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Fuses and Protective Devices: Rating, Breaking Capacity and the Wrong Replacement
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The protective device in an instrument limits the energy released when a fault occurs inside the enclosure. The assessment asks whether the device operates before the wiring or the enclosure is damaged, and whether it can interrupt the fault current without rupturing.
Rating and derating
A fuse must carry the rated current of the instrument continuously at the maximum rated ambient, which means derating from its nominal value for temperature. It must also not open on the inrush of a transformer or a motor at switch-on. Those two requirements pull in opposite directions and the margin between them is often small, which is why fuse selection is verified by test rather than by calculation alone.
Breaking capacity
The fuse must be able to interrupt the prospective fault current at the point of installation without sustaining an arc. A small glass fuse in a circuit capable of a high fault current can rupture and expel material, which turns a protective device into a hazard. The breaking capacity has to be at least the prospective fault current, and that is a property of the installation as much as of the instrument.
The replacement problem
Where the user can replace a fuse, the instructions and the marking must specify the exact type and rating, and the holder should make a larger fuse physically difficult to fit. A user who fits a fuse of double the rating has removed the protection, and the standard expects the design to resist that as far as practicable.
