Circuit Breaking Rating

Table of Contents
1. What is a Circuit Breaker?
2. Standard Current Ratings
3. Rated Short Circuit Breaking Current
4. Rated Short Circuit Making Current
5. Rated Operating Sequence of Circuit Breaker
6. Rated Short Time Current
7. Rated Voltage of Circuit Breaker
8. Momentary current rating of circuit breaker
9. Which Type of Circuit Breaker Has a Higher Rating?
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The rating of a circuit breaker is determined by the duties it performs. For a complete specification, including standard ratings and various tests, one should consult the relevant standards for switches and circuit breakers. A circuit breaker is an automatic device used in electrical power systems, equipment, and devices to protect against overload and short circuits. It detects faults and interrupts the continuity of electric power flow. However, for a circuit breaker to effectively protect electrical circuits and power system components, it is necessary that the ratings of the circuit breaker match the requirements of the circuit or system it is meant to protect. Beyond its normal operation, a circuit breaker must perform three major duties under short circuit conditions:

• Breaking Capacity: The circuit breaker must be capable of interrupting the faulty section of the system. This is described as its breaking capacity.
• Making Capacity: The circuit breaker must be capable of closing onto the circuit under the maximum asymmetrical current in the current wave. This refers to its making capacity.
• Short-Time Capacity: The circuit breaker must be capable of safely carrying the fault current for a short period while another breaker clears the fault. This is known as its short-time capacity.

This article discusses in detail the various ratings of a circuit breaker to help understand how circuit breakers are selected based on the intended application. 

What is a Circuit Breaker?

A circuit breaker is a protective automatic switching device that can be used to open or close a circuit. It is used to open the circuit automatically and safely in case of faults like short circuit or overcurrent. 
Some key characteristics of a circuit breaker are:

• It can interrupt the fault current automatically and safely.
• It can be reclosed manually or automatically after the fault is cleared.
• It makes use of electromagnetic, thermal, and magnetic forces to operate depending on the type of circuit breaker.

What is the Meaning of the Rating of Circuit Breaker?

The ratings of a circuit breaker indicate its capability to safely interrupt fault currents, carry continuous load current, withstand fault currents momentarily without damage, and isolate the faulty section of the electrical network. The rating of a circuit breaker indicates its capability to handle electrical parameters safely and effectively. These ratings include current, voltage, and breaking capacity, which determine the maximum levels at which the circuit breaker can operate without sustaining damage or failing. The ratings are necessary to select an appropriate circuit breaker based on the fault levels and operating conditions of the electrical system it is meant to protect. 
Some important ratings of a circuit breaker are discussed below.

Standard Current Ratings

Circuit breakers are available in various current ratings, typically measured in amperes (A). The standard current ratings are essential for matching the circuit breaker's capacity to the expected load of the electrical circuit it protects. Common ratings include:

• 10A, 20A, 30A, 50A, 100A

Some standard current ratings as defined in Indian Standards are:

• Low voltage circuit breakers (up to 1000V): 16A, 25A, 32A, 40A, 63A, 100A, 200A, 400A
• Medium voltage circuit breakers (1kV to 52kV): 250A, 400A, 630A, 800A, 1250A, 1600A, 2000A, 2500A, 4000A
• High voltage circuit breakers (above 52kV): 1250A, 2000A, 2500A, 3000A, 4000A, 5000A

Rated Short Circuit Breaking Current

The rated short circuit breaking current is the maximum current that the circuit breaker can interrupt without damage. This rating is crucial for ensuring that the breaker can handle extreme fault conditions. The rated breaking capacity of a circuit breaker is equal to the maximum fault current it can safely interrupt. For instance, a circuit breaker with a breaking capacity of 25kA can safely interrupt a fault current of up to 25,000 amperes.

• It expresses the highest number of short circuit currents that the breakers are capable of breaking under specified conditions of transient recovery voltage and power frequency voltage.
• It is expressed in kA (RMS) at contact separation.

The breaking capacities are divided into two ways:

• Symmetrical breaking capacity of a circuit breaker 
• Asymmetrical breaking capacity of a circuit breaker

Rated symmetrical breaking current I_B = 

Rated Short Circuit Making Current

The rated short circuit making current refers to the maximum current that the circuit breaker can handle when it first closes onto a fault. This rating is typically higher than the breaking current due to the high initial inrush current. It ensures that the circuit breaker can withstand the initial impact of closing onto a short circuit without sustaining damage.
This indicates the peak value of asymmetrical short-circuit current that the circuit breaker can safely withstand when closed. It is usually 1.5 to 2.5 times the rated short-circuit breaking current.

• Making current of a circuit is the peak value of the maximum current loop during sub transient condition including the DC component when the breaker closes. 
• Symmetrical making current = 2.55∗symmetrical breaking current

Rated Operating Sequence of Circuit Breaker

The operating sequence of a circuit breaker defines the sequence and timing of opening and closing operations it can perform under specified conditions. It includes terms like O-CO (Open-Close-Open) and CO-CO (Close-Open-Close-Open), representing the breaker's ability to handle repeated operations within a short period. This rating is important for applications requiring rapid switching cycles. It specifies the number and time interval between switching operations that the circuit breaker can perform without failure or reduction in contact part life. It is usually CO-3 min-CO.

