I'll start off by saying that I know this not a recommend practice.

I'm looking to build a regulated power supply that can supply 1200VDC at 300mA. The supply consists of a transformer, xenon tube rectifier bridge, a choke input filter (5H), and the pass tubes of the regulator.

I have found 315mA 1000VDC fuses, but have been unable to find any fuses of a higher voltage rating below 2.5A. Being that the maximum fault current isn't very high (limited by the transformer windings, rectifier impedance, choke winding, and pass tubes), would fuses in series be effective?

From what I can see even if one were to blow and maintain an arc, the second fuse would blow shortly. The two should then be enough of an arc gap to extinguish it, right? The 1000V fuses are the ceramic shell sand filled type, and since I'd only be exceeding the voltage rating slightly I'd be surprised if the first fuse didn't quench the arc itself, but I'd rather be safe.

In the event both fuses fail to break the circuit, it would inevitably blow the mains fuse. Also if anyone knows where to get fuses rated for a higher voltage at that current and they're not obscenely expensive I'm all ears.

Thank You

  • \$\begingroup\$ Rated voltage should be higher than the maximum voltage source it would have to disconnect. Connecting fuses in series does not increase the rated voltage of the combination, nor of any one fuse. \$\endgroup\$ Commented Jan 12, 2020 at 18:43
  • 3
    \$\begingroup\$ If the first fuse blows and strikes an arc ... adding enough impedance to reduce the current enough to stop the second fuse blowing ... what then? \$\endgroup\$
    – user16324
    Commented Jan 12, 2020 at 19:11
  • \$\begingroup\$ Then the fuse would maintain an arc, smolder in its ceramic holder on a metal chassis until I noticed smoke coming from the supply and/or meters flickering wildly or it shorts to the chassis and blows the mains fuse. Unlikely to damage the supply either way. \$\endgroup\$
    – Sparky95
    Commented Jan 12, 2020 at 19:15
  • 2
    \$\begingroup\$ You said yourself that "this is not a recommended practice" but then you want to know if we will recommend it. No. If it is not a recommended practice then you should assume that it is not recommended for a good reason. \$\endgroup\$ Commented Jan 12, 2020 at 19:23
  • \$\begingroup\$ Not having a fuse is also not a recommend practice. Those are my only two options, since the fuse I need doesn't appear to exist. I also didn't ask if you would recommend it, I asked if it would in theory be more effective than a single fuse that is rated 200V less than the supply, or no fuse at all. \$\endgroup\$
    – Sparky95
    Commented Jan 12, 2020 at 19:29

1 Answer 1


This is bad idea.

If one of fuses blows, the arc will be striked, what results in lowering the voltage drop on this fuse to circa 50...150V. Then, if current will remain sufficient to blow the second fuse, the second fuse blows, and near full voltage (minus arc voltage drop on first fuse) will strike the arc in this second fuse too.

As the result you will have arc in both fuses.

  • 1
    \$\begingroup\$ If it dropped the voltage by 200V in the arc, then the second one wouldn't arc at all because it'd be within its 1kV rating. But counting on that voltage drop is risky. Guess I won't fuse it at all. The worst that will happen is that I burn up a few tubes until the mains fuse pops. \$\endgroup\$
    – Sparky95
    Commented Jan 12, 2020 at 21:03

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