Circuit breakers are designed to protect electrical systems from damage during fault conditions. When a short circuit occurs in an electrical network, the current flowing through the circuit can increase to extremely high levels within milliseconds. The circuit breaker must be able to handle this enormous current for a brief period before it trips and disconnects the faulty circuit. This ability is defined by a parameter called the rated short-time withstand current.
In this technical guide, we will discuss in detail about rated short-time withstand current, its importance, standards, relationship with different current ratings and testing of rated short-time withstand current
1. What is Rated Short-Time Withstand Current?
The rated short-time withstand current (Icw) is the maximum current that a circuit breaker can carry in its closed position for a specified short duration without suffering any damage. During this time, the circuit breaker remains closed and conducts the fault current through its contacts. The breaker does not trip during this period. It simply withstands the thermal and electromagnetic stresses caused by the high current flow.
This rating is expressed in kiloamperes (kA) and is associated with a specific time duration. The standard durations are usually 0.5 seconds, 1 second, or 3 seconds.
For example, if a circuit breaker has Icw = 50 kA for 1 second, it means the breaker can safely carry 50,000 amperes for 1 second without any damage to its internal components.
2. Why is This Rating Important?
In many industrial and commercial electrical installations, system designers require selective coordination between protective devices. When a fault occurs downstream, the upstream circuit breaker should not trip immediately. Instead, it should wait for a short time to allow the downstream breaker to clear the fault first. This waiting period is called the delay time or intentional time delay.
During this delay period, the upstream circuit breaker must carry the full fault current without tripping and without getting damaged. The rated short-time withstand current defines how much current the breaker can handle during this delay period. If the fault current exceeds the Icw rating of the breaker, the breaker may suffer thermal damage to its contacts and internal insulation.
This selectivity coordination helps in maintaining power supply to healthy sections of the electrical network while only isolating the faulty section.
3. Standards Governing Rated Short-Time Withstand Current
Several international standards define the requirements and testing procedures for rated short-time withstand current. The most widely referenced standards include:
- IEC 60947-2: This is the primary international standard for low-voltage switchgear and controlgear. It specifies the requirements for circuit breakers including the short-time withstand current rating and associated test procedures.
- IEEE C37 Series: These standards are commonly used in North America and define similar requirements for medium and high-voltage circuit breakers.
- ANSI/NEMA Standards: These American standards also address the short-time withstand current requirements for industrial circuit breakers.
- According to IEC 60947-2, the standard time durations for short-time withstand current are 0.05s, 0.1s, 0.25s, 0.5s, 1s, and 3s. The most common duration used in practice is 1 second.
4. Relationship Between Icw and Time Duration
The rated short-time withstand current has an inverse relationship with time. A circuit breaker that can withstand 65 kA for 1 second may be able to withstand a higher current for a shorter duration. This relationship follows the thermal energy principle where the heat generated in the conductor is proportional to I²t (current squared multiplied by time).
For example:
- A circuit breaker rated at 50 kA for 1 second
- The same breaker might handle approximately 70 kA for 0.5 seconds
- For 3 seconds, the rating might drop to around 40 kA
Manufacturers provide these values in their technical datasheets. Engineers must check these specifications carefully when designing selective protection schemes.
5. Difference Between Icw and Other Current Ratings
Circuit breakers have several current ratings. It is important to distinguish between them to avoid confusion.
5.1 Rated Short-Time Withstand Current (Icw) vs Rated Ultimate Breaking Capacity (Icu)
Icu is the maximum fault current that a circuit breaker can interrupt and then remain functional afterward. The breaker trips and opens its contacts to interrupt this current.
Icw is the current that the breaker can carry without tripping. The breaker stays closed during this period.
Both ratings may have the same numerical value for some circuit breakers. However, they represent different capabilities.
5.2 Rated Short-Time Withstand Current (Icw) vs Making Capacity (Icm)
Icm is the maximum current that a circuit breaker can close onto during a fault condition. This is the peak value of the first current loop during a short circuit.
Icw is an RMS (root mean square) value that the breaker can carry for a specified duration.
Icm is usually 2.2 times the Icu value for AC circuits due to the asymmetrical nature of the fault current during the first few cycles.
6. Testing for Short-Time Withstand Current
Circuit breakers undergo rigorous testing to verify their Icw rating. The test procedure according to IEC 60947-2 involves the following steps:
- The circuit breaker is connected in a test circuit that can generate the required fault current level.
