Mercury Fuses

Fit the same type as the fuse you are replacing. Slow blow fuses such as the 787.021UK allow a time delay for heavy start-up currents, so equipment that draws a surge at switch-on does not blow them. Quick blow fuses such as the 787.321UK are designed to blow quickly when the current exceeds the rated amperage, and fitting one where a slow blow fuse belongs can make it blow at switch-on.

Measure the length of the glass body: a 5 x 20mm fuse is 20mm long and a 6 x 32mm fuse is 32mm long. Both sizes are sold here in slow blow and quick blow versions, for example the 787.260UK quick blow in 5 x 20mm and the 787.329UK quick blow in 6 x 32mm. A holder accepts just its own size, so match the length before the rating.

The 799.050UK holds 160 quick blow glass fuses in the 5 x 20mm size. Its current ratings are 0.5, 1, 2, 3, 4, 5, 8 and 10A, and the fuses are packed in a plastic storage case for easy organisation. It suits a technician who replaces equipment fuses often, but it has no slow blow or 6 x 32mm fuses.

The 787.408UK takes a 5 x 20mm glass fuse and has a current rating of 2.5A. The 787.408UK has a plastic housing with a bayonet fixing and 18cm of wire, so it can be fitted into a cable run rather than a panel. Fuses are not included with the holders, which come in a pack of 10.

The Mercury glass fuses 787.263UK (quick blow) and 787.012UK (slow blow) comply with IEC 127, sheets 1 and 11, as do the 6 x 32mm 787.147UK and 787.334UK. Check the marking on the old fuse if an equipment manual names a standard.

No, fit the rating the equipment specifies, never a higher one. A fuse that blows repeatedly usually points to a fault, a wrong fuse type or an overloaded circuit, and a larger fuse may let wiring overheat before it breaks. If a quick blow fuse fails at switch-on, check whether the equipment was built for a slow blow fuse such as the 787.018UK.