Medium-voltage breaker switchgear example (800 amp service)

Below is an example on an MV breaker switchgear feeding a loop distribution system.


Figure 5: Time-current curves illustrate a typical transformer MV distribution protection scheme with six loop-fed transformers. Courtesy: JBA Consulting EngineersFigure 5 illustrates a medium-voltage (MV) breaker switchgear feeding a loop distribution system with six-unit substation 1500 kVA transformers. It also includes two utility 800 amp service, main-tie-main MV metal clad switchgear, and 400 amp loop distribution feeders. The distribution circuit consists of:

  • Utility sources: Two 12.47 kV 800 amp service
  • Incoming mains: 1200 amp 15 kV 750 MVA vacuum breakers
  • Incoming mains CTs: 1200 amps (1.5 x 800 amps)
  • Loop circuit breakers: 1200 amp 15 kV 750 MVA vacuum breakers
  • Loop distribution wiring: U/G 3 to 500 KCM with #2 copper-ground wire
  • Loop circuit CTs: 600 amps (incoming main CTs: 600 amps (1.5 x 400 amps)
  • Main and feeder protective relays: SEL 351 with 50 and 51 3-Phase, 50/51 GP ground fault

The following protective features are illustrated:

  • Loop overcurrent curve is to the right of the combined six transformer inrush point; under emergency conditions, either breaker can feed all six transformers. Under normal conditions, each loop circuit breaker will serve half of the loop’s transformers.
  • The loop overcurrent curve is to the left of the cable damage curve.
  • NEC Art 210.9(B) (1) requires the loop ampacity of the conductors shall not be less than 125% of the design load. The design is for 400 amp distribution, which should be sufficient for six transformers loaded to 90% or 375 amps. Per Table 310.609(C)(75) 500 KCM MV 105 cable is rated for 525 amps, which equates to 133%—well within the NEC requirements.
  • The minimum clear time between the main breakers and loop breakers is 0.312 s, greater than the minimum 0.2 s guidelines.

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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