You're right to be skeptical. There is no blanket requirement in the NEC (National Electrical Code) mandating LSI (Long-Time, Short-Time, Instantaneous) circuit breakers on all amperages for life safety loads.Here's a breakdown: Where the Confusion Might Come From



Selective Coordination

Selective coordination is a major driver for using LSI breakers on life safety systems. The NEC, specifically Section 700.27 (Emergency Systems), Section 701.32 (Legally Required Standby Systems), and Section 708.54 (Critical Operations Power Systems (COPS)), requires selective coordination under specific circumstances. Selective coordination means that a fault downstream should only trip the closest upstream breaker, allowing the rest of the system to remain energized.

High-Ampacity Systems

For large systems, especially those using transfer switches and generators, achieving selective coordination often necessitates using breakers with adjustable trip units, including LSI functionality. It's harder to achieve selective coordination with simple thermal-magnetic breakers in these scenarios.

Healthcare Facilities

NFPA 99 (Healthcare Facilities Code) has very strict selective coordination requirements for essential electrical systems. This often leads to the specification of LSI breakers.

Engineering Best Practices

Even if not strictly required by the NEC, many engineers specify LSI breakers for life safety loads as a design choice for improved system reliability and maintainability. The ability to adjust trip curves provides greater flexibility in coordinating the system.

Why a Blanket Requirement is Unlikely and Potentially Problematic



Cost

LSI breakers are significantly more expensive than standard thermal-magnetic breakers.

Complexity

Adjusting the trip settings on LSI breakers requires knowledge and careful consideration. Improper settings can be worse than using standard breakers.

Smaller Loads

For very small loads (e.g., a single emergency light), adding an LSI breaker is overkill and not practical. The fault current available at the end of a long branch circuit might not even reach the short-time or instantaneous trip settings, rendering those functions useless.

Potential for Mis-Coordination

As you noted, adding a short-time delay can be detrimental if not coordinated properly with other breakers in the system. It can lead to nuisance tripping and system failures.

Arc Flash Hazard

Adding short time delay to the breaker can increase the incident energy potential during an arc flash event.

What the NEC Does Say (relevant sections)



700.27 (Emergency Systems), 701.32 (Legally Required Standby Systems), 708.54 (Critical Operations Power Systems)

These sections require selective coordination with all supply-side overcurrent protective devices. This is the key requirement that often leads to the use of LSI breakers, but it's not a direct mandate for them.

NEC Article 240

Covers overcurrent protection in general, but doesn't specifically require LSI breakers for life safety loads.

NEC Article 517

Covers healthcare facilities, and although it doesn't explicitly say "use LSI breakers," the strict coordination requirements in NFPA 99 often make them necessary.

How to Respond to the Inquiry


Clarify the Scope

Ask the person who made the statement to provide the specific code reference they are relying on. It's possible they are misinterpreting a requirement or applying a specific project requirement too broadly.2.

Explain Selective Coordination

Explain the concept of selective coordination and why it's important for life safety systems.3.

Point Out the NEC Sections

Refer to NEC 700.27, 701.32, and 708.54 regarding selective coordination requirements.
4.

Highlight Cost and Complexity

Discuss the increased cost and complexity of using LSI breakers for all amperages.5.

Advocate for Proper Engineering Analysis

Emphasize the need for a proper short-circuit and coordination study to determine the appropriate overcurrent protection devices for each circuit. A qualified electrical engineer should perform this analysis. The engineer will evaluate the fault current levels and time-current curves (TCCs) to ensure selective coordination is achieved without compromising safety or reliability. Sometimes, a simple thermal-magnetic breaker is the best solution.6.

Consider System Size and Design

For smaller systems with limited fault current availability, standard breakers might be sufficient and more cost-effective.

In summary: The idea that the NEC now requires LSI circuit breakers on all amperages for life safety loads is incorrect. Selective coordination is the driving factor, and while LSI breakers are often a good solution for achieving that, they are not always necessary or appropriate. A proper engineering analysis is crucial to determine the best overcurrent protection strategy for each specific application. Flag for review