The ratings of a circuit breaker are determined based on the duties it is designed to perform. For a comprehensive understanding of the specifications, standard ratings, and various tests of switches and circuit breakers should be consulted.
What is a Circuit Breaker?
A circuit breaker is an electrical switch designed to open the circuit during fault conditions. It receives a trip signal from protection relays, which monitor current through current transformers (CTs).
Standards for Circuit Breaker Ratings
Circuit breaker ratings are governed by international standards such as:
- IEC 60947-2 – for Low-Voltage Circuit Breakers
- IEC 62271-100 – for High-Voltage Circuit Breakers
These standards ensure safe operation, consistent performance, and global compliance.
Standard Circuit Breaker Ratings
To ensure reliable performance and proper selection, circuit breakers are rated based on several key parameters, including breaking capacity, making capacity, short-time withstand current, and others. The key rating terms are listed below and explained in detail afterward:
- The number of poles
- Rated voltage
- Operating Voltage
- Rated current
- Rated frequency
- Operating duty
- Breaking Capacity
- Making Capacity
- Rated Short-Time Current
Number of Poles
The number of poles refers to the number of separate circuits that a circuit breaker can control. In three-phase systems, circuit breakers are typically 3-pole or 4-pole. A 3-pole breaker interrupts all three phases simultaneously, while a 4-pole breaker also interrupts the neutral wire along with the three phases.
Rated Voltage
The rated voltage of a circuit breaker is the highest RMS voltage that it is designed to handle, above the nominal voltage. This voltage is represented in kVrms and is measured phase-to-phase for a three-phase circuit. It sets the upper limit for operation.
The rated voltage also depends on the insulation system of the circuit breaker. For systems operating below 400 kV, breakers are designed to handle up to 10% above the nominal voltage. For 400 kV and above, the insulation is designed to withstand 5% above the normal voltage.
This ensures that the breaker can operate safely even when the system voltage rises—such as during light-load or no-load conditions, when the actual voltage may reach the system’s maximum allowable limit.
As per Indian Standards (IS):
- For systems < 1000V, rated voltage = 1.1 pu of the highest system voltage.
- For systems > 1000V, rated voltage = 1.05 pu of the highest system voltage.
Some common rated voltages in practice include:
0.415 kV, 3.3 kV, 6.6 kV, 11 kV, 33 kV, 66 kV, 110 kV, and 220 kV.
At higher voltages, circuit breakers often use SF₆ gas or vacuum insulation to maintain dielectric strength and ensure reliable performance.
Operating Voltage
The operating voltage of a circuit breaker refers to the voltage level at which the breaker is intended to function under normal conditions. It must be suitable for the system voltage to ensure reliable operation and safe interruption of the circuit during fault or maintenance conditions.
While rated voltage defines the maximum voltage a breaker can safely handle (including overvoltage margins), the operating voltage is typically equal to or slightly below the system’s nominal voltage. This ensures the breaker functions efficiently without overstressing its insulation system or internal components.
For example, in a system with a rated voltage of 11 kV, the operating voltage might normally be around 10.5 to 11 kV, depending on load conditions. Maintaining the correct operating voltage within this range ensures proper arc quenching, mechanical endurance, and longevity of the circuit breaker.
Rated Current
The rated current of a circuit breaker is the RMS value of the current that it can continuously carry, under specified conditions, at rated frequency and voltage.
Circuit breakers are available in various current ratings, typically measured in amperes (A), to suit different applications and load requirements.
These standard current ratings play a crucial role in ensuring that the circuit breaker matches the expected electrical load. Common ratings include 10A, 20A, 30A, 50A, and 100A for domestic and light commercial use.
As per Indian Standards, current ratings vary based on the voltage class:
- 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
For example, an 800A circuit breaker is commonly used in 33kV substation feeder panels to safely handle medium-voltage power distribution.
Choosing the right current rating is essential not only for overload protection but also for maintaining the long-term reliability and safety of the electrical system.
Rated Frequency
The rated frequency of a circuit breaker is its designed operating frequency. For most regions, including India and Europe, the standard frequency is 50 Hz. However, in countries like the United States, Canada, and parts of South America, circuit breakers are commonly designed for 60 Hz systems.
Selecting a breaker that matches the system frequency is essential for proper operation, timing, and coordination with other equipment in the electrical network.
Operating Duty
The circuit breaker duty requires a specific number of operations at set intervals. The operating sequence refers to the opening and closing of the circuit breaker contacts.
In simple terms, the operating duty defines how many times a circuit breaker can open and close in a defined sequence under specified conditions. Common sequences include:
- O–CO (Open–Close–Open)
- CO–3 min–CO (Close–Open, wait 3 minutes, then Close–Open again)
- O–0.3 sec–CO–3 min–CO, where the breaker performs an open operation, re-closes after 0.3 seconds, then repeats the cycle after a 3-minute pause.
This rating is essential for applications requiring rapid or multiple switching cycles. It ensures the breaker can perform safely and reliably without compromising the life of its contacts or internal components.
Breaking Capacity
The term breaking capacity of circuit breaker refers to the maximum short-circuit current that it can interrupt under specific conditions of transient recovery voltage and power frequency voltage. This capacity is measured in kA RMS at the point of contact separation.
There are two types of breaking capacities.
Symmetrical Breaking Capacity of a Circuit Breaker
This is the RMS value of the AC component of the fault current without any DC offset. It represents the breaking capacity under steady-state conditions.
Asymmetrical Breaking Capacity of a Circuit Breaker
This includes both the AC and DC components of the fault current. It usually occurs in the first cycle after a fault and represents a more severe, peak-breaking condition.
Making Capacity
It is possible that the circuit breaker may be closed under short circuit conditions. The making capacity of the circuit breaker refers to its ability to withstand electromagnetic forces that are directly proportional to the square of the peak value of current.
