# How do large cartridge buss fuses work?

My son found one of these and he was curious what it was.

I showed him how to test it with a multimeter to show if it was blown and then proceeded to take it apart to show him the insides.

I expected to just find something similar to a glass fuse, but instead found lots of interesting things...

• Sand of some sort
• Fiberglass wrap (at least that's what my now prickly fingers say it was!)
• A thin flat copper wire with a hole or two in it
• A block of copper on a spring that appeared to be once soldered to a copper base

I have some ideas of what purpose each of these does, but I was hoping to get a larger idea of the design of these. Is the flat copper wire also meant as redundancy for blowing the circuit? Is the soldered piece the primary? Why the hole? How do they calibrate the solder release with the amperage? What makes this fuse a time delay?

• Is that an HRC fuse? (try google "HRC fuse inside) – George Herold Aug 31 '17 at 18:06
• Yes ... seems it was an HRC since they both have the sand/silica. – rrauenza Aug 31 '17 at 18:10

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