Type SMD-20 Power Fuses and Type SMD-40 Power Fuses

S&C SMD® Outdoor Distribution Fuses are internationally recognized for their superb protection of transformers, lines, cables, and capacitor banks in distribution substations and on overhead feeders. SMD-20 and SMD-40 Power Fuses, with their SMU Fuse Units, provide full-fault¬spectrum protection. They detect and interrupt all faults— large, medium, and small (even down to minimum melting current); with line-to-line or line-to-ground voltage across the fuse; whether the fault is on the primary or secondary side of the transformer; and regardless of the transformer winding connection.

SMD-20 Power Fuses are designed expressly for today’s distribution systems, where higher fault currents, higher voltages, and higher loads commonly push cutouts beyond their capabilities. SMD-40 Power Fuses are specially suited to the high continuous-current and fault-interrupting requirements of distribution substations.

SMD-20 Power Fuses employ a unique solid-material, low-arc-energy technique of fault interruption that produces a mild exhaust, with only 20% the arc energy of a cutout. Clearances can be keyed to fuse-handling requirements, not fuse-exhaust behavior, allowing structures to shrink to minimum phase spacings.

SMD-40 Power Fuses use the same time-proven low-arc-energy fault-interrupting technique as SMD-20 Power Fuses. A stainless-steel outdoor silencer virtually eliminates the noise and reaction forces ordinarily associated with outdoor power fuses. These lightweight fuses are easily handled using a universal pole equipped with an S&C Large Clamp.

SMU Fuse Units . . . Precise Performance
SMU-20® and SMU-40® Fuse Units have silver or pretensioned nickel-chrome fusible elements that are drawn through precision dies to very accurate diameters and are of solderless construction, brazed into their terminals. Melting time-current characteristics are precise, with only 10% total tolerance in melting current, compared to the 20% tolerance of most fuses.

. . . Nondamageable Construction—Helical coiling of the fusible elements and the absence of constraining filler materials result in elements that are free from mechanical and thermal stresses. SMU Fuse Units conform to their time-current characteristics—not only initially, but on a sustained basis. Neither age, corrosion, or vibration, nor surges that heat the element nearly to the severing point, will affect the characteristics of these fuse units. With nondamageable construction, there is no need to adjust the minimum melting curves for “safety zones” or “setback allowances” as required for other fuses.

. . . Superior Loading Capability—SMU Fuse Units are designed to accommodate, not interrupt, all anticipated loading levels including daily and repetitive peak loads, as well as hot- and cold-load inrush currents. Unlike current-limiting fuses, SMU Fuse Units have peak-load capabilities in excess of their normal ampere rating. This generous loading capability, combined with precise conformance to TCCs, nondamageability, and a broad selection of fuse-unit ampere ratings and speeds, permits the ultimate in close fusing for maximum protection and optimal coordination.

. . . Outstanding Protection for Transformers
The exceptional peak-load capabilities of SMU Fuse Units permit the use of low fusing ratios while accommodating the high loading and inrush currents of distribution transformers. These low fusing ratios mean fast clearing of faults. Transformer mechanical and thermal stresses are minimized, extending transformer life, and the faulted transformer is quickly isolated, minimizing the impact on the distribution system. In contrast, current-limiting fuses provide significantly less through-fault protection because their susceptibility to damage, limited loadability, and steep TCCs result in the need for markedly higher ampere ratings.

. . . For Capacitor Banks—In protecting pole-top and small station capacitor banks, the high peak-load capabilities again allow exceptionally close fusing—so that even evolving faults within individual capacitor units can be detected and cleared.

. . . And for Sectionalizing Points—When installed at sectionalizing points and cable risers, these fuses protect against permanent faults and coordinate easily with reclosers. The low fusing ratios possible with SMU-20 and SMU- 40 Fuse Units permit upstream protective devices to be set for faster operation . . . reducing system short-circuit stresses. And, SMU Fuse Units’ TCC curve shapes facilitate complete coordination with downstream protective devices.

Every SMD-20 and SMD-40 Power Fuse is a Load-Switching Device —When used with Loadbuster®, S&C’s portable loadbreak tool, SMD-20 and SMD-40 Power Fuses become switching devices for no-external-arc live switching of transformers, lines, cables, and capacitors. There’s no need for complex system switching procedures—you can minimize the length of planned outages and the number of customers involved.