NEC 240.87: Arc Energy Reduction

A Guide for Electrical Professionals for the 2023 NEC edition.

What is NEC 240.87?

NEC 240.87 is a code section in the National Electrical Code (NEC) that requires arc energy reduction methods for circuit breakers rated at 1200 amps or higher. Arc energy is the amount of heat and light produced by an electrical arc fault, which can cause severe damage to equipment and personnel. Arc energy reduction methods are designed to reduce the duration and intensity of an arc fault, thereby minimizing the potential hazards.

Why is NEC 240.87 important?

NEC 240.87 is important because it addresses a serious safety issue in electrical systems. Arc faults can occur due to various reasons, such as insulation failure, loose connections, foreign objects, or human error. When an arc fault occurs, it can generate temperatures of up to 35,000 degrees Fahrenheit, which can melt metal, ignite combustible materials, and cause severe burns and injuries. Arc faults can also create pressure waves that can rupture eardrums, damage lungs, and shatter windows. According to the National Fire Protection Association (NFPA), arc faults are responsible for more than 2,000 injuries and 400 deaths per year in the United States.

What are the arc energy reduction methods required by NEC 240.87?

NEC 240.87 requires one of the following arc energy reduction methods for circuit breakers rated at 1200 amps or higher:

• Zone-selective interlocking (ZSI): This method uses communication signals between circuit breakers to coordinate their tripping times and isolate the faulted zone as quickly as possible. ZSI can reduce the arc energy by up to 50% compared to conventional coordination. ZSI requires compatible circuit breakers and wiring between them.
• Differential relaying: This method uses current transformers to measure the difference between the incoming and outgoing currents of a circuit breaker and trip the breaker if the difference exceeds a set value. Differential relaying can reduce the arc energy by up to 80% compared to conventional coordination. Differential relaying requires additional current transformers and relays.
• Energy-reducing maintenance switching with local status indicator: This method uses a switch that can be manually operated to reduce the trip threshold of the circuit breaker and a light that indicates the status of the switch. Energy-reducing maintenance switching can reduce the arc energy by up to 70% compared to conventional coordination. Energy-reducing maintenance switching requires a switch and a light on or near the circuit breaker.
• Energy-reducing active arc flash mitigation system: This method uses a device that can detect and extinguish an arc fault by creating a low-impedance path for the fault current and applying a counter voltage to the arc. Energy-reducing active arc flash mitigation system can reduce the arc energy by up to 90% compared to conventional coordination. Energy-reducing active arc flash mitigation system requires a device and a power source.
• Instantaneous trip setting: This method uses a circuit breaker that can trip within one cycle or less of the fault current, reaching a preset value. Instantaneous trip setting can reduce the arc energy by up to 40% compared to conventional coordination. Instantaneous trip setting requires a circuit breaker with an adjustable instantaneous trip function.
• Instantaneous override: This method uses a circuit breaker that can override the normal time delay and trip immediately when the fault current exceeds a certain level. Instantaneous override can reduce the arc energy by up to 60% compared to conventional coordination. An instantaneous override requires a circuit breaker with an instantaneous override feature.
• Approved equivalent means: This method uses any other technology that can reduce the arc energy to a level below the incident energy at the working distance. Approved equivalent means can vary in their performance and installation requirements. Approved equivalent means must be approved by the authority having jurisdiction. Sometimes creativity is needed, and the application of special relaying functions or other engineered solutions can achieve the same or better results for arc fault reduction. This will require engineering calculations in order to achieve this. Consult an electrical engineer to help you with other solutions.

How to comply with NEC 240.87?

To comply with NEC 240.87, electrical professionals should follow these steps:

• Identify the circuit breakers rated at 1200 amps or higher in the electrical system.
• Select and install one of the arc energy reduction methods for each circuit breaker.
• Label the circuit breaker with the arc energy reduction method and the location of the documentation.
• Document the arc energy reduction method and the settings of the circuit breaker.
• Test and verify the functionality of the arc energy reduction method.
• Update the arc flash hazard analysis and the arc flash labels of the electrical system.

NEC 240.87 is a code section that requires arc energy reduction methods for circuit breakers rated at 1200 amps or higher. It aims to improve the safety of electrical systems and reduce the risks of arc faults. Electrical professionals should be familiar with the arc energy reduction methods and the steps to comply with NEC 240.87.