I'm trying to select fuses for a 3 phase generator. It's a specialty generator and the manufacturer rates it at 360Vrms and 50Arms under full load. I'm fusing the 3 individual phases before they feed into a bridge rectifier. The rectifiers, 3 individual, are rated at 400A with a 50% duty cycle. The rectifier is also capable of absorbing a non recurring 3000A pulse for 8.3mS.

The voltage rating is easy, 600V standard size.

The current gets harder. The general rule of thumb is to oversize the fuse 125% from full load current. This gives us 62.5A rounded up to 70A. Then things get more complicated. The generator runs at 4kHz. That is not a typo it is 4kHz. Because of the high frequency and skin effect the fuse gets derated 0.8. This now makes the needed fuse 87.5A rounded up to 90A.

At that current and voltage all i'm finding are branch rated, current limiting fuses. These fuses are rated to interrupt 10000A or more and are the size of my forearm. I don't think i need branch rated fuses. The generator is a finite supply and i don't think that it's capable of achieving even a 1000A.

I would like to use a small fuses (space is kind of limited). Are there small, not branch rated fuses that i could use?

  • \$\begingroup\$ It's highly dependent on the short circuit current rating of the generator. \$\endgroup\$
    – winny
    Jul 23, 2016 at 8:20

1 Answer 1


This is a link to fuses that we used in surge suppressors. They only needed a 30 amp operation rating, but a 200KA interrupt rating. I have scanned the Internet and came up with the same fuses you likely did. Fuses rated for 100 amps are about 7.8 inch's long.

I do not see a safe work-around for this issue. I picked Bussmann because they were predictable under our testing procedures, even if a bit expensive. Considering the cost of the generator I would think the cost of the fuses and fuse holders would be justified.

These fuses are slow-blow with mild current limiting built in, and are best used for AC generators and motors. The right value fuse will NOT blow unless a short circuit happens or a motor stalls that has no electronic current limiting.

In the power industry you cannot cheat or go cheap on safety.

  • \$\begingroup\$ Use a semiconductor rated fuse. They are typically fast blowing, not time delayed, but you can use a slightly larger value. Specifically, look at the Bussman FWH series. \$\endgroup\$
    – R Drast
    Jul 22, 2016 at 10:42

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