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In 2011, the National Electrical Code (NEC) introduced Section 240.87 for arc energy reduction. Across numerous iterations of the NEC, new approved arc flash mitigation methods have been added. This list is current as of the NEC 2020.
It’s important to know that while all the methods are approved, they don’t all yield the same energy reduction levels. Because they are not interchangeable solutions, knowing each of them, and understanding how best to use them, will help you apply the right solution.
ZSI enables the nearest upstream circuit breaker to a short-circuit or ground-fault to override any of the delays programmed for coordination. This protection scheme helps reduce arc flash energy by tripping the breakers faster than the programmed settings.
This method, which is available on the electronic trip units in Eaton’s Series C & G molded-case circuit breakers and Magnum DS/SB power breakers, offers a traditional ZSI performance.
Visual indication of ZSI, when it is enabled and if a signal occurs, is available in Eaton’s Power Xpert Release Trip (PXR) electronic trip units installed in our Power Defense circuit breakers.
This builds on the traditional ZSI experience by offering a visual indication of the system’s status and test functionality that ensures all connections are properly wired and working.
Differential relaying is a method for arc flash reduction that compares two quantities of current flowing in and out of a system. This method works by calculating and monitoring the sum of all the currents flowing into the protected object. Under non-fault conditions the sum is zero. Any differential current, however, is forced through the high impedance of the high impedance relay.
Devices such as Eaton’s Bus Differential Relay, combined with the EBR-Z (EBR-3Z) provide a high impedance differential protection scheme that offers an intuitive operating system with validation checks and commissioning functions such as a built-in fault simulator. The devices calculate and monitor the sum of all the currents flowing into the protected area to provide a fast trip when any differential current is detected.
An energy-reducing maintenance switch is a maintenance-only solution that must be activated when a user approaches the equipment and deactivated once work is completed.
The switch is typically mounted on the front of the equipment but also can be mounted remotely or activated over communications or via a hard-wired signal. When the maintenance switch is activated, it temporarily overrides all programmed delays in the relay or trip unit, causing the overcurrent protective device to trip based on the instantaneous settings in the event of a fault.
Eaton’s Arcflash Reduction Maintenance System uses a separate bypass circuit. This circuit acts in parallel with the electronic trip unit to save critical milliseconds over using instantaneous trip settings. The circuit provides the fastest possible breaker clearing time which results in the lowest arc energy exposure to the worker. The Arcflash Reduction Maintenance System is available on Magnum and Power Defense circuit breakers, as well as Eaton’s motor, distribution and transformer protective relays. Additionally, an Integrated Arcflash Reduction Maintenance System feature is available in LV replacement circuit breakers for multiple generations of Westinghouse, Allis-Chalmers, Federal Pacific, General Electric and ITE.
Active arc flash mitigation systems are designed to directly detect arcing faults, typically by sensing the bright light that is emitted when an arc fault occurs. Most systems also use a current signal to make them more immune to nuisance operation that could occur from other sources of bright light. Since active arc flash mitigation systems do not rely on circuit breaker settings to detect the fault, complete circuit breaker coordination can be achieved, thereby maximizing system reliability while minimizing incident energy.
Eaton’s Arc Flash Relay (EAFR) uses light and current inputs to detect the presence of an arc fault and sends a trip signal to the closest upstream device to clear the fault. Pairing an arc flash detection system with an arc quenching device, such as in Eaton’s Arc Quenching Switchgear, will reduce incident energy to a level lower than any of the other approved methods.
These methods involve first calculating arcing currents at the equipment and then ensuring that current is within the instantaneous trip region of the circuit breaker by comparing it to the Time Current Characteristic (TCC) curve of the circuit breaker.
These methods do not require user intervention since there are no local indicators or a means to engage or disengage. But, reducing the instantaneous setting only during maintenance times is prohibited by the NEC 2020 revision and wouldn’t be safe because there would be no way for an operator to know that the arc flash reduction method was disabled. The arc flash label would be incorrect in that instance. That’s why these methods should only be used if the maximum possible instantaneous setting or the maximum instantaneous override setting is always below the minimum possible arcing current (i.e., arc energy reduction is always present regardless of the settings).
Those interested in having selective coordination and arc flash safety utilizing one of these methods may be unable to achieve both. Specifically, if you need to maintain device coordination for code compliance and system uptime, this approach may prevent device coordination.
This method leaves the door open for innovative new solutions that may be developed in the future. Whether now or in the future, it’s important to work with a supplier that has a solid reputation for delivering reliable solutions. This mitigation method also puts additional responsibility on inspectors, who must be extremely precise and diligent when assessing facilities.
Need help determining the best mitigation plan for your facility? Eaton can help.
