HRC fuse construction; fuse vs MCB

Construction of HRC fuse

An HRC (high rupturing capacity) cartridge fuse can safely interrupt very high fault currents (up to 80–120 kA) without external effects.

   end cap              end cap
  ┌──┐ ┌────────────────┐ ┌──┐
  │  ├─┤ ·:·:·:·:·:·:·  ├─┤  │
  │  │ │ ══╪══╪══╪══╪══ │ │  │ silver
  │  ├─┤ ·:·:·:·:·:·:·  ├─┤  │ element
  └──┘ └────────────────┘ └──┘
      ceramic body filled
      with quartz sand (·:)

• Ceramic (steatite) body: strong, heat-resistant tube that withstands arc pressure.
• Metal end caps with contact blades/tags, welded to the fuse element.
• Fuse element: silver (or copper) strips with reduced sections (notches) in parallel; sometimes a tin/low-melting-point bead (M-effect) for overload.
• Filling powder: pure quartz (silica) sand packed around the element.
• Indicator: a fine wire in parallel that melts and releases a striker/indicator.

Operation

1. Under normal current, the heat produced is dissipated and the element stays below melting temperature.
2. On fault, the reduced sections melt quickly (pre-arcing) and several arcs form in series.
3. The heat of the arcs vaporises silver, which reacts with/fuses the quartz sand to form a high-resistance glass-like substance (fulgurite).
4. The arc resistance rises rapidly, the current is forced to zero, and the arc is quenched (arcing time).
5. For high faults the element melts before the first current peak, so the fuse limits current (cut-off).

Difference between fuse and MCB