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SMD® Power Fuses

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.

Features

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.

 

Ratings

Fuse Type kV Amperes, RMS, Symmetrical
Nominal Maximum BIL Maximum Interrupting
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

 

Operation

Here’s How It Works

  1. Overcurrent melts 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 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.
  3. 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.

 

1 and 3E

1 and 3E

5E and 7E

5E and 7E

10E and above

10E and above

Styles

Vertical 45° Opening Mounting
Vertical Mounting Style
115-kV SMD-2B Power Fuse shown.

Vertical-Offset Mounting
Vertical-Offset Mounting Style


69-kV SMD-2B Power Fuse shown.

Upright Mounting

Upright Mounting Style


34.5-kV SMD-1A Power Fuse shown.

Right-Angle Mounting

Right-Angle
 Mounting Style


34.5-kV SMD-1A Power Fuse shown.

Inverted Mounting

Inverted
Mounting Style


34.5-kV SMD-1A Power Fuse shown.