Introduction

The high-speed fuse is the last line of defence for a power semiconductor. A diode, thyristor, IGBT or SCR has a very small thermal mass and fails within milliseconds of an overcurrent, so the fuse must open in less than half a cycle and limit the energy the device sees. That is a very different job from protecting a cable, and it is why a high-speed fuse is selected with the device, not the load, in mind. This application note explains coordination, I-squared-t and layout for Bussmann high-speed fuses in drives, rectifiers and inverters.

Coordination with the Device

Coordination means the fuse clears the fault before the semiconductor is damaged. The comparison is made with the I-squared-t, the energy let through during the clearing time: the fuse clearing I-squared-t must be lower than the device withstand I-squared-t at the prospective fault current. Because both figures vary with current, the comparison must be made across the whole fault range, not at a single point. A fuse that coordinates at one current may not coordinate at another, so the curves are examined together.

Current Limitation

A current-limiting fuse opens so quickly that the fault current is interrupted before it reaches its first major peak. The device therefore sees a much lower let-through current and let-through energy than the prospective fault current would imply. That current limitation is the property that lets a small fuse protect a large semiconductor, and it is the reason a high-speed fuse can often be coordinated where a circuit breaker cannot.

Amp Rating and Voltage Class

The amp rating is set above the normal load current and below the device rating, then corrected for the ambient and the enclosure, because a fuse carries less current when it is hot. The voltage class must exceed the circuit voltage, including the DC case, with margin for the recovery voltage after clearing. A fuse must never be used at a voltage above its rating, and an AC fuse must not be used in a DC circuit where the fault current has no zero crossing.

Mounting and Layout

The square-body fuse bolts directly onto the busbar, so the connection has the lowest practical inductance and the fuse cools through the bar. Tighten to the specified torque: a loose connection adds resistance and heat and can cause a nuisance failure, while the low-inductance mount is what keeps the fault loop small. In a DC link, place the fuse so the commutation loop is not enlarged, and check the holder, if one is used, for the same voltage and current rating as the fuse.

Diagnosing a Blown Fuse

When a high-speed fuse operates, it is worth reading the evidence before replacing it. A fuse that clears a genuine short circuit will show a distinct arc mark, while a fuse that fails from a poor connection or a thermal problem may look different. Because the fuse is the last line of defence, a blown high-speed fuse usually means the device it protected was saved, and the fault should be found before the fuse is replaced. The BeiLuo FAE team helps with the coordination, the derating and the diagnosis.