Selective coordination studies for mission critical environments

Isolating an electrical fault condition to the smallest area possible is essential in providing the most reliable electrical distribution system with maximum uptime for your facility.


Learning objectives

  • Learn the basics of protective device coordination studies.
  • Know the proper sizing of the transformer primary breaker.
  • Understand selective coordination impacts on arc fault.
  • Understand NEC Article 517 and ground fault coordination studies required for health care facilities. 

Figure 4: This is an aerial shot of the construction of the SABEY Intergate Quincy Data Center Facility. Lane Coburn & Assocs. worked closely with the owner, electrical contractor, and switchgear vendors to ensure proper coordination between all overcurreAn unexpected loss of power can have a significant effect on business, especially in a mission critical environment. Isolating an electrical fault condition to the smallest area possible is essential in providing the most reliable electrical distribution system with maximum uptime for your facility. Expensive electronic distribution protection equipment is not worth the extra cost unless a proper protective device coordination study is provided by an experienced electrical engineer. 

A properly coordinated system will limit an electrical fault to the nearest upstream protective device. After a one-line diagram of an electrical distribution system is completed and the brand and model of the protective devices are selected, an overcurrent protective coordination study can be completed. Protective devices can consist of both fuses and breakers. Evaluating the merits of choosing to use fuses or circuit breakers is beyond the scope of this article. The primary focus of this article is adjustable trip circuit breakers as the protective device. 

Severalparameters can be selected for each protective device. The total number, type, and sensitivity of the settings will depend on the specific device. Adjustment of these parameters allows for what is referred to as “curve shaping.” Curve shaping allows for better coordination between upstream and downstream overcurrent protection devices. Below is a list of the common possible parameters.

Continuous current rating 

Continuous current rating is often called the current sensor or plug. There are several possibilities: 

  • Long-time pickup (long time per unit): This is the long-time trip setting of the overcurrent protective device. This parameter, also known as continuous amps, is a percentage of the breaker’s nominal rating and can typically be set at 20% to 100%. This setting is usually achieved with a thermal overload in a molded case circuit breaker.
  • Long-time delay: This setting allows for inrush from motors to pass without tripping the breaker. This setting effects the position of the I squared T slope just below the continuous current setting.
  • Short-time pickup: This is typically provided with an adjustment of 5 to 10 times. This setting allows downstream overcurrent protection devices to clear faults without tripping upstream devices. It can also be adjusted to allow for transformer inrush current. 
  • Short-time delay and instantaneous override: This setting postpones the short-time pickup. It can be a fixed setting or an I squared T ramp setting. This allows for better coordination between upstream and downstream devices. An instantaneous override can be set at high current to override this function and protect electrical equipment. The I square T function of the short-time delay can provide better coordination when coordinating a breaker with a fuse.
  • Instantaneous: This setting will trip the overcurrent protective device with no intentional delay.
  • Ground fault setting (ground fault per unit): This is the percentage of the rating of the breaker for the ground fault setting. Per the NFPA 70: National Electrical Code, ground fault cannot exceed 1,200 amps, regardless of the size of the breaker.
  • Ground fault delay: This setting allows for a time delay before ground fault pickup, which allows for better selective coordination between multiple levels of ground fault protection. In addition, the time delay cannot exceed 1 second (60 cycles) for ground fault currents of 3,000 amps or more. 

Before beginning a coordination study, the electrical engineer should design a one-line diagram and coordinate with the electrical contractor and/or the equipment provider to determine the actual equipment to be installed. The following are required to provide an accurate protective coordination study:

  • Description, make, and catalog numbers of protective devices
  • Full load current at the protective device
  • Transformer kVA, impedance, and inrush data
  • Available fault current at the protective device
  • Conductor cable information including current carrying capacity and insulation type
  • Protective device design requirements from the serving utility.

It is common to perform complicated electrical protection coordination studies with computer software. These software platforms typically contain libraries that include most of the common overcurrent protective device required settings. Sometimes new device settings have to be developed by the electrical engineer in the software program. 

As noted above, with the review of protective coordination study basics, an electrical system’s reliability can be assured only if proper coordination is implemented between protective devices. The next portion of this article will review instances where the National Electrical Code requires a protective coordination study and where K-rated transformers are employed to deal with electronics and nonlinear loads can reduce reliability if not properly coordinated.

Using K-rated transformers 

On a typical transformer, the current and associated magnetic field is 90 deg out of phase with the voltage. When you close a breaker and turn on a transformer, the instantaneous magnetic field can be twice as high as normal. In an “ideal” transformer, the current required to supply this magnetic field would also be twice as high. However, in a real transformer, the core is saturated and the actual current required to create the field can be 12 times as high as normal. Factors such as the size of the transformers’ cores and the time the voltage is applied play roles in determining the amount of inrush current. 

Figure 1: This indicates a 30 kVA transformer protected by a 45 amp circuit breaker. The “Tx” refers to the transformer inrush. The 45 amp breaker curve is represented by the red hash marks. This breaker curve is to the right of the “Tx” ensuring that theThe actual inrush current mentioned above is different depending on the actual transformer manufacturer. It is critical to contact the specific manufacturer of the transformer supplied in the field. If actual transformer inrush data is not known, common industry standard is to assume the inrush is 12 times for 0.1 seconds and 25 times for 0.01 seconds. Figure 1 illustrates the transformer inrush at 12 times for 0.1 seconds. 

Electrical engineers were running into trouble some years back when the K13-rated transformer was becoming more prolific in regular office and mission critical facilities. A K13-rated transformer is often just a larger transformer with a smaller rating to compensate for harmonics. The same 110 amp breaker typically on the primary side of a regular 75 kVA transformer may trip when protecting a 75 kVA, K13 transformer. For sizing of the primary side overcurrent protective device for K13 or higher rated transformers, I recommend multiplying the input full load amps of a transformer by 125% and going to the next common size up. In addition, a breaker with the instantaneous setting is often required to allow for the transformer current inrush. As a final step, I recommend a coordination study to ensure the system will work before it is too late, after construction is complete and the engineer is stuck with an angry owner.

<< First < Previous Page 1 Page 2 Next > Last >>

The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
Pipe fabrication and IIoT; 2017 Product of the Year finalists
The future of electrical safety; Four keys to RPM success; Picking the right weld fume option
A new approach to the Skills Gap; Community colleges may hold the key for manufacturing; 2017 Engineering Leaders Under 40
Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
The cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Power system design for high-performance buildings; mitigating arc flash hazards
VFDs improving motion control applications; Powering automation and IIoT wirelessly; Connecting the dots
Natural gas engines; New applications for fuel cells; Large engines become more efficient; Extending boiler life

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

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.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me