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What is NEC 240.87?

Originally introduced in 2011, the National Electrical Code (NEC) Section 240.87 was developed to reduce clearing times when a circuit breaker without an instantaneous trip unit was installed. This section was titled “Non-Instantaneous Trip.” Later, in the 2014 revision cycle, the NEC changed this section to “Arc Energy Reduction,” and the scope was slightly changed. Any breaker whose “highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1200 amperes or higher” now fell under the scope of this article, not just breakers without an instantaneous trip function. In layman’s terms: A breaker that can be set to 1200A or greater must have a way to reduce clearing time. This applies to main breakers as well as feeder breakers.

How can I meet this requirement?

Fortunately, NEC 240.87 lists seven approved methods for reducing clearing times:

(1)   Zone Selective Interlocking (ZSI): ZSI uses restraint signals to provide faster clearing times within the desired “zone.” While typically a more complex solution due to increased communications and wiring, ZSI can provide greatly reduced clearing times without sacrificing system coordination. Consider a Low Voltage (LV) switchgear with ZSI communications between the main breaker and feeder breakers. For a bus fault, ZSI permits the main to trip fast, as no restraint signal is received from the feeders. However, for a through fault, the main receives a restraint signal from the feeder that the fault is on and allows the feeder to trip the fault before the main. Thus, system coordination is maintained, but the clearing time for a bus fault (and subsequently, the incident energy), is greatly reduced.

(2) Differential Relaying: Similar to ZSI, this method provides faster clearing times within the zone of protection. Current Transformers (CTs) are used to develop the boundaries for this zone, and should a fault occur within this region, high speed signals are sent to the relay to initiate breaker opening. Bus differential and transformer differential are the most popular methods within this option for reducing clearing time. Transformer differential, if the secondary CTs are installed in the proper location, are a great way to reduce clearing times (and subsequently incident energy) for the main breaker sections in LV switchgear, notorious for having over 40 cal/cm2 available.

(3) Energy-reducing maintenance switching with local status indicator: This method is equivalent to a temporary instantaneous setting, as activation of the switch reduces the instantaneous setting of a breaker to a setting below the available arcing current. While a simpler and typically cost-effective solution, the method has limitations. It requires worker interaction to activate the switch and it is only available with certain breaker trip units.

(4) Energy-reducing active arc flash mitigation system: Typical industry solutions that fall under this category are optical arc flash detection and “crowbar” devices. Optical arc flash detection systems rely on the use of light sensors installed within equipment compartments that are programmed to look for high levels of light, indicating a fault event is taking place. When high light is seen at the same time as current, the tripping device is programmed to trip faster than the normal setting. Crowbar devices involve displacing the fault to another area within the system, providing the fault current a redirected path to ground.

(5) An instantaneous trip setting that is less than the available arcing current: This one is simple enough: if the breaker instantaneous function is set below the available arcing current, no additional changes are required! This is typically the lowest cost solution. However, it may not always be possible to set an instantaneous function low enough, either due to coordination restrictions or limitations of the instantaneous setting range of the breaker. It is recommended that a short circuit, coordination, and arc flash study be performed to determine whether a breaker is currently set, or can be set, to meet this requirement.

(6) An instantaneous override that is less than the available arcing current: Similar to (5), an instantaneous override (an instantaneous function that is designed to protect the breaker from fault currents and is not adjustable) set below the available arcing current may also meet this requirement.

(7) An approved equivalent: This method is listed to mitigate potential differences in interpretations from AHJs. Sometimes, the methods listed in this section may meet the NEC requirement, but have no impact on the incident energy calculation due to equipment construction or low arcing currents.

What changed in 2020?

There are two primary modifications to this section. The first emphasizes the importance of having the method of choice operate below the available arcing current. NEC 240.87 (B) now reads: “One of the following means shall be provided and shall be set to operate at less than the available arcing current.” This new language reinforces the original intent of the code and applies it to all seven methods listed.

The second modification eliminates temporary adjustment of the instantaneous trip setting to achieve arc energy reduction. The fifth method now reads:

(5) An instantaneous trip setting. Temporary adjustment of the instantaneous trip setting to achieve arc energy reduction shall not be permitted.

Significance

Choosing the proper method to meet the requirements of NEC 240.87 can be difficult. Equipment construction and available arcing fault current must be considered in the early stages of developing a design solution. In most cases, performing a short circuit, coordination, and arc flash study will play a key role in developing a customized solution that both meets NEC requirements and effectively reduces incident energy. This study will also identify problem areas in your electrical system that have high available incident energy, whether mitigation by the NEC is required or not!

Written By: Josh Brandenburg, PE and Ron Fair