SMD^® Power Fuses

S&C’s SMD Power Fuses provide reliable, economical protection for transformers and capacitor banks in outdoor substations through 138 kV. They incorporate precision-engineered nondamageable 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 SMD 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.

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SMD Power Fuses are offered with maximum continuous current ratings through 300E amperes, in a variety of fault-interrupting ratings. They’re available in a wide range of ampere ratings, in three different speeds: S&C Standard, Slow, and Very Slow. So they can be readily coordinated with protective relays, circuit reclosers, and other fuses. This broad selection of ampere ratings and speeds permits close fusing to achieve maximum protection and optimum coordination.

Fuse Type	kV			Amperes, RMS, Symmetrical
	Nominal	Maximum	BIL	Maximum	Interrupting
	Nominal	Maximum	BIL	Maximum	60 Hz	50 Hz
SMD-50	34.5 46 69	38 48.3 72.5	200 250 350	100E 100E 100E	6 700 5 000 3 350	6 700 5 000 3 350
SMD-1A	34.5 46 69	38 48.3 72.5	200 250 350	200E 200E 200E	17 500 13 100 8 750	17 500 13 100 8 750
SMD-2C	34.5 46	38 48.3	200 250	300E 300E	33 500 31 500	33 500 31 500
SMD-2B	69 115 138	72.5 121 145	350 550 650 & 750	300E 250E 250E	17 500 10 500 8 750	17 500 10 500 8 750
SMD-3	69	72.5	350	300E	25 000	25 000

Here’s How It Works

Overcurrent melts fusible element. The strain wire severs, initiating arcing.
Released force of drive spring accelerates arcing rod upward, causing rapid elongation of the arc in the solid-material-lined bore. Under high-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 the small-diameter upper section of the arc-extinguishing chamber, where deionizing gases are effectively concentrated for efficient arc extinction.
Continued upward travel of the arcing rod after arc extinction causes arcing rod to drive release tube upward, tripping latch mechanism and initiating drop-out of the fuse unit.

Construction Detail — Fusible Element

SMD Fuse Units feature silver or pretensioned nickel-chrome current-responsive elements that are drawn through precision dies to very accurate diameters. They’re 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 many fuses.

These design and construction features assure that SMD Fuse Units conform to their time-current characteristics on a sustained basis. They’re corrosion-resistant and nondamageable. Age, vibration, and surges that heat the element nearly to the severing point won’t affect their characteristics.

The nondamageability of SMD Fuse Units provides these important advantages:

Superior transformer protection. You can fuse close to the transformer full-load current — thus providing protection against a broad range of secondary-side faults.
Higher levels of service continuity. “Sneakouts” (unnecessary fuse operations) are eliminated.
Close coordination with other protective devices. No “safety zones” or “set-back allowances” are needed to the published TCCs to protect element against damage.
Operating economies. No need to replace unblown companion fuses on suspicion of damage following a fuse operation.

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