S&C Electric Company
Contact Us
SMD® Power Fuses

SMD® Power Fuses

SMD Power Fuses are especially suited for protecting transformers, capacitor banks, and cables in outdoor distribution substations through 34.5 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-20 Power Fuses, rated 200 amperes continuous, utilize SMU-20 Fuse Units. These fuse units are offered in a variety of ampere ratings, in five different speeds — S&C “K,” Standard, Slow, Very Slow, and “DR.” They provide interrupting ratings of up to 14,000 amperes symmetrical at 60 Hz.

SMD-40 Power Fuses, rated 400 amperes continuous, utilize SMU-40 Fuse Units. These fuse units are similarly offered in a variety of ampere ratings, in four different speeds — S&C “K”, Standard, Slow, and Very Slow. They provide interrupting ratings of up to 25,000 amperes symmetrical at 60 Hz.

This broad selection of ampere ratings and speeds permits close fusing to achieve maximum protection and optimum coordination. SMD-20 and SMD-40 Power Fuses are manufactured in accordance with a quality system certified to ISO9001:2000.

 

Ratings

Fuse Type kV Amperes, RMS, Symmetrical
Nominal Maximum BIL Maximum Interrupting
60 Hz 50 Hz
SMD-20 14.4
14.4
14.4
25
34.5
17.0
17.0
17.0
27
38
110
125
150
150
200
200E or 200K
200E or 200K
200E or 200K
200E or 200K
200E or 200K
14 000
14 000
14 000
12 500
10 000
11 200
11 200
11 200
10 000
8 000
SMD-40 4.8
14.4
25
5.5
17.0
29
95
110
150
400E
400E
400E
25 000
25 000
20 000
20 000
20 000
16 000

 

 

Fault Interruption

Fast, positive fault interruption is achieved in SMU-20 and SMU-40 Fuse Units 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. The resultant high rate of dielectric recovery voltage more than matches the transient recovery voltage severity of the circuit.

Here’s How it Works

  1. Overcurrent melts the fusible element, shown below. The strain wire severs, initiating arcing.
  2. Released force of the drive spring accelerates the arcing rod upward, causing rapid elongation of the arc in the solid-material-lined bore. Under maximum fault conditions, heat from the 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, the 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 the actuating pin to penetrate the upper seal, initiating positive dropout of the fuse unit.

SMU Fuse Units — Fusible Element

SMU 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 SMU Fuse Units conform to their time-current characteristics on a sustained basis. SMU Fuse Units are 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 SMU Fuse Units provides these important advantages:

  • Superior transformer protection. You can fuse close to the transformer full-load current and thus protect 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 the element against damage.
  • Operating economies. No need to replace unblown companion fuses on suspicion of damage following a fuse operation.

 

1 ampere

1 ampere element illustration.

3K, 5E, and 7E

3K, 5E, and 7E  element illustration.

6K through 200K, 10E through 200E, and 10DR through 20DR 6K through 200K, 10E through 200E, and 10DR through 20DR  element illustration.

Styles

Overhead-Pole-Top Style
14.4-kV SMD-20 Power Fuse

(25 kV is similar)
Overhead-Pole-Top Style

Overhead-Pole-Top Style
34.5-kV SMD-20 Power Fuse

Overhead-Pole-Top Style

Station-Inverted Style
34.5-kV SMD-20 Power Fuse

Station-Inverted Style

Station-Right-Angle Style
14.4-kV SMD-20 Power Fuse

Station-Right-Angle Style

Station-Vertical-Offset Style
14.4-kV SMD-20 Power Fuse

Station-Vertical-Offset Style

Station-Vertical Style
14.4-kV SMD-40 Power Fuse

Station-Vertical Style

 

 

Fuse Handling

Opening and Closing Fuse Units

SMD-20 Power Fuses are equipped with attachment hooks for universal load switching using Loadbuster®, S&C’s portable loadbreak tool. SMD-40 Power Fuses are available with optional attachment hooks for full-load live switching with Loadbuster®.

SMD-20 and SMD-40 Power Fuses must not be opened under load without use of a Loadbuster.

Installing or Removing Fuse Units

SMU-20 Fuse Units can be installed in or removed from their mountings using a universal pole equipped with an S&C Talon™, Distribution Prong, or Station Prong. SMU-40 Fuse Units can be installed in or removed from their mountings using a Universal Pole equipped with an S&C Large Clamp.

Switching Overhead-Pole-Top Style SMD-20 Power Fuse using Loadbuster.
Switching Overhead-Pole-Top Style SMD-20 Power Fuse using Loadbuster.

Installing the fuse unit in Overhead-Pole-Top Style SMD-20 Power Fuse using Talon.
Installing the fuse unit in Overhead-Pole-Top Style SMD-20 Power Fuse using Talon.