-circuit or fault current that the circuit breaker can interrupt without breakdown.

Rated transient recovery voltage for terminal faults - The maximum value of voltage that can appear across the circuit breaker contacts after the fault current has been interrupted.

Rated short-circuit making capacity - It is the maximum value of current that the circuit breaker can conduct without being damage at the instant of closing and considering there is a short-circuit fault in the system.

Rated short time withstand current - It the RMS value of current that the circuit breaker can conduct for a specified short time without tripping.

Rated operating sequence - It is the specified sequence of operation of the circuit breaker. An example of rated operating sequence of a circuit breaker is O-0.4sec-CO-4min-CO, here O and C represent opening and closing respectively, 0.4sec and 4min represent the time delays.

Rated characteristics to be given in specific cases -

Rated characteristics for short-line fault - These are some specific ratings of a circuit breaker related to its performance under short-circuit conditions. Typical short-line fault characteristics include rated short-circuit breaking capacity, rated transient recovery voltage, rated short-circuit making capacity, rated short-circuit duration, etc.

Rated line charging breaking current - It is defined as the maximum value of line charging current that the circuit breaker can break at its rated voltage.

Rated supply voltage of closing and opening devices - It is the rated supply voltage that required for operating the closing and opening mechanism of the circuit breaker.

Rated supply frequency of closing and opening devices - It is the rated frequency of supply voltage used for closing and opening the circuit breaker contacts.

Rated pressure of compressed gas for operation and interruption - It is the specified pressure of compressed gas used for successful operation and interruption of the circuit breaker.

Optional rated characteristics of circuit breakers:

Rated out of phase breaking current - It is the maximum value of unequal current that the circuit breaker can carry and interrupt safely.

Rated cable charging breaking current - It is the maximum value of cable charging current that the circuit breaker can interrupt at its rated voltage.

Rated single capacitor bank breaking current - It is the maximum value of capacitor bank current that the circuit breaker can interrupt at its rated voltage with no capacitor bank is connected at the supply end of the breaker.

Rated small inductive breaking current - It is defined as the maximum value of current that the circuit breaker can interrupt when a small inductive load is disconnected.

Rated supply voltage of auxiliary circuit - It is the maximum voltage level at which the auxiliary circuit or control circuit of the circuit breaker can operate safely.

Rated supply frequency of auxiliary circuit - It is the specified frequency of supply voltage required for safe and reliable operation of auxiliary circuit of the breaker.

Types of Circuit Breaker and Their Specifications

Circuit breakers are mainly classified depending on the medium used for arc extinction.

The following table highlights all the common types of circuit breakers and their voltage ratings −

Conclusion

A circuit breaker is an automatic electrical switch that can be used for switching and protection purposes. Circuit breakers can be classified into different types depending on the medium of arc extinction used.

The most common types of circuit breakers used in power systems are air circuit breaker, vacuum circuit breaker, oil circuit breaker, and $\mathrm{SF_6}$ circuit breaker.

Each type of circuit has its own voltage capacity and breaker capacity depending on the insulation medium used. For example, an $\mathrm{SF_6}$ circuit breaker can handle a voltage of up to 765 kV. After going through this article, you will be able to understand the fundamentals of circuit breakers.

FAQs Related to Circuit Breakers

The following are some commonly asked questions related to circuit breakers and their answers.

1. Why do we use circuit breakers?

We use a circuit breaker in electrical power systems for the following two purposes :

• During operating conditions, circuit breakers are used for switching power supply.
• During fault or abnormal conditions, circuit breakers are used to isolate the faulty part from the healthy section.

2. Why do we lock circuit breakers?

The major reasons why we should lock a circuit breaker is to prevent unauthorized or accidental operation of the circuit breaker which can cause a serious consequence.

3. How do circuit breakers keep you safe?

A circuit breaker keeps us safe by providing the following common protections and operations :

• Short-circuit protection
• Over-current or over-voltage protection
• Isolation during maintenance and repair

4. Where is ACB used?

ACB or Air Circuit Breaker is used in low voltage applications like distribution panels, motor control circuits, power control circuits, etc.

5. What is the maximum rating of ACB?

An Air Circuit Breaker (ACB) can have a maximum rating of 6300 A.