Friday, July 27, 2012

Power Fuse


Power fuses  are especially suited for protecting transformers, capacitor banks, and cables in outdoor distribution substations through 34.5 kV. They incorporate precision-engineered, non damageable silver or nickel-chrome fusible elements with time-current characteristics that are precise and permanently accurate, assuring not only dependable performance, but also continued reliability of system coordination plans. With power fuses, source-side devices may be set for faster operation than practical with other power fuses or circuit breakers, thereby providing better system protection without compromising coordination.

 
The power fuses are offered with maximum continuous current ratings of 200 A, 300 A, 400 A, and 720 A in a variety of fault-interrupting ratings. They are available in a wide variety of ampere ratings, in three different speeds: standard, slow, and coordinating. This broad selection of ampere ratings and speeds permits close fusing to achieve maximum protection.


S&C Fused Disconnect
Advantages

  • High interrupting capacity, up to 200 kA

  • Reduces let-through energy and thus provides better protection for downstream elements.
  • Low initial cost.

2500 A Fuse-GE Class L

55kV S&C Fuse

Disadvantages
  • Not effective in resistance-grounded system (fuse will not see line-to ground [L-G] faults); upstream breaker with ground-fault element will sense/trip, causing blackout in a larger area
  • Source for single phasing (one blown fuse), which poses danger to electric motors.
  • High operating expenses
Fuse Cutout

Fuse Size for Motor Branch Circuit

Manufacturers provide selection tables. Their selection is generally based on:


  • 6.0-P.U. (per unit) motor inrush current
  • 6 s starting time
  • Two starts per hour
Fuse size selected on the above basis will not be suitable for high-inertia loads (blowers, etc.) or high-efficiency motors (starting inrush & 6.00 P.U.).
 Combination starter with fused disconnect. (Courtesy Square D Company.)


Fuse Let-Through Charts

Fuse let-through charts, as shown in, are used to determine the prospective fault current that will be available at the load side of the fuse. Thus, a lower rated device, i.e., molded-case circuit breaker, can be used in the circuit.
Prospective rms current should not be used to check the downstream equipment interrupting capacity. Compare I2 × t value or ensure that the combination has been tested and certified.
 




Fuse let-through chart
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