Ever thought about ... ?

Cheesecloths etc...  No sign of anything so modern in this 1958 masterpiece.  The concern appears only to be that the ejected material shouldn't cause arcing to a metal enclosure or to neighbouring fuses on other phases.  Part of the test is a fine wire connecting a metal enclosure to the 'suitable point' of the source (e.g. neutral) - this wire mustn't melt. 

Probably the user was supposed to turn off the switch, change the fuse, and turn on. A cap sounds a good start if going the quicker route without switching off.


The upper bound set by a service fuse can be a bit misleading, but it should be good for currents in the several kA level and up.  If we take, e.g. this bussman service fuse spec we'd find that the melting (pre-arcing) I2t for their 60 A service fuse is about half of what Andy and Graham have shown as lower bounds for the 30 A BS3036 based on t/I curves.  This makes sense because the values are for different timescales, and the cartridge fuse varies in I2t between these timescales.  The quoted I2t is for short times such as 10 ms. If one uses t/I curves from the same spec to calculate I2t for longer times, the result increases to about 4 times as much at 200 ms. The restrictions (necks) in a cartridge fuse element allow the bend of the t/I curve at short times to be designed. A fast fuse (semiconductor) has short but thin restrictions that can melt very quickly at one current, but can conduct their heat away to the surrounding element at rather lower current, whereas a general-purpose one has thicker, longer restrictions. Meanwhile, the simple cylindrical wire of a BS3036 fuse gives no control over the t/I curve for times at which heat transfer from the element can be neglected, which for a 30 A size would be times up to several hundred milliseconds: over this range the melting needs roughly constant I2t.  Cartridge fuses overtake the simple fuses in speed at the higher currents, because the shortness of their restrictions means that one has to come down to only milliseconds or tens of milliseconds before the melting I2t becomes roughly constant.  As long as most faults that pit a smaller BS3036 against a bigger cartridge fuse have rather low current, e.g. 1 kA or 500 A due to typical impedances of source, cable and possibly fault, the bigger fuse survives. But at higher current there comes a point where the curves will cross unless the cartridge fuse is much bigger or is designed for slow blowing.

Cheesecloths etc...  No sign of anything so modern in this 1958 masterpiece.  The concern appears only to be that the ejected material shouldn't cause arcing to a metal enclosure or to neighbouring fuses on other phases.  Part of the test is a fine wire connecting a metal enclosure to the 'suitable point' of the source (e.g. neutral) - this wire mustn't melt. 

Probably the user was supposed to turn off the switch, change the fuse, and turn on. A cap sounds a good start if going the quicker route without switching off.


The upper bound set by a service fuse can be a bit misleading, but it should be good for currents in the several kA level and up.  If we take, e.g. this bussman service fuse spec we'd find that the melting (pre-arcing) I2t for their 60 A service fuse is about half of what Andy and Graham have shown as lower bounds for the 30 A BS3036 based on t/I curves.  This makes sense because the values are for different timescales, and the cartridge fuse varies in I2t between these timescales.  The quoted I2t is for short times such as 10 ms. If one uses t/I curves from the same spec to calculate I2t for longer times, the result increases to about 4 times as much at 200 ms. The restrictions (necks) in a cartridge fuse element allow the bend of the t/I curve at short times to be designed. A fast fuse (semiconductor) has short but thin restrictions that can melt very quickly at one current, but can conduct their heat away to the surrounding element at rather lower current, whereas a general-purpose one has thicker, longer restrictions. Meanwhile, the simple cylindrical wire of a BS3036 fuse gives no control over the t/I curve for times at which heat transfer from the element can be neglected, which for a 30 A size would be times up to several hundred milliseconds: over this range the melting needs roughly constant I2t.  Cartridge fuses overtake the simple fuses in speed at the higher currents, because the shortness of their restrictions means that one has to come down to only milliseconds or tens of milliseconds before the melting I2t becomes roughly constant.  As long as most faults that pit a smaller BS3036 against a bigger cartridge fuse have rather low current, e.g. 1 kA or 500 A due to typical impedances of source, cable and possibly fault, the bigger fuse survives. But at higher current there comes a point where the curves will cross unless the cartridge fuse is much bigger or is designed for slow blowing.