Unfused spur.

The "instantaneous" part of the current curves for MCBs comes from the following:

A short circuit high current fault occurs which generates a magnetic field in the trip coil. It exists for 1/2 cycle when the magnetic field collapses and is reversed. It could be that the short occurs partway through a cycle but we will ignore that for the moment.  1/2 cycle lasts for 10ms at 50 Hz. In this 1/2 cycle, the energy let through is the RMS current squared x 10 ms. A 6kA breaker then has 6kA squared x 10 ms which is 360,000 A²seconds. This number is not that given by the manufacturer, so what is wrong? They say typically 45,000A² seconds. The only variable to be incorrect is the trip time, which therefore must be substantially less than 10 ms. I² is 36 million Amps The let-through of 45,000 is divided by 36 million giving a trip time of 1.25 ms. I have deliberately simplified this as the waveform over a part cycle is not a sine wave, the worst case is a small section of the sine wave approaching the peak, the width of which is related to the trip time. As you see, all this is very complicated to exactly define and calculate, which is why one gets the worst-case figure from the manufacturer. In mechanical terms 1ms is very long, consider what happens inside a car engine during 10 milliseconds at 6000 RPM.


Does this help you to see what is wrong with the graphs in BS7671, and more to the point why those for fuses are different?

The "instantaneous" part of the current curves for MCBs comes from the following:

A short circuit high current fault occurs which generates a magnetic field in the trip coil. It exists for 1/2 cycle when the magnetic field collapses and is reversed. It could be that the short occurs partway through a cycle but we will ignore that for the moment.  1/2 cycle lasts for 10ms at 50 Hz. In this 1/2 cycle, the energy let through is the RMS current squared x 10 ms. A 6kA breaker then has 6kA squared x 10 ms which is 360,000 A²seconds. This number is not that given by the manufacturer, so what is wrong? They say typically 45,000A² seconds. The only variable to be incorrect is the trip time, which therefore must be substantially less than 10 ms. I² is 36 million Amps The let-through of 45,000 is divided by 36 million giving a trip time of 1.25 ms. I have deliberately simplified this as the waveform over a part cycle is not a sine wave, the worst case is a small section of the sine wave approaching the peak, the width of which is related to the trip time. As you see, all this is very complicated to exactly define and calculate, which is why one gets the worst-case figure from the manufacturer. In mechanical terms 1ms is very long, consider what happens inside a car engine during 10 milliseconds at 6000 RPM.


Does this help you to see what is wrong with the graphs in BS7671, and more to the point why those for fuses are different?