Medium-voltage fused switchgear example

This example illustrates the results from an MV fused switchgear feeding a pad-mounted or dry-type transformer.


Figure 1 illustrates medium-voltage (MV) fused switchgear feeding either a pad-mounted or a dry-type transformer. The NEC does not require the secondary overcurrent device to be close coupled. Where the primary and secondary overcurrent devices are located remotely, consideration must be given to protection of both feeder conductors between the transformer terminals and the terminals of the overcurrent devices.

Figure 1: The single line diagram and time-current curves illustrate a typical transformer protection scheme using primary MV fuses and low-voltage secondary breaker protection. Courtesy: JBA Consulting Engineers

Figure 1: The single line diagram and time-current curves illustrate a typical transformer protection scheme using primary MV fuses and low-voltage secondary breaker protection. Courtesy: JBA Consulting EngineersThe transformer circuit consists of:

  • Originating source: Customer-owned 12.47 kV metal fused switchgear
  • Primary fuse: 100E
  • Secondary circuit breaker: 2000 amp
  • Primary cable: #2-15 kV EPR MV 105 cables
  • Secondary cables: 9 sets of underground cables of 500 KCMIL per phase
  • Transformer rating: 1500 kVA, 12.47 kV D to 480 VU, Z=5.75% 

The following protective features are illustrated:

  • The 100E fuse curve is to the right of the transformer 12 times FLA inrush current point and to the left of the cable damage curve.
  • For a transformer of 6% impedance or less, the NEC requires both primary and secondary protection. The primary fuse must not be larger than 300% of primary FLA and the secondary 480 V overcurrent device must not be larger than 125% of secondary FLA.
    • Primary: The continuous rating of a 100E fuse is 130 amps; the FLA of a 1500 kVA transformer is 70 amps, which is 180%—well within the NEC requirements.
    • Secondary: A 2000 amp main breaker is 111% of the transformer FLA.
  • Both the primary and secondary cable damage curves are above the maximum fault current at 0.01 s.

Leslie Fernandez is senior project engineer, electrical at JBA Consulting Engineers. He has more than 28 years of engineering and design and field experience that includes MV distribution systems for military, mining, tunneling, food manufacturing, power production facilities, high-rise facilities, and casino resort complexes. 

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