SM & SML Power Fuses for Underground Distribution, Indoor

Fault Interruption

Fast, positive fault interruption of SM Refill Units and SMU Fuse Units is achieved through high-speed elongation of the arc in the solid-material-lined bore, and by the efficient deionizing action of gases generated through thermal reaction of the solid material due to the heat of the confined arc. Here’s how it works.

Fault Interruption of SM-4Z, SML-4Z, SM-5S, and SM-5SS Power Fuses:

1. Overcurrent melts the silver fusible element. The strain wire severs, initiating arcing.
2. Both main and auxiliary arcing rods are drawn upward by spring-and-cable assembly in the holder. After approximately 1/8-inch travel, lower section of auxiliary arcing rod engages the auxiliary contact momentarily shorting out the arc.
3. For low-magnitude faults, arcing is reinitiated in small-diameter auxiliary bore when the tip of the auxiliary arcing rod travels about one inch (and clears auxiliary contact).
   
   For moderate-to-high-faults, auxiliary arcing rod — momentarily providing the only path for the fault current — quickly melts at reduced section and separates from the one-inch long arcing tip. Any arcing in the auxiliary bore can’t persist, and quickly transfers to main arcing rod in the main bore.
4. For low-magnitude faults, large-diameter section of auxiliary bore delays arc extinction until a sufficient gap is attained to preclude reignition in the main bore.
   
   For moderate-to-high faults, arc is lengthened as main rod is drawn upward into the main bore. The large circumference of main bore provides greater surface exposure of the arc-extinguishing medium to the heating effects of the arc, thereby enhancing generation of arc-quenching deionizing gases.
5. For low-magnitude faults, after auxiliary arcing rod has traveled about one-half stroke, sufficient deionization has occurred to extinguish the arc.
   
   For moderate-to-high faults, after main arcing rod has traveled about one-half stroke, sufficient deionization has occurred to extinguish the arc. The fluorescent fire-orange target is visible in the “blown-fuse” indicator window of the holder.

Fault Interruption of SM-20, SML-20, and SM-40 Power Fuses:

1. Overcurrent melts the silver fusible element. The strain wire severs, initiating arcing.
2. Released force of drive spring accelerates arcing rod upward, causing rapid elongation of the arc in the solids-material-lined bore.
   
   Under maximum fault conditions, heat from confined arc causes solid material in the large-diameter lower section of the arc-extinguishing chamber to undergo thermal reaction, generating turbulent gases and effectively enlarging the bore diameter so that the arc energy is released with a mild exhaust.
   
   Under low-to-moderate fault conditions, arc is extinguished in small-diameter upper section of the arc-extinguishing chamber, where deionizing gases are effectively concentrated for efficient arc extinction.
3. Continued upper travel of the arcing rod after arc extinction causes actuating pin to penetrate the upper seal, resulting in projection of the brilliant-red blown-fuse target from the upper end fitting.