- The breaker is closed and the test current is applied for the rated duration (usually 1 second).
- During the test, the breaker must remain closed and carry the current without tripping.
- After the test, the circuit breaker is inspected for any signs of damage.
- The breaker must still be able to perform its normal functions including carrying rated current and interrupting fault currents.
- Multiple tests may be performed to verify consistency in performance.
The test current is maintained at the specified RMS value throughout the test duration. The circuit breaker must not show any deterioration in its dielectric properties or mechanical operation after the test.
7. Thermal and Electromagnetic Effects During Short-Time Current Flow
When a high current flows through a circuit breaker for a short time, two main effects occur.
7.1 Thermal Effect
The current flowing through the contacts and conductors generates heat according to the formula Q = I²Rt. This heat can cause:
- Softening or melting of contact materials
- Degradation of insulation materials
- Expansion of metal parts
- Welding of contacts
The circuit breaker design must account for this heat generation. The mass of the contacts and conductors must be sufficient to absorb the heat without reaching dangerous temperatures. Heat sinks and special contact materials are used to manage thermal stress.
7.2 Electromagnetic Effect
High currents create strong magnetic fields. These fields exert forces on current-carrying conductors. The electromagnetic forces can cause:
- Mechanical deformation of busbars
- Movement of contacts
- Stress on mounting structures
- Repulsion between parallel conductors carrying current in opposite directions
Circuit breakers are designed with robust mechanical structures to withstand these forces. The contacts are held together with strong springs or other mechanisms to prevent them from blowing apart during fault conditions.
8. Categories of Circuit Breakers Based on Icw
IEC 60947-2 classifies circuit breakers into two categories based on their short-time withstand capability.
8.1 Category A Circuit Breakers
Category A circuit breakers do not have a rated short-time withstand current. They are designed to trip instantaneously when a fault occurs. These breakers are used in applications where selectivity is not required. They provide fast protection but cannot be used for selective coordination schemes.
8.2 Category B Circuit Breakers
Category B circuit breakers have a defined rated short-time withstand current. They can carry fault current for a specified time without tripping. These breakers are used in applications where selective coordination is required. They are more expensive than Category A breakers due to their enhanced construction.
Most industrial circuit breakers used in main distribution boards and important feeders are Category B type.
9. Common Values of Rated Short-Time Withstand Current
Circuit breakers are available with various Icw ratings depending on their design and intended application.
Molded Case Circuit Breakers (MCCBs): These are available with Icw ratings ranging from 5 kA to 100 kA for 1 second. Higher ratings are available in premium models.
Air Circuit Breakers (ACBs): These heavy-duty breakers offer Icw ratings from 25 kA up to 150 kA or more for 1 second. They are used in main switchboards and large industrial installations.
Miniature Circuit Breakers (MCBs): Most MCBs are Category A and do not have a defined Icw rating. They trip instantaneously on short circuits.
10. Conclusion
The rated short-time withstand current is a fundamental specification that defines the ability of a circuit breaker to carry high fault currents without tripping or suffering damage. This rating plays a key role in designing selective protection schemes where upstream breakers must wait for downstream devices to clear faults first.
11. Frequently Asked Questions (FAQs)
Icw is the current that a circuit breaker can carry without tripping for a specified short time. Icu is the maximum current that the breaker can interrupt by tripping. With Icw, the breaker stays closed. With Icu, the breaker opens to interrupt the current.
The most commonly used standard duration is 1 second. Other standard values include 0.5 seconds and 3 seconds. IEC 60947-2 also recognizes 0.05s, 0.1s, and 0.25s as standard durations.
If the fault current exceeds the Icw rating, the circuit breaker may suffer damage when operating with time delay. In such cases, the breaker should be set to trip instantaneously or a breaker with a higher Icw rating should be selected.
Main circuit breakers are located upstream of many protective devices. They need to wait for downstream breakers to clear faults first. During this waiting period, they must carry the full fault current without damage. This requires a high Icw rating.
Category B circuit breakers are defined by IEC 60947-2 as breakers that have a specified rated short-time withstand current. They are designed for use in selective protection schemes where time-graded coordination is required.
The circuit breaker is subjected to a current equal to its Icw rating for the specified duration while in the closed position. After the test, the breaker must show no damage and must still function normally.
No. The Icw rating is determined by the physical design of the circuit breaker. It cannot be increased without replacing the breaker with a higher-rated model.