The making current is the peak value of the first current wave (including the DC component) that flows during the first cycle after the circuit breaker closes onto a short circuit. This current causes a significant mechanical and thermal stress, and the circuit breaker must be able to handle it safely.
The symmetrical making current is higher than the symmetrical breaking current and is expressed as:
Symmetrical making current = 2.55 × symmetrical breaking current
Read more: For detailed explanation, formula derivation, and design factors, see our full guide on Making Capacity of Circuit Breaker: Meaning, Formula, and Design Factors.
Rated Short-Time Current
The rated short-time current of a circuit breaker is the maximum RMS value of current that the breaker can safely withstand in the closed position for a specified short time interval, under defined conditions, without suffering damage. This rating is typically expressed in kA for 1 or 4 seconds and is based on the thermal withstand capacity of the circuit breaker.
During fault conditions, circuit breakers must carry high fault current for a brief period before tripping. This is because of time delays introduced by:
- The operation time of protection relays
- The mechanical response time of the circuit breaker
- Any intentional delay settings for protection coordination
These delays can extend fault-clearing time up to 3 seconds. Hence, a circuit breaker must be designed to carry the maximum short-circuit current during this interval without damage.
This temporary current flow can cause the following stresses:
- Thermal Stress: High current generates intense heat, putting stress on conductive elements and insulation.
- Mechanical Stress: Large electromagnetic forces act on internal components, which may result in structural strain.
To ensure reliability, the rated short-time current is typically equal to or greater than the circuit breaker’s rated breaking capacity. It confirms the breaker’s ability to withstand these stresses without compromising performance or lifespan.
Summary of Circuit Breaker Ratings
The table below provides a quick reference to the key ratings and parameters of a circuit breaker discussed in this article.
| Rating Parameter | Description |
|---|---|
| Number of Poles | Indicates how many separate circuits a breaker can interrupt (e.g., 3-pole or 4-pole). |
| Rated Voltage | Highest RMS voltage the breaker can handle above nominal voltage, measured phase-to-phase. |
| Operating Voltage | The voltage level at which the breaker normally operates in the system. |
| Rated Current | Maximum continuous RMS current the breaker can carry under rated conditions. |
| Rated Frequency | Frequency for which the breaker is designed (usually 50 Hz). |
| Operating Duty | Defined number of open/close operations the breaker must perform under specified intervals. |
| Breaking Capacity | Maximum fault current the breaker can interrupt under defined conditions. |
| Symmetrical Breaking Capacity | RMS value of fault current (AC only) without DC offset — steady state. |
| Asymmetrical Breaking Capacity | Peak fault current including AC and DC components — typically in the first cycle. |
| Making Capacity | Peak current the breaker can close onto under short circuit conditions, including DC component. |
| Rated Short-Time Current | Maximum RMS current the breaker can withstand while closed for a short interval (e.g., 1 or 4 s). |
Example: Typical Circuit Breaker Ratings
The following is an example of a medium-voltage circuit breaker used in an industrial application. It illustrates how various rating parameters are specified:
- Number of Poles: 3
- Rated Voltage: 11 kV
- Operating Voltage: 10.5 kV
- Rated Current: 1250 A
- Rated Frequency: 50 Hz
- Operating Duty: O–0.3 sec–CO–3 min–CO
- Breaking Capacity: 25 kA (Symmetrical)
- Making Capacity: 62.5 kA (Peak)
- Rated Short-Time Current: 25 kA for 3 seconds
Different Types of Circuit Breakers and Their Ratings
| Type | Voltage Range | Rated Breaking Current | Rated Making Current |
|---|---|---|---|
| Air Circuit Breaker (ACB) | Up to 36kV | 16kA to 50kA | 40kA to 125kA |
| Vacuum Circuit Breaker (VCB) | 36kV to 220kV | 16kA to 63kA | 40kA to 160kA |
| SF6 Circuit Breaker | 72.5kV to 765kV | 16kA to 63kA | 40kA to 160kA |
| Oil Circuit Breaker (OCB) | Up to 170kV | 16kA to 50kA | 40kA to 125kA |
Types of Circuit Breakers Based on Voltage Rating
In addition to individual parameters like current, voltage, and breaking capacity, circuit breakers are also classified based on the voltage levels they are designed to operate in. This classification helps in selecting the right type of breaker for residential, commercial, industrial, or transmission systems.
Low Voltage Circuit Breakers
- Rated Voltage: Less than 1000V
- Common Current Ratings: 10A, 20A, 50A
- Breaking Capacity: Typically up to 25kA
- Applications: Residential and small commercial installations
Medium Voltage Circuit Breakers
- Rated Voltage: 1000V to 72kV
- Common Current Ratings: 630A, 1250A
- Breaking Capacity: Typically up to 40kA
- Applications: Industrial plants and utility substations
High Voltage Circuit Breakers
- Rated Voltage: Above 72kV
- Common Current Ratings: 1200A, 2000A
- Breaking Capacity: Typically up to 63kA
- Applications: Transmission and large-scale power distribution networks
Special-Purpose Circuit Breakers
- Rated Voltage & Current: Vary as per application
- Examples: Vacuum circuit breakers, gas-insulated SF6 circuit breakers
- Applications: Specific industrial needs requiring compact, high-performance designs
Conclusion
Understanding the various ratings of a circuit breaker is essential for ensuring safety, reliability, and compatibility within electrical systems. Parameters like breaking capacity, making capacity, rated current, and short-time withstand current define how a breaker performs under normal and fault conditions.
By adhering to international standards and evaluating each rating carefully, engineers and professionals can select the right circuit breaker for any application—whether for low-voltage or high-voltage environments.
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