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I have a lighting fixture with a blown fuse on the PSU PCB. The blown fuse fitted was a "T3.15AL250C" 932 (red-coloured) box fuse. I replaced it with a "T3.15A 250V CQ MST" of the same form factor (but black.) The fuse blows instantly.

Before I start investigating if there's a fault elsewhere on the PCB, I'm trying to establish if I've fitted a different fuse that is blowing during a "normal" condition.

Could anyone help to say if there is a difference? The breaking capacity ('L') isn't mentioned on the new fuse. I'm not sure if that is related.

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    \$\begingroup\$ A gross overload will vapourise the fuse whereas a more moderate overload will cause the filament to open. What is your fuse telling you? \$\endgroup\$
    – Kartman
    Mar 30, 2023 at 12:35
  • \$\begingroup\$ Thanks, it was vaporised so as you and Justme say it's likely to be a fault. Looks like the Bridge rectifier is not reading correctly with a diode checker, again that may be a symptom not a cause. Thanks for your help \$\endgroup\$
    – Ian
    Mar 31, 2023 at 14:52
  • \$\begingroup\$ It is not uncommon for one or more of the diodes to fail in a bridge. \$\endgroup\$
    – Kartman
    Apr 1, 2023 at 11:12
  • \$\begingroup\$ Thanks, would you say that the bridge diodes typically fail because of a fault elsewhere (i.e. could there something else that will cause my replacement rectifier to also fail) or are they prone to failing and causing excessive current draw (and therefore blowing the fuse) ? \$\endgroup\$
    – Ian
    Apr 2, 2023 at 15:00
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    \$\begingroup\$ Both scenarios are possible and common. I’ve had many bridge rectifiers just fail with one diode. Replace it and the fuse may get the unit working again. In the case where the bridge rect is collateral damage, measure the resistance of the DC side of the bridge. If that is short circuit then you need to dig further. In the case of a switchedmode power supply you will likely have a capacitor which will initially measure as a short circuit but will then charge up, so measure for 30 seconds or so. \$\endgroup\$
    – Kartman
    Apr 3, 2023 at 20:53

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The old fuse had low breaking capacity, that is what the "L" means

So the new fuse can't really have lower breaking capacity.

There is a fault in the circuit.

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