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. 

No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
A cool solution: Collaboration, chemistry leads to foundry coat product development; See the 2015 Product of the Year Finalists
Raising the standard: What's new with NFPA 70E; A global view of manufacturing; Maintenance data; Fit bearings properly
Sister act: Building on their father's legacy, a new generation moves Bales Metal Surface Solutions forward; Meet the 2015 Engineering Leaders Under 40
Cyber security cost-efficient for industrial control systems; Extracting full value from operational data; Managing cyber security risks
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Pipeline vulnerabilities? Securing hydrocarbon transit; Predictive analytics hit the mainstream; Dirty pipelines decrease flow, production—pig your line; Ensuring pipeline physical and cyber security
Upgrading secondary control systems; Keeping enclosures conditioned; Diagnostics increase equipment uptime; Mechatronics simplifies machine design
Designing positive-energy buildings; Ensuring power quality; Complying with NFPA 110; Minimizing arc flash hazards
Building high availability into industrial computers; Of key metrics and myth busting; The truth about five common VFD myths

Annual Salary Survey

After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.

The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.

Read more: 2014 Salary Survey: Confidence rises amid the challenges

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.