How do large cartridge buss fuses work?
Sand of some sort



hiV insulation, low thermal resistance



  Fibreglass wrap (at least that's what my now prickly fingers say it was!)



high thermal resistance and some acoustic resistance with packing pressure.



  A thin flat copper wire with a hole or two in it



a low cost conductive terminal with tooling holes for automated assembly.



  A block of copper on a spring that appeared to be once soldered to a copper base



ambient heat sink for the thermal spring to maintain a relatively cool end 


Theory


the resistance of the copper spring increases with temperature (PTC) thus dissipating more power not only with current but temperature. The  heat rise is slowed down by the sand and thus temperature accelerates more rapidly near threshold by Pd= I squared (R+delta R) to melting point with rising currents at rated for slow blow at high current.


Not as precise as smart current sensing, but cheap fuses that can withstand motor surge currents e.g. 6x rated current for brief periods.
biggest benefit is the dual heating element fuses open fast for short circuits and slow for motor surges that are too long.



example datasheet

http://m.littelfuse.com/~/media/electrical/datasheets/fuses/industrial-and-ul-fuses/littelfuse_fuse_idsr_datasheet.pdf