Seven Methods for Reducing Arc Flash Energy

Seven Methods for Reducing Arc Flash Energy

Arc flash study mitigation is required before running powered electrical equipment. When powered electrical equipment arcs, energy reduction minimizes arc energy. These techniques range from de-energize to costly technology solutions.

Section 240.87, formerly known as Coinstantaneous Trip, was added to the 2011 NEC to trip breakers without purposeful delay. The 2014 NEC revised the term to Arc Energy Reduction in order to improve worker safety by decreasing arc flash incident energy (AFIE) on circuit breakers that can be changed at 1,200A or above.

The 2020 NEC Part 240.87 describes seven methods for limiting arc flash energy during design/construction or retrofit. Each of the seven approaches has advantages and disadvantages.

Approach 1—zone selective interlocking (ZSI)

ZSI uses upstream and downstream breakers to eliminate purposeful short-time and ground-fault delays in order to shorten fault-clearing time. This kind of communication allows the nearest upstream device to isolate and correct problems in a selectively coordinated system without delay.

Pro: Allows for targeted coordination for external problems and speedy reaction for internal ones (no intentional time delay).

Con: Hard wiring linking equipment is time-consuming and costly to install and test.

Approach 2—Differential relaying is the second approach.

To address difficulties, programmable relays detect and compare several currents. The current in the protected zone corresponds to its outflow. It is common to use medium-voltage differential relaying. This method is created to order and installed.

Site-specific.

Relays and large current transformers (CTs) add cost and space.

Approach 3—an energy-saving maintenance switch (ERMS) with local status indicator

This capacity is provided by circuit breakers with remote switches and indicator lights. During an arcing fault event, an ERMS switch on a trip unit immediately trips a breaker. Protective settings A are often selectively coordinated with upstream and downstream equipment in routine operations. This coordinated system has a high AFIE. The maintenance switch engages protective settings B to reduce the AFIE while maintaining downstream equipment. After maintenance, the ERMS will resume normal functioning with precautionary settings A enabled (along with the higher AFIE). This simple design has one flaw: the maintenance switch occasionally remains on, activating protection settings B. As a result, the local status indicator light allows for establishing overlap or mis-coordination between that device and downstream devices. Depending on the load, this omission may result in nuisance tripping.

Manufacturers’ protection settings B philosophies differ. Some manufacturers’ protection settings B set instantaneous settings to low by default, whilst others enable B settings to be adjusted, allowing research engineers more options. Your research engineer must understand these various concepts and owner expectations.

The pricing is reasonable. The ERMS indicator light makes this decision easier.

Mistakes. For miscoordination and nuisance tripping, leave the switch in maintenance mode. Certain manufacturers are unable to program.

Coordination software assists research engineers in creating protected environments.
To develop protected settings, research engineers employ coordinating software.

Approach 4—energy-saving active arc-flash study mitigation

This is one of several unique methods that limit arc-fault energy discharge automatically. The following less-known principles are supported by NFPA 70E, Standard for Electrical Safety in the Workplace, Annex O:

By establishing a low-impedance current channel for the arcing fault, the upstream breaker can clear the fault quickly. This process generates a bolted fault on the circuit, shifting electrical energy to a separate channel and immediately activating the upstream breaker.
Another solution is an arc-flash relay with light sensors and a current sensor (often a CT) to detect overcurrent. When these two components are identified together, the protective system is instantly activated.
Line-side isolation barriers save energy by enclosing line-side conductors. This option protects personnel on the line from shock and arc-flash.
Arc-resistant equipment has the ability to deflect arc energy away from employees.
End-user or application-specific design is advantageous.

Con: Design, installation, and testing are all expensive.

Approach 5—immediate trip setup with no transient alteration allowed

This technology was first utilized as a temporary maintenance switch, decreasing the instantaneous setting to decrease the AFIE during maintenance. Yet, personnel changed random protection settings without a research engineer detecting the true arcing cause and proposing setting changes. As a consequence, settings that did not minimize the AFIE and wrongly protected employees were implemented. Because there was no indicator light (as in Method 3), settings were frequently not changed back or reapply at random because the worker could not recollect them. Human errors disrupted the electrical system and may have caused the arc-flash label to be incorrect, resulting in hazardous working conditions. As a precaution, the 2020 NEC prohibits temporary changes to the instantaneous trip setting to decrease arc energy.

A trained engineer calculates the arcing-fault current and permanently adjusts the protection settings to trip on it in this method. The low settings of this strategy may result in nuisance tripping and downstream coordination difficulties.

Pro: A low-cost option for current protection settings.

Difficulties with downstream coordination and tripping.

Approach 6—immediate override

A factory-set instantaneous override guards against fault currents that exceed the withstand capability of a breaker. This option generally, but not always, prevents mistakes. This process should be used only when designing and installing new breakers with owner involvement. If the override function trips the breaker, coordination and nuisance tripping may occur.

Pro: There are no equipment costs, therefore it is less expensive.

Con: Coordination issues; equipment may not be compatible.

Approach 7—approved similar means

This technique allows for future technologies. As technology evolves, the Authority Having Jurisdiction may allow any technique comparable to the six listed above (AHJ). This approval is usually negotiated by the studies engineer and the AHJ.

What is the best way? If not de-energizing offers a greater danger, the procedures outlined above are employed. Section 110.4(A) describes the aim of NFPA 70E: The term “celebration” refers to the act of celebrating a special occasion. It is best to de-energize the apparatus.

What is more risky than a worker being electrocuted or killed by an arc flash? If an arc flash occurs at a facility and injures a worker, the plant owner must explain why they did not de-energize or risk severe penalties and lawsuits. De-energization must come first. Owners and operators must shut down and work safely. These mitigating methods, like PPE, should be utilized last.