I am supplying a 30 kW load through a 3 phase socket. The socket is fused and the same fuse (same phase) is always blowing after a few hours of usage.

What are the possible reasons for a single line fuse blown in a 3 phase system?

EDIT: load is a DC power supply

  • 1
    \$\begingroup\$ Have you checked the other two fuses? Have you replaced all with the same fuse and retested? You used to be really good at accepting answers in the early days. What happened? \$\endgroup\$
    – Andy aka
    Mar 25, 2021 at 13:06
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    \$\begingroup\$ I'd measure the phase currents with a quality AC clamp meter. Could be uneven load, or intermittent short on that leg. \$\endgroup\$
    – rdtsc
    Mar 25, 2021 at 13:13
  • \$\begingroup\$ If the correct type of fuse is used and all 3 have the same fuse, the issue is likely in the equipment you connected to it. As for the consistent timing, that could be a function of the fuse getting slowly worn out from the problem or something in the equipment heating up. If you can, check equipment for faulty/degraded insulation. \$\endgroup\$
    – Abel
    Mar 25, 2021 at 13:25
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    \$\begingroup\$ He means that you have asked about 60 questions on the site but only accepted answers to ten of them. You used to take the trouble to accept answers but seem to have stopped. If you are not getting good answers to that many questions then it suggests that the questions are poor or not clear. If you are getting good answer then you should acknowledge the work put in by others at no cost to you. This is something you can fix. \$\endgroup\$
    – Transistor
    Mar 26, 2021 at 8:44
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    \$\begingroup\$ @Transistor and the reason is simple. I mostly stopped using the website and forgot to accept answers where appropriate. I am going through some of my old questions at the moment. \$\endgroup\$
    – mickkk
    Mar 26, 2021 at 8:46

1 Answer 1


To be sure if the problem comes from the load, I should shift the phases of wires. That is, if existing wiring connection is Fuse1-Wire1,Fuse2-Wire2, Fuse3-Wire3, then change to Fuse1-Wire2,Fuse2-Wire3, Fuse3-Wire1. Rotation will keep the same, and check again. If same fuse is blowing, could be an issue in fuse holder (bad contact is heating the fuse). If the fuse blown is another one, the fault is in the load, may be a ground fault or unbalanced current.

  • \$\begingroup\$ If the load is a 3-phase motor, the motor rotation will change if you swap phases. \$\endgroup\$ Mar 25, 2021 at 20:27
  • \$\begingroup\$ "the motor rotation will change if you swap phases." If you swap any two phases, yes. It won't if you rotate all the phases one step as @Bravale suggested. \$\endgroup\$
    – Graham Nye
    Mar 25, 2021 at 23:01

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