Understanding NEC 240.87: Arc Energy Reduction Guide

Your Expert Guide to Arc Energy Reduction

As a licensed electrician, ensuring the safety of yourself and your colleagues is paramount. The National Electrical Code (NEC) provides the framework for safe electrical installations, and one of its most critical sections for worker safety is nec 240.87. This provision mandates arc energy reduction for large circuit breakers, specifically those rated 1200A or higher. The core purpose of nec 240.87 is to minimize the immense danger posed by arc flash events by reducing the total energy released. This is achieved by decreasing the circuit breaker clearing time when a fault occurs. Understanding and correctly applying these requirements is not just about compliance; it’s about preventing catastrophic injuries. This article provides an in-depth look at the methods for achieving compliance, the importance of an arc flash hazard analysis, and how these rules integrate with broader electrical safety standards like NFPA 70E.

What is NEC 240.87 and Why is it Critical for Safety?

First introduced in the 2011 NEC (NFPA 70), Section 240.87 was created to protect a qualified electrical worker from the severe hazards of high incident energy during an arc flash event. An arc flash is a violent explosion of energy that happens during a fault, releasing intense light, heat, and pressure. The amount of energy in an arc flash is directly proportional to the time it takes for an overcurrent protective device (OCPD) to clear the fault. For large circuit breakers (rated or adjustable to 1200 amps or more), the intentional delays set for selective coordination can result in dangerously high incident energy levels.

NEC 240.87 addresses this by requiring one of several listed methods to be implemented to reduce the clearing time. This is crucial when maintenance or servicing is performed on energized equipment. The documentation for the chosen method must be available to those who design, install, and inspect the system. For certain methods, such as using an instantaneous trip or override, this documentation must demonstrate that the setting is below the available arcing current. This focus on reducing clearing time is a fundamental shift toward engineering safer work environments.

The Relationship Between NEC 240.87, NFPA 70E, and NEC 110.16

These three standards form a triad of electrical safety. While nec 240.87 mandates the “how” of reducing arc energy in equipment, NFPA 70E, the “Standard for Electrical Safety in the Workplace,” dictates safe work practices. It requires employers to perform an arc flash hazard analysis to identify hazards and determine safe work boundaries and required PPE. The results of this analysis directly inform the need for and the settings of the arc reduction methods required by nec 240.87. Furthermore, NEC 110.16 mandates arc-flash hazard warning labels on equipment that is likely to be worked on while energized, providing a visual alert to the qualified electrical worker. Together, these standards create a comprehensive system to protect workers from arc flash dangers.

Methods for Arc Energy Reduction per NEC 240.87(B)

Section 240.87(B) provides several approved methods to reduce the clearing time of an OCPD. The selection of a method depends on system design, cost, and the need for selective coordination. An arc flash hazard analysis is essential to determine the available arcing current and ensure the chosen method is effective.

• Zone-Selective Interlocking (ZSI): This is an intelligent system where circuit breakers communicate. If a fault occurs, the breaker closest to the fault signals the upstream breakers to hold their tripping, allowing it to clear the fault instantaneously without an intentional delay. This preserves selective coordination while drastically reducing incident energy.
• Differential Relaying: This method uses current transformers to compare the current entering and leaving a piece of equipment. If a difference is detected (indicating a fault within the zone), it trips the breaker with no intentional delay. It’s highly effective but can be more complex and costly to implement.
• Energy-Reducing Maintenance Switching (ERMS): An energy-reducing maintenance switching device allows a worker to temporarily enable a faster, more sensitive trip setting (like an instantaneous trip) before working on equipment. A local status indicator light confirms the protective mode is active. This is a practical and widely used method.
• Energy-Reducing Active Arc Flash Mitigation System: These advanced systems use sensors (often light-based) to detect the signature of an arc flash and trip the breaker in milliseconds, significantly faster than standard overcurrent detection.
• Instantaneous Trip Setting: A permanent instantaneous trip setting can be used if it is set below the calculated available arcing current. The code prohibits using temporary adjustments for this purpose to ensure reliability.
• Instantaneous Override: Some breakers have a built-in, non-adjustable override that trips the breaker at a very high current level to protect itself. If an analysis proves this override setting is below the available arcing current, it can be a compliant method.
• Approved Equivalent Means: This allows for other technologies or engineered solutions, provided they are approved by the authority having jurisdiction (AHJ) and achieve the goal of arc energy reduction.

