Know the definitions for selective coordination in mission critical systems

Selective coordination insights

• The text discusses the importance of selective coordination in mission critical facilities, emphasizing the need for electrical engineers to design systems that prioritize reliability and safety during outages.
• It highlights the NEC’s specific guidance on selective coordination and the various requirements for different types of critical facilities, underscoring the crucial role it plays in ensuring uninterrupted operation and safeguarding human life.

Mission critical facilities, such as hospitals and data centers, necessitate electrical engineers to design robust power systems for reliability, safety and redundancy. These designs must adhere to strict codes and practices, including selective coordination, to ensure timely protection during electrical faults. Engineers must carefully coordinate protective devices to ensure that the closest one to the fault operates swiftly, preventing disruptions to upstream systems.

This edited transcript of the Jan. 31, 2024, webcast Critical power: How to design for selective coordination in mission critical systems can be used to help electrical engineers understand the definitions.

Presenters:

• Radames Cocco, PE, LEED AP, Principal Electrical Engineer, Spectrum Engineers
• Joshua Fluecke, PE, LEED AP, Senior Principal, Syska Hennessy Group

Joshua Fluecke: Mission critical facilities typically demand electrical engineers to design and specify significant power amounts to enhance reliability, life safety, outage limitation and redundancy provision. Various types of critical facilities are inherently designed to safeguard either human lives or business functions, including hospitals, laboratories, security operation centers and data centers. Today, we will delve into the considerations and criteria for planning the design of mission critical facilities concerning overcurrent devices and electrical systems to ensure selective coordination.

Now, regarding key definitions, the NFPA 70: National Electrical Code (NEC) provides specific guidance on selective coordination. However, determining ratings and settings for protected devices to establish proper selective coordination entails decisions among competing objectives. The primary objectives, selectivity and equipment protection remain consistent.

In a selective system, a fault or overload condition is isolated by the operation of the protective device nearest to the fault location, thereby maximizing service continuity for the remainder of the system. Many mission critical systems install emergency backup and standby generator systems to ensure maximum reliability and continuity of electrical power during outages caused by equipment failures or human error. The aim is to keep critical systems operational for as long as possible during these events. However, merely installing backup generator systems is not sufficient; the electrical system must be designed to maximize power to critical loads and minimize outages even during physical catastrophes. Analyzing the characteristics of overcurrent protected devices and the overall electrical system is crucial to ensure they perform as intended.

The NEC mandates special requirements for these electrical systems to ensure reliable operation, minimize faults and mitigate the impact of outages as much as possible, with the goal of ensuring human safety during emergencies or essential health care functions.

Visually, the code describes selective coordination as follows: In an electrical distribution system, a fault location is indicated on the left, with protective devices opening in the event of a fault, resulting in an unnecessary loss of power without selective coordination. Conversely, in a selectively coordinated system, only the protective device directly upstream of the fault opens, isolating the fault while keeping other upstream devices operational.

Figure 1: NFPA 70: National Electrical Code provides definitions for selective coordination. Courtesy: Consulting-Specifying Engineer

Radames Cocco: For mission critical facilities, coordination is imperative. However, the NEC lists certain areas where it’s not just a matter of preference but a mandate. These include emergency systems, legally required standby systems, health care facilities, fire pumps, elevators, critical operation data centers and critical operations power systems.

Overcurrent devices serving these systems must be selectively coordinated to ensure proper functioning during faults or outages. Coordination requirements vary depending on the type of facility and system, but the overarching objective is to maintain reliability and safety in critical situations.

Coordination requirements vary depending on the type of facility and system, but the overarching objective is to maintain reliability and safety in critical situations. The NEC provides specific guidance on selective coordination, ensuring that overcurrent devices are effectively coordinated to mitigate risks and ensure continuity of operations. Through careful planning and adherence to NEC guidelines, mission critical facilities can enhance their resilience and minimize the impact of electrical failures.

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