For what it is worth .... I measured the voltage difference between the caps of a 13 amp fuse when supplying a fan heater .... Estimating that a 3 kW load will take 12 amps and produce a Vd across the fuse of 0.102 Volt ( 3 times 0.034 ) ... 3 kW load--...--0.102 Volts x 12 Amp = 1.22 Watt ...
That's in a similar ballpark (albeit about double the answer) as theory ...
Per tables, 13A rated fuse wire is 27 SWG, which has a CSA of 0.1363 mm². A BS1362 fuse wire is about 26 mm long, and that translates to a resistance of 26 mm of 27 SWG copper wire of 0.003262 Ω. At 13A that would result in a power dissipation of 0.5513 W.
If I understand you correctly, ... "3 kW load--------0.102 Volts x 12 Amp = 1.22 Watt ... A fuse in good order can release about as much heat as a 1W lightbulb ... Which is trivial.
Not in a totally enclosed space where the only route for heat to exit the space is by conduction along the Live pin into the metal of the socket's receptacle.
Really! 1 Watt on the metal pins of any plug is a fairly trivial amount of heat.
that assumes a low thermal resistance from fuse to pin allowing the heat to be conducted to the pin. And of course if the thermal path is degraded then almost certainly the electrical path is compromised with voltage drops across weak connections ( such as the clips in fuse holders ) creating more heat.
As bernard has said, whether 1 W (or even 0.55W) is "trivial" (in terms of temperature rise) is totally dependent upon the opportunities for heat to be removed from its place of generation (i.e. the fuse wire). If heat cannot be removed at least as fast as it is being generated, the temp will continue rising 'indefinitely' (until it reaches the melting point of the fuse wire, at which point the heat generation will cease).
One sees that in the ('adiabatic') situation in which heat is generated so quickly that there is no time for it to be removed 'to anywhere'. A 13A-rated copper fuse wire has a fusing current of about 23A. If the resistance of the fuse wire is, above, 0.003262 Ω, the power dissipated at 23A will only be about 1.73 W, yet that results in the temp of the fuse wire rising from ambient temp to the melting point of copper (about 1,085 °C) in a fraction of a second.
Hence, the thermal implications of 1W (or 0.55W) being dissipated in a 13A fuse depend entirely upon how well heat can be removed - and that can really only be determined by experiment, in various different situations. Although one would hope that the temp rise of a plug would be fairly low with the 'trivial' amount of heat being generated by the fuse, one cannot be certain of that without 'doing the experiments'.
Kind Regards, John