Step-by-Step: Implementing an Arc Energy Reduction Method

For a qualified electrical worker, understanding the practical application is key. Here is a simplified process for ensuring compliance with nec 240.87:

1. Identify Applicable Equipment: Review the single-line diagram to identify all circuit breakers rated or adjustable to 1200A or higher. These fall under the scope of nec 240.87. These requirements typically apply to large commercial and industrial services, not residential dwelling services.
2. Perform an Arc Flash Hazard Analysis: An analysis based on standards like IEEE 1584 is required. This study calculates the prospective incident energy and the available arcing current at each relevant point in the system. This data is fundamental to selecting and setting a protection method.
3. Select a Compliant Method: Based on the analysis, choose one of the seven methods listed in 240.87(B). The decision will balance safety, cost, complexity, and the need to maintain selective coordination with other devices.
4. Document and Set the Device: Implement the chosen method. For example, if using an instantaneous trip setting, adjust the breaker’s electronic trip unit accordingly. Document the setting, the method used, and the location of the breaker as required by 240.87(A).
5. Performance Testing: While not universally mandated by NEC 240.87 at installation, performance testing is a critical best practice to verify the system works as designed. Primary current injection testing is a common and thorough method for this, as it verifies the entire trip circuit, from sensors to the breaker mechanism.
6. Label the Equipment: Ensure the equipment has the proper nec 110.16 arc flash warning label, updated with the new, lower incident energy values resulting from the arc reduction method. This informs workers of the reduced hazard. For more on worker safety, read about how the 2023 NEC improves worker safety.

Following these steps ensures not only compliance with the code but a genuinely safer working environment. As a professional, enhancing your expertise in these areas is crucial. For further development, consider specialized training. Enhance your safety knowledge with our NFPA 70E and Arc Flash courses.

Key Takeaways for Electricians

• NEC 240.87 applies to circuit breakers rated or adjustable to 1200A or higher. Its sole purpose is to reduce the massive energy released in an arc flash event.
• Reducing Circuit Breaker Clearing Time is the Goal. All approved methods focus on tripping the breaker faster than its coordinated settings would normally allow.
• An Arc Flash Study is Essential. You cannot effectively comply with nec 240.87, especially when using instantaneous trip methods, without knowing the available arcing current.
• Documentation is Mandatory. You must document the method used, its settings, and its location, and this documentation must be available on-site.
• Testing is a Best Practice. While not mandated in all circumstances, performance verification, such as with primary current injection testing, is a crucial step to ensure the system operates as designed.
• Impact on Safety and PPE: A properly implemented system reduces the calculated incident energy, which often allows for a lower level of required PPE, a critical topic also linked to NFPA 70E hearing protection requirements.

Primary Sources

• NFPA 70, National Electrical Code (NEC)
• NFPA 70E, Standard for Electrical Safety in the Workplace
• IEEE 1584, Guide for Performing Arc-Flash Hazard Calculations

Frequently Asked Questions (FAQ) about NEC 240.87

What is the primary purpose of nec 240.87?

The primary purpose of nec 240.87 is to enhance safety for a qualified electrical worker by mandating arc energy reduction for large circuit breakers (1200A or higher). It requires implementing a method to reduce the circuit breaker clearing time during a fault, thereby lowering the dangerous incident energy of an arc flash.

Does an energy-reducing maintenance switching (ERMS) system satisfy nec 240.87?

Yes, an energy-reducing maintenance switching (ERMS) system with a local status indicator is one of the explicitly listed and approved methods in nec 240.87(B). It allows a worker to put the OCPD into a state with no intentional time delay, which significantly reduces arc energy while work is being performed.

How does zone-selective interlocking (ZSI) work for arc energy reduction?

Zone-selective interlocking (ZSI) is a system where breakers communicate. The breaker nearest a fault trips instantaneously while sending a “restraint” or blocking signal to the upstream breaker to prevent it from tripping. This maintains selective coordination for the rest of the system but ensures the fastest possible clearing time for the fault, directly meeting the goal of nec 240.87.

Is an arc flash hazard analysis required to comply with nec 240.87?

While the text of nec 240.87 doesn’t explicitly state “thou shalt perform an analysis,” compliance is often impossible without one. For certain methods, like using an instantaneous trip or override, the code requires that the setting operate below the available arcing current. The only way to determine this value and document compliance is through an arc flash hazard analysis, making it an essential prerequisite.

What other NEC sections are relevant to worker safety and overcurrent protection?

Many sections work in concert with nec 240.87. Key related sections include nec 110.16 (Arc-Flash Hazard Warning), NEC 240.67 (Arc-Energy Reduction for Fuses), nec 230.71 (Maximum Number of Disconnects), and even new requirements like nec 230.85 (Emergency Disconnects) for dwelling units, which also prioritize first responder safety. Other relevant sections like nec 310.15 (Ampacities) and nec 210.63 (Equipment Servicing Receptacles) also contribute to a holistic approach to electrical safety standards.