When selecting a circuit breaker for any electrical installation engineers must evaluate several current ratings. One of the most important ratings is the rated peak withstand current. This parameter defines the maximum instantaneous current that a circuit breaker can handle without getting damaged.
In this technical guide, we will explore everything about rated peak withstand current. We will cover its definition, calculation methods, standards, testing procedures, and practical applications.
1. What is Rated Peak Withstand Current?
Rated peak withstand current is the maximum value of the first major peak of fault current that a circuit breaker can withstand in its closed position. When a short circuit occurs in an electrical system, the fault current rises rapidly. This current reaches its peak value during the first half cycle of the fault. The rated peak withstand current tells us how much peak current the circuit breaker can tolerate without any mechanical or electrical damage.
This rating is expressed in kiloamperes (kA). For example, if a circuit breaker has a rated peak withstand current of 105 kA, it means the breaker can handle a peak fault current up to 105 kA without failure.
2. The Physics Behind Peak Current
To understand rated peak withstand current, we need to look at what happens during a fault. When a short circuit occurs, the fault current has two components. These are the symmetrical AC component and the asymmetrical DC component.
The AC component is the steady-state short circuit current that oscillates around zero. The DC component is a decaying exponential current that appears due to the stored magnetic energy in the system inductance at the moment of fault initiation.
The combination of these two components creates an asymmetrical waveform. The first peak of this asymmetrical current is much higher than the steady-state AC peak. This first peak is what the rated peak withstand current addresses.
3. How Peak Current Develops
Let us take an example to make this concept clearer. Suppose a power system has a prospective short circuit current of 40 kA RMS. If we calculate only the AC peak, it would be 40 × √2 = 56.57 kA. However, the actual first peak will be much higher due to the DC component.
The worst-case scenario occurs when the fault happens at the instant when the voltage is crossing zero. At this moment, the DC component is at its maximum. This causes the first peak current to reach values between 1.8 to 2.5 times the RMS short circuit current depending on the X/R ratio of the system.
4. Relationship with Rated Short Circuit Breaking Current
Many students confuse rated peak withstand current with rated short circuit breaking current. These are two different parameters that serve different purposes.
The rated short circuit breaking current is the maximum RMS value of short circuit current that the circuit breaker can interrupt. This is measured as the AC component of the fault current at the instant of contact separation.
The rated peak withstand current is the maximum instantaneous peak value that the breaker can withstand while remaining closed. This value is always higher than the rated short circuit breaking current because it accounts for the asymmetry in the fault current.
5. The Peak Factor
The relationship between rated short circuit breaking current and rated peak withstand current is defined by the peak factor. The peak factor is a multiplier that converts RMS current to peak current.
According to IEC 62271-100, the standard peak factors are:
- Peak factor of 2.5 for rated short circuit breaking currents up to 25 kA
- Peak factor of 2.6 for rated short circuit breaking currents above 25 kA and up to 50 kA
- Peak factor of 2.7 for rated short circuit breaking currents above 50 kA
These peak factors account for the DC component in the asymmetrical fault current.
6. Calculation of Rated Peak Withstand Current
The calculation of rated peak withstand current follows a straightforward formula:
Ip = n × Isc
Where:
- Ip = Rated peak withstand current
- n = Peak factor (2.5, 2.6, or 2.7 as per IEC standards)
- Isc = Rated short circuit breaking current
6.1 Example Calculation 1
A circuit breaker has a rated short circuit breaking current of 20 kA. What is its rated peak withstand current?
Since 20 kA is below 25 kA, we use a peak factor of 2.5.
Ip = 2.5 × 20 = 50 kA
The rated peak withstand current is 50 kA.
6.2 Example Calculation 2
A circuit breaker has a rated short circuit breaking current of 40 kA. Calculate its rated peak withstand current.
Since 40 kA falls between 25 kA and 50 kA, we use a peak factor of 2.6.
Ip = 2.6 × 40 = 104 kA
The rated peak withstand current is 104 kA.
6.3 Example Calculation 3
A circuit breaker has a rated short circuit breaking current of 63 kA. Find its rated peak withstand current.
Since 63 kA is above 50 kA, we use a peak factor of 2.7.
Ip = 2.7 × 63 = 170.1 kA
The rated peak withstand current is 170.1 kA.
7. X/R Ratio and Its Effect on Peak Current
The peak factor depends on the X/R ratio of the power system. The X/R ratio is the ratio of system reactance to system resistance. Higher X/R ratios result in higher DC components and therefore higher peak currents.
In high voltage transmission systems, the X/R ratio can be as high as 20 or more. In low voltage distribution systems, the X/R ratio is often between 1 and 10.
The time constant of the DC component is given by:
τ = L/R = X/(2πf × R)
Where f is the system frequency. A longer time constant means the DC component decays slowly and the peak current is higher.
8. International Standards for Rated Peak Withstand Current
Several international standards govern the specifications and testing of rated peak withstand current.
8.1 IEC Standards
IEC 62271-100 is the primary standard for high voltage circuit breakers. This standard defines the rated peak withstand current as part of the short circuit performance requirements. It specifies test procedures and acceptance criteria for type testing.
IEC 60947-2 covers low voltage circuit breakers. This standard uses the term “rated short time withstand current” along with peak current values.
8.2 IEEE Standards
IEEE C37.04 provides rating structure for AC high voltage circuit breakers. This standard also defines peak current requirements similar to IEC standards.
IEEE C37.09 covers test procedures for AC high voltage circuit breakers. It includes detailed methods for peak current testing.
8.3 ANSI Standards
ANSI standards use terms like “closing and latching capability” which is equivalent to the rated peak withstand current concept. ANSI C37.06 lists standard ratings for AC high voltage circuit breakers including peak current values.
