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.

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