HRC fuse construction and advantages

An HRC (high rupturing capacity) fuse is a totally enclosed cartridge fuse that can safely interrupt very large fault currents (tens of kA) without any external flame or explosion.

Constructional details

  contact            contact
   blade ┌──────────────┐ blade
  ═══╦═══╡ ::::::::::::: ╞═══╦═══
     ║   │ ═╦═╦═╦═╦═╦═  │   ║
     ║   │ ::::::::::::: │   ║
  end cap└──────────────┘ end cap
      ceramic body + silica sand
      silver element with notches
      ● indicator / striker

1. Body: a cylindrical tube of high-grade ceramic (steatite or porcelain), strong enough to withstand pressure and heat during arcing.
2. End caps and contacts: metal (brass/copper) caps fixed to the ends, with knife-blade or bolted tags; the fuse element is welded to them.
3. Fuse element: usually silver (sometimes copper) because silver does not oxidise, has low resistivity and a well-defined melting point. It is made as a strip with several reduced sections (notches); for large ratings, two or more strips in parallel.
4. Overload "M-effect": a small tin or solder bead on the element; at moderate overloads it melts first and alloys with silver, lowering its melting point, giving controlled time delay.
5. Filler: pure, dry quartz (silica) sand packed tightly around the element. It absorbs arc heat and, with the vaporised silver, forms a high-resistance fulgurite that quenches the arc.
6. Indicator/striker: a thin resistance wire in parallel with the element; it melts after the main element and releases a spring-loaded indicator or striker that can also trip a switch.

Operation (brief)

On fault, the notches melt almost at once, several series arcs are formed, the sand cools them and the arc resistance increases, forcing the current to zero, often before the first peak.

Advantages of HRC fuse

1. Very high breaking capacity at low cost.
2. Current-limiting: cuts off before the prospective peak, reducing thermal and mechanical stress.
3. Very fast operation on heavy faults (a few ms).
4. Inverse time-current characteristic: time delay for small overloads, fast for short circuits.
5. Consistent, non-deteriorating performance; calibration does not change with age.
6. No maintenance; totally enclosed, so no external flame or noise.
7. Good discrimination with other fuses and breakers.
8. Simple, compact and cheaper than a circuit breaker of equal breaking capacity.

Disadvantages: must be replaced after operation; heating of contacts may cause discrimination problems.