9. Testing of Rated Peak Withstand Current
Circuit breakers undergo rigorous testing to verify their peak current withstand capability. The test is called the peak withstand current test or making capacity test.
9.1 Test Setup
The test circuit must be capable of producing the required peak current. This requires high power test laboratories with sufficient short circuit power. The test circuit includes a power source, impedance elements to adjust fault current, and measuring equipment.
9.2 Test Procedure
- The circuit breaker is closed onto a fault with the test circuit energized
- The fault is timed to occur at the worst-case instant (voltage zero crossing)
- The peak current during the first half cycle is measured
- The breaker must remain closed and intact after the test
- The breaker must then be able to carry rated current without overheating
9.3 Acceptance Criteria
After the peak withstand current test, the circuit breaker must show no visible damage. The contacts must not weld together. The operating mechanism must function properly. The insulation must maintain its properties.
10. Selection Guidelines for Peak Withstand Current
When selecting a circuit breaker, the rated peak withstand current must exceed the prospective peak fault current of the system.
Step 1: Calculate Prospective Short Circuit Current
First, calculate the three-phase symmetrical short circuit current at the installation point. This can be done using fault analysis software or manual calculations.
Step 2: Determine System X/R Ratio
Find the X/R ratio of the power system at the installation point. This information may come from the utility or can be calculated from system impedance data.
Step 3: Calculate Prospective Peak Current
Using the X/R ratio, calculate the prospective peak current:
Ipeak = k × √2 × Isc
Where k is a factor depending on X/R ratio. For X/R = 10, k is approximately 1.7. For X/R = 20, k is approximately 1.8.
Step 4: Apply Safety Margin
Add a safety margin of 10% to 20% to account for system changes and calculation uncertainties.
Step 5: Select Circuit Breaker
Choose a circuit breaker with rated peak withstand current higher than the calculated value.
10.1 Practical Example of Selection
A 33 kV substation has a prospective three-phase short circuit current of 31.5 kA. The system X/R ratio is 15. Select an appropriate circuit breaker.
Step 1: Isc = 31.5 kA (given)
Step 2: X/R = 15 (given)
Step 3: For X/R = 15, the peak factor is approximately 1.85
Ipeak = 1.85 × √2 × 31.5 = 82.4 kA
Step 4: With 15% safety margin
Ipeak(design) = 82.4 × 1.15 = 94.76 kA
Step 5: Select a circuit breaker with rated peak withstand current of at least 95 kA. A standard circuit breaker with 40 kA breaking capacity has a rated peak withstand current of 104 kA (40 × 2.6). This would be suitable.
11. Difference Between Peak Withstand Current and Short Time Withstand Current
These two parameters are sometimes confused. Let us clarify the difference.
Rated Peak Withstand Current:
- Measured as an instantaneous peak value
- Applies to the first peak of fault current (first half cycle)
- Primarily tests mechanical strength
- Duration is fraction of a cycle
Rated Short Time Withstand Current:
- Measured as an RMS value
- Applies for a specified duration (1 second or 3 seconds)
- Primarily tests thermal capability
- Duration is much longer
12. Peak Withstand Current in Different Voltage Classes
The rated peak withstand current varies with voltage class and application.
12.1 Low Voltage Circuit Breakers (up to 1000V)
Low voltage molded case circuit breakers (MCCBs) have peak withstand current ratings from 10 kA to 200 kA. Air circuit breakers (ACBs) can have ratings up to 254 kA or higher.
12.1 Medium Voltage Circuit Breakers (1 kV to 52 kV)
Medium voltage vacuum and SF6 circuit breakers have peak withstand current ratings from 50 kA to 200 kA. These are used in industrial plants, utility substations, and commercial buildings.
12.3 High Voltage Circuit Breakers (52 kV and above)
High voltage circuit breakers for transmission systems have peak withstand current ratings from 100 kA to 250 kA or higher. These are massive devices with robust mechanical construction.
13. Conclusion
Rated peak withstand current is a key specification for circuit breakers. It defines the maximum instantaneous fault current that the breaker can handle in its closed position. This rating addresses the mechanical stresses caused by electromagnetic forces during the first peak of fault current.
When selecting circuit breakers, engineers must calculate the prospective peak fault current and select breakers with adequate ratings. The peak factor specified in standards accounts for the asymmetry in fault currents due to the DC component.
14 Frequently Asked Questions (FAQs)
The rated short circuit breaking current is the maximum RMS current that a circuit breaker can interrupt. The rated peak withstand current is the maximum instantaneous peak current the breaker can withstand while closed. The peak value is always higher than the breaking current due to the asymmetrical DC component in fault current.
Higher fault current levels are associated with systems having higher X/R ratios. Higher X/R ratios result in larger DC components and higher peak currents relative to RMS values. Therefore, standards specify higher peak factors (2.6 or 2.7) for higher breaking currents to account for this effect.
The rated peak withstand current is listed on the circuit breaker nameplate and in manufacturer datasheets. It may be shown as “Ip” or “peak making current” or “making capacity.” You can also calculate it by multiplying the rated short circuit breaking current by the appropriate peak factor (2.5, 2.6, or 2.7).
No. The peak withstand current test is a type test performed on representative samples during product development. Routine tests performed on each manufactured unit do not include this test.
The rated peak withstand current as defined in IEC and IEEE standards applies to AC systems. In DC systems, there is no AC component and the fault current behavior is different. DC circuit breakers have separate specifications for fault current withstand capability.
No. Using a circuit breaker with insufficient peak current rating is unsafe. The breaker may fail during a fault condition. This could result in equipment damage, fire, and safety hazards.
No. These are different ratings. Rated peak withstand current is an instantaneous peak value for the first half cycle. Short time current rating is an RMS value for a duration of 1 or 3 seconds.