Selective coordination is a critical design requirement for emergency electrical systems. It ensures faults are isolated to the smallest possible section while allowing the rest to function. Failure to implement selective coordination correctly can lead to catastrophic failures and compromised safety, operations and compliance. This case study examines a real-world incident where the absence of proper selective coordination resulted in a major power failure in a high-rise building, putting lives at risk and causing severe repercussions.

A high-rise office building underwent extensive renovations across multiple floors and tenants. The building’s electrical infrastructure included emergency and legally required standby power systems supporting life safety equipment such as fire alarms, elevators, elevator recall, egress lighting and exit signage. Despite National Electrical Code requirements for selective coordination, a failure to design, implement and verify compliance led to a disastrous outage.

Understanding requirements

As with many code requirements, the use of proper terminology is key. The first piece of the terminology “puzzle” is understanding what type of system it is. Terms such as “emergency,” “legally required,” “standby power” and “backup power” are often used interchangeably, but that is a mistake. Understanding the purpose of each of these systems and electrical loads required to be connected to each is the starting point for success. This information is not found in the Code, so familiarity with Chapter 27 of the International Building Code (IBC) becomes crucial when designing, installing and inspecting these systems. 

Once the system is identified, we need to use Article 700 for wiring methods, protection and performance requirements of the emergency power system and Article 701 for similar requirements of legally required standby systems. These articles will focus on requirements related to selective coordination as found in sections 700.32 and 701.32.

Let’s look at what it means to have a selectively coordinated electrical system. Selective coordination is defined in Article 100 as localization of an overcurrent condition to restrict outages to the circuit or equipment affected, which is accomplished by the selection and installation of overcurrent protective devices (OCPDs) and their ratings or settings for the full range of available overcurrents, from overload to the available fault current, and for the full range of OCPD opening times associated with those overcurrents. Full range of time and fault current are the key elements of this definition and an important distinction between a system that is selectively coordinated and one that is not.

The incident

During a severe thunderstorm, a power outage affected the building’s normal power supply. Flooding from the storm compromised portions of the building’s legally required standby and emergency electrical system. When the generator came online, a fault in the portions of the legally required standby and emergency system caused the upstream breakers to trip, leading to widespread failure across both systems. The immediate consequences included:

• Building evacuations: The purpose of the emergency system is to aid in the safe evacuation of building occupants. The emergency system’s failure prevented communication of evacuation instructions, which led to confusion.
• Compromised egress lighting: The loss of emergency lighting and exit signage resulted in mass panic, causing stampedes as people rushed to exits in darkness.
• Delayed rescue efforts: The legally required standby system powers critical first responder systems. Its failure affected elevator recall and area-of-refuge communications, which delayed rescue efforts.

Root cause analysis

A forensic electrical engineering team identified three key issues:

1. Improper OCPD selective coordination: The system failed to ensure downstream protective devices would trip before upstream devices. As a result, branch-circuit and feeder level faults led to widespread power outages.
2. Inadequate system studies: An engineer evaluated the emergency system and legally required standby systems, and a selective coordination study was done. However, the study showed coordination of OCPDs to a time of 0.1 seconds. Selective coordination is only achieved when the OCPDs are coordinated for the full range of fault currents and the time associated with them. 
3. Noncompliance with the Code: Sections 700.32 and 701.32 mandate selective coordination for emergency and legally required standby systems. This requirement was not met, directly contributing to the system failure.

Financial and operational impact

Beyond immediate life safety risks, the failure had long-term repercussions:

• Injuries: Thankfully, in this instance there was no loss of life. However, many occupants suffered minor injuries and some suffered severe ones that will permanently affect their quality of life. 
• Lawsuits and liability: Several occupants and families filed legal actions citing negligence by the electrical contractor, inspection department and building owner, leading to costly settlements.
• Costly system redesign: A system overhaul was necessary to bring these systems into compliance, costing hundreds of thousands of dollars for redesign and installation.

Lessons learned and best practices

This case highlights the severe risks of neglecting selective coordination. To prevent similar incidents, industry professionals should follow these best practices.

Conduct comprehensive selective coordination studies: Before installation and after any system modifications, engineers should conduct full-range selective coordination studies to ensure reliability and Code compliance.

Train key personnel: Facility managers, electrical engineers, contractors and inspectors must be trained in:

• Emergency and legally required power system operations
• Reading coordination studies and one-line diagram
• Applicable codes and regulations
• The consequences of failing to achieve and maintain selective coordination

Perform periodic reviews and audits: Emergency and legally required standby power systems should be reviewed to detect potential weaknesses before failures occur.

Conclusion

The failure to implement selective coordination in emergency electrical systems can lead to life-threatening consequences, regulatory penalties and financial burdens. This case study highlights proper planning, compliance with the NEC and IBC standards and routine system testing. By prioritizing selective coordination, building owners and electrical professionals can prevent cascading failures, protect lives and ensure reliable power to critical systems when it matters most.

Eaton