Impact of DC Components of Short Circuit Current and Circuit Breaker Interruption

As per IEC 62271.1, the rated short circuit current has a time constant of 45ms (approximately X/R=14.1 at 50 Hz). If your application requires a higher time constants, you may purchase a breaker designed to operate on short circuit currents with a time constant of 120ms. Thus if you assume that the relay takes 10mS to detect the fault and actuate circuit breaker shunt trip, it will be interrupting more DC current if the time constant is 120mS, while the AC component remains same.

When a short circuit occurs, there's basically an AC component and a DC component. The time constant determines how quickly the DC component dies down. So when a breaker operates, it opens against both the AC component and DC component.

The exact DC component of a short circuit component is calculated using the X/R ratio (or time constant) of the system, at the point where short-circuit occurs. The exact value will depend on line parameters like inductance, capacitance and resistance. There is plenty of literature to cover how to calculate the DC-component of short circuit currents.

Also note, CT's may or may not be designed to pick up DC component of the fault currents. In general CT's for over current protection are designed not to pick up DC component of the fault current (to keep size and cost small), whereas differential CT's are always designed to pick up the DC component of fault current.

The DC component is seen as an offset that where the magnitude is dependent upon the X/R ratio. This is important as it can result in higher instantaneous trip currents than the steady state short circuit currents. In the US we use an ANSI multiplier to determine the breaker interrupting current based on the calculated X/R compared to the test X/R ratio for the circuit breaker (The test X/R ratio is function of the breaker type and interrupting current.). The equation can be found in many books and most software has an option for doing the ANSI calculations.

Breakers are only marked with interrupting ratings (they do not show the test X/R ratio). It is up to the specifying engineer to compare the calculated fault duty with appropriate multiplier with the published interrupting rating and select the proper class of device to allow for adequate protection.

Basics

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