Coordinating protective devices in mission critical facilities

A coordination study ensures that the most reliable electrical system has been installed. Applicable codes and standards help engineers get it right.

12/04/2015


This article is peer-reviewed.Learning objectives:

  • Illustrate the basics of protective-device coordination studies.
  • Outline elevator protection coordination as required per the NEC.
  • Apply NEC Article 517 and ground-fault coordination studies required for health care facilities. 

A sudden power failure will have a dramatic effect on business, especially in a critical environment. Isolating a fault condition to the smallest area possible is essential in providing the most reliable electrical 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 engineer.

Figure 1: The TCC graph and one-line diagram indicate a 150-kVA transformer protected by a 225-A circuit breaker. The “Tx” refers to the transformer inrush in red. The 225-A breaker curve is represented by the blue curve. This breaker curve is to the righ

NFPA 70-2014: National Electrical Code (NEC) defines selective coordination as: "Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent protective device opening times associated with those overcurrents."

In other words, a properly coordinated system will limit disconnection to the nearest upstream protective device.

Protective-device coordination study basics

The main types of overcurrent protection used in mission critical environments are circuit breakers, fuses, and relays. This article focuses on circuit breakers and fuses. Relays are not addressed due to space constraints.

Depending on the circuit breaker type, there may be several parameters that can be selected for each protective device. A thermal magnetic breaker may have no adjustment at all, or only minimal adjustment to the instantaneous region, whereas a fully adjustable electronic trip breaker may have many.

Adjustment of these parameters allows for what is referred to as "curve shaping." Curve shaping enables better coordination between upstream and downstream overcurrent-protection devices. Typical parameters include:

  • Overload region (long time trip unit): This is the long time trip setting of an overcurrent protective device. This parameter, also known as continuous amperes, is a percentage of the breaker's nominal rating.
  • Long time delay: This setting allows for inrush from motors to pass without tripping the breaker. This setting affects the position of the I2t slope just below the continuous-current setting.
  • Short-time pickup: This setting is typically provided with an adjustment of 5 to 10 times the inrush current. This setting allows downstream overcurrent-protection devices to clear faults without tripping upstream devices. This setting can also be adjusted to allow for transformer inrush current.
  • Short time delay, instantaneous override: This setting postpones the short-time pickup. Setting this parameter can be done on a fixed setting or an I2t ramp setting. This allows for better coordination between upstream and downstream devices. An instantaneous override can be set at high-current value to override this function and to protect electrical equipment. The I2t 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 trip unit): This is the percentage of the rating of the breaker for the ground fault setting. According to the NEC, ground fault cannot exceed 1,200 A, regardless of the size of the breaker.
  • Ground fault delay: This setting allows for a time delay before ground fault pickup. This allows for better selective coordination between multiple levels of ground fault protection. In addition, the time delay cannot exceed 1 sec (60 cycles) for ground-fault currents of 3,000 A or more.
  • Reduced arc flash mode: This setting allows the breaker to be manually taken out of coordination for short periods of time during maintenance to reduce the arc flash hazards on the system.

When performing electrical engineering studies for mission critical environments, the required documentation includes:

  • Description, rating, make, and catalog numbers of protective devices
  • Full-load current at the protective device (3-phase and line-to-ground)
  • Transformer kVA, impedance, and inrush current data and connection type (delta-wye, etc.)
  • Available fault current at the protective device
  • Cable and conductor sizes
  • Protective-device design requirements from the serving utility
  • Voltage at each bus.

After the aforementioned critical information is typed into the software database, the function of protective devices can be graphically presented. The resulting graphic representation is called a time-current curve (TCC). When more than one electrical device is overlaid on a single graph, the relationship of the characteristics among the devices is presented. Any potential issue, such as overlapping of curves or long time intervals between devices, are illustrated. Fault-current conditions can be illustrated by indicating on the current scale the maximum and minimum value of short-circuit currents (3-phase and line-to-ground) that can occur at various points in the circuit (see Figure 1). It is common today to perform complicated electrical protection coordination studies with computer software. These software platforms typically contain libraries that include most of the common overcurrent protective devices and their available adjustments.


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Steven , AE, United Kingdom, 01/21/16 11:32 AM:

I am based in the UK so have used British Standard orientated software to conduct cable protection and overcurrent discrimination/selectivity studies. What are the most common software packages used to complete these studies according the NEC regulations?
GENE , NM, United States, 01/28/16 09:26 AM:

Why does the article indicate that coordination study "ensures that the most reliable electrical system has been installed" rather than simply that coordination ensures that the installed system is operating at the optimum level for the installation. A marginal system cannot be made more reliable (redundancy levels and such) by providing better coordination. It can operate as well as can be expected. One of the biggest unknowns (increasingly so) is the utility and limited information they typically provide regarding the characteristics of the service.
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