Short Time Rating Of Circuit Breaker Formula

Short Time Rating of Circuit Breaker Formula

The short time rating (STR) of a circuit breaker is the maximum current that the breaker can carry for a specified period of time without tripping. The STR is typically expressed in kiloamperes (kA) and the time period is typically expressed in seconds (s). The STR is important because it determines the maximum fault current that the breaker can withstand without tripping. If the fault current exceeds the STR, the breaker will trip, which will interrupt the flow of electricity and protect the circuit from damage. The STR is determined by the following factors: * The interrupting rating of the breaker * The type of fault * The duration of the fault The interrupting rating of the breaker is the maximum current that the breaker can interrupt without damage. The type of fault refers to the way in which the fault occurs. There are two types of faults: * Line-to-line faults * Line-to-ground faults The duration of the fault refers to the length of time that the fault occurs. The STR is calculated by multiplying the interrupting rating of the breaker by the factor for the type of fault and the duration of the fault. The following table shows the factors for different types of faults and durations: | Fault Type | Duration | Factor | |---|---|---| | Line-to-line | 0.5 s | 1.0 | | Line-to-line | 1.0 s | 1.2 | | Line-to-line | 3.0 s | 1.5 | | Line-to-ground | 0.5 s | 0.8 | | Line-to-ground | 1.0 s | 1.0 | | Line-to-ground | 3.0 s | 1.2 | The STR is an important safety factor to consider when selecting a circuit breaker. The breaker should have a STR that is greater than or equal to the maximum fault current that is likely to occur in the circuit. ## Example Let's say that we have a circuit breaker with an interrupting rating of 10 kA. We want to determine the STR for a line-to-line fault that lasts for 1.0 second. The factor for a line-to-line fault that lasts for 1.0 second is 1.2. So, the STR for this circuit breaker is 10 kA * 1.2 = 12 kA. This means that the circuit breaker can withstand a fault current of up to 12 kA for 1.0 second without tripping. ## Conclusion The short time rating of a circuit breaker is an important safety factor to consider when selecting a circuit breaker. The breaker should have a STR that is greater than or equal to the maximum fault current that is likely to occur in the circuit. ## References * [IEC 60898-1:2016, Circuit-breakers for ac circuits – Part 1: General rules](https://www.iec.ch/en/standards/standard/60898-1ed2.html)

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