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Old 7th Jul 2019, 10:56 am   #88
Lucien Nunes
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Join Date: Apr 2008
Location: London, UK.
Posts: 1,773
Default Re: 13 amp fuse for 1.5mm mains flex?

They routinely carried 1.5 times their rated current for many minutes before failing. (I assume the default format for plug fuses is 'slow blow')
That does not make them 'slow blow', the BS1363 specification I believe places them somewhere between 'fast' and 'medium' using the modern categories.

An overload of 1.5xIn is a very low overload that is hard for a fuse to detect, and most fuses will not react for a long time if at all. The heat input that causes the temperature rise to melt the element follows the square of the current, so over an extended period when the fuse is in thermal equilibrium, i.e. as much heat is escaping as is being produced, a 1.5x overload only produces a rise of 2.25x normal working temperature. The maximum power dissipation of a plug fuse is typically 1W, so the additional 1.25W has got to take it from permanent endurance to melting, which is hard to achieve consistently with a copper wire fuse.

Where the distniction between fast and slow fuses comes in is with a surge of perhaps 5xIn, typical of PSU inrush, producing a momentary heat input of 25x normal. If that takes the element over its melting point, as it often will with a simple copper wire fuse, it blows before the heat has had time to dissipate. If instead the element has high thermal mass, typified by the traditional slow-blow fuse with a coiled spring and slug of solder, it can absorb that heat impulse without a corresponding rise to 25x normal working temperature and then dissipate it over a longer period. It will still react to a low overload of 2xIn over a longer period, as thermal equilbrium will be achieved and the melting part of the element (the solder) will catch up with the heat dissipating part (the wire).

Ultra-fast fuses are designed to have very low pre-arc values to protect semiconductors. They have specially shaped elements often of foil with one or more slots in. Under thermal equilbrium, the wider areas of foil work as heatsinks for the narrow areas, but in the event of a short-circuit the adiabatic heating at the isthmus very rapidly raises it beyond melting point.
Three anodes good, six anodes better!

Last edited by Lucien Nunes; 7th Jul 2019 at 11:08 am.
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