Rated Short Time Current

The rated short time current is the maximum current that the circuit breaker can carry for a short period, typically 1 to 3 seconds, without sustaining damage. This rating ensures that the breaker can handle temporary overload conditions that may occur during certain fault scenarios or operational conditions. A circuit breaker's rating defines the current limit it can safely carry for a specific period without sustaining damage. Circuit breakers do not immediately clear the short circuit current when a fault occurs in the system. There are intentional and unintentional time delays between the occurrence of the fault and the circuit breaker clearing it.
These delays occur due to the operation time of protection relays, the circuit breaker's operation time, and any intentional time delays imposed for proper coordination of the power system protection. If a circuit breaker fails to trip, the fault will be cleared by the next higher-positioned circuit breaker, resulting in a longer fault-clearing time. Therefore, after a fault, a circuit breaker must carry the short circuit current for a certain period. 
The total of all time delays should not exceed 3 seconds; thus, a circuit breaker should be capable of carrying the maximum fault current for at least this short period.

The short circuit current can have two major effects within a circuit breaker:

• Thermal Stress: Due to the high electric current, there may be significant thermal stress on the insulation and conducting parts of the circuit breaker.
• Mechanical Stress: The high short circuit current produces considerable mechanical stress on the various current-carrying parts of the circuit breaker.

A circuit breaker is designed to withstand these stresses, but it should not carry a short circuit current for longer than the specified short period. The rated short-time current of a circuit breaker is at least equal to its rated short circuit breaking current.

Rated Voltage of Circuit Breaker

The rated voltage of a circuit breaker depends on its insulation system. For systems below 400 KV, the circuit breaker is designed to withstand voltages 10% above the normal system voltage. For systems at or above 400 KV, the circuit breaker's insulation must withstand voltages 5% above the normal system voltage.
This means the rated voltage of a circuit breaker corresponds to the highest system voltage. This consideration is necessary because, during no-load or low-load conditions, the power system's voltage level can rise to the highest voltage rating of the system. As per Indian Standards, it is considered as:

• 1.1 pu of highest system voltage for systems with voltage <1000V
• 1.05 pu of highest system voltage for systems with voltage > 1000V

Some voltage ratings as per IS are 0.415kV, 3.3kV, 6.6kV, 11kV, 33kV, 66kV, 110kV, 220kV etc. Higher voltage CBs may have SF6 or vacuum insulation.

Momentary current rating of circuit breaker

It indicates the peak let-through current in kA that the circuit breaker can withstand when tripping under fault conditions. It is important from an arc interruption and contact erosion point of view. For air breakers, it is around 5-10 times the rated current, and for oil/SF6 circuit breakers it is around 15 times the rated current.
Low voltage circuit breakers have a rated voltage of less than 1000V. They come with standard current ratings as defined in IS/IEC and have thermal-magnetic or microcontroller-based trips.

Different Types of Circuit Breakers and Their Ratings

Different types of circuit breakers and their typical ratings:
Air Circuit Breakers (ACBs)

• Used for medium voltages up to 36kV
• Rated breaking current: 16kA to 50kA
• Rated making current: 40kA to 125kA

Vacuum Circuit Breakers (VCBs)

• Used from 36kV up to 220kV
• Rated breaking current: 16kA to 63kA
• Rated making current: 40kA to 160kA

Sulfur Hexafluoride (SF6) Circuit Breakers

• Used above 72.5kV up to 765kV
• Rated breaking current: 16kA to 63kA
• Rated making current: 40kA to 160kA

Oil Circuit Breakers (OCBs)

• Used up to 170kV
• Rated breaking current: 16kA to 50kA
• Rated making current: 40kA to 125kA

Types of Circuit Breakers on the Basis of Voltage Rating

So basically there are four types of circuit breakers based on voltage rating:

Low Voltage Circuit Breakers

• Applications: Residential and commercial installations
• Rated Voltage: Less than 1000V
• Common Current Ratings: 10A, 20A, 50A
• Breaking Capacity: Typically up to 25kA

Medium Voltage Circuit Breakers

• Applications: Industrial and utility sectors
• Rated Voltage: 1000V to 72kV
• Common Current Ratings: 630A, 1250A
• Breaking Capacity: Typically up to 40kA

High Voltage Circuit Breakers

• Applications: Power transmission and distribution
• Rated Voltage: Above 72kV
• Common Current Ratings: 1200A, 2000A
• Breaking Capacity: Typically up to 63kA

Special-Purpose Circuit Breakers

• Applications: Specific industrial needs
• Rated Voltage and Current: Vary widely based on application
  Examples: Vacuum circuit breakers, gas-insulated breakers

Which Type of Circuit Breaker Has a Higher Rating?

Among the various types, high-voltage circuit breakers generally have the highest ratings due to their application in power transmission and distribution systems. These breakers are designed to handle significant fault currents and high operating voltages, making them suitable for large-scale electrical infrastructure.

Rating of Circuit Breaker Depends Upon

The rating of a circuit breaker depends upon several factors, including:

• System Voltage: The operating voltage of the electrical system
• Load Current: The expected current load of the circuit
• Fault Current: The potential fault current that may occur
• Application Requirements: Specific needs such as rapid switching or high-reliability

Conclusion

Understanding the rating of a circuit breaker is fundamental for ensuring the safe and efficient operation of electrical systems. To summarize, the key factors determining the rating of a circuit breaker are its interrupting capacity, load current carrying ability, withstand capacity, and insulation level. Proper coordination between ratings of circuit breakers installed at different voltage levels ensures reliable protection of equipment as well as the safety of operating personnel. Understanding circuit breaker ratings is important for engineers to select equipment matching electrical requirements.