{"id":47884,"date":"2026-01-14T16:01:06","date_gmt":"2026-01-14T23:01:06","guid":{"rendered":"https:\/\/brainfiller.com\/?post_type=docs&p=47884"},"modified":"2026-01-20T14:02:59","modified_gmt":"2026-01-20T21:02:59","password":"","slug":"ieee-1584-nfpa-70e-and-the-2-second-arc-duration-myth","status":"publish","type":"docs","link":"https:\/\/brainfiller.com\/es\/knowledge-base\/nfpa-70e\/ieee-1584-nfpa-70e-and-the-2-second-arc-duration-myth\/","title":{"rendered":"I\u2019ve heard that an arc flash can\u2019t last longer than two seconds. Is that true?"},"content":{"rendered":"
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Pregunta<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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I\u2019ve heard that an arc flash can\u2019t last longer than two seconds. Is that true?<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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Short answer<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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No. An arc flash can absolutely last longer than two seconds. Neither IEEE 1584 nor NFPA 70E says an arc flash is physically limited to two seconds. The two-second value is an assumption used for exposure time, not a limit on how long an arc can actually exist.<\/p>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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Where the misunderstanding comes from<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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The confusion often comes from a statement in IEEE 1584-2018 that says \u201c2 seconds usually is a reasonable assumption for the arc duration.\u201d This gets repeated frequently, but it is commonly taken out of context.<\/p>\n

IEEE 1584 is addressing how long to assume a worker is exposed to the arc for incident energy calculations, not how long the arc itself can physically persist.<\/p>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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What the 2-second assumption actually means<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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In an arc-flash study, arc duration is normally based on protective device clearing time\u2014how long it takes a breaker, relay, or fuse to detect the fault and open the circuit. In some systems, especially where devices are slow or coordination is poor, that clearing time can exceed two seconds.<\/p>\n

IEEE 1584 permits capping the arc duration at 2 seconds for calculation purposes only when it is reasonable to assume that a person can move out of the arc-flash exposure area.<\/p>\n

In other words, the arc may still be burning and the equipment may still be energized, but the worker is assumed to no longer be in front of it.<\/p>\n

Two seconds represents human reaction and escape time, not a physical limit on arc duration.<\/p>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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Arc flashes can last much longer<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Actual arc duration is generally unpredictable and depends on many factors (for example: arcing current, electrode gap, and voltage). That is why studies often use protective device clearing time: it is a known value.<\/p>\n

In real-world conditions, an arc flash can potentially last several seconds, tens of seconds, or until the equipment fails catastrophically or the power source is removed.<\/p>\n

Longer-duration events are especially possible in systems with failed or miscoordinated protection, low arcing fault current that causes long response times, or utility-fed equipment without fast local protection (relying only on upstream utility devices).<\/p>\n

This is why treating \u201ctwo seconds\u201d as a hard limit is incorrect\u2014and potentially dangerous.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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How NFPA 70E uses the same concept<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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NFPA 70E also references a two-second arc duration assumption in specific places.<\/p>\n

For example, NFPA 70E Table 130.7(C)(15)(b), which provides arc-flash PPE categories for DC systems, includes Informational Note 2 stating that a two-second arc duration is assumed if there is no overcurrent protective device to limit the duration of the arc.<\/p>\n

This does not mean the arc will self-extinguish after two seconds, that DC arc flashes are limited to two seconds, or that two seconds is inherently safe.<\/p>\n

It means that, for PPE category guidance, NFPA 70E assumes a two-second exposure duration when no overcurrent protective device exists to clear the fault.<\/p>\n

Similar to AC arcs, a DC arc can potentially continue well beyond two seconds.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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Engineering judgment is required<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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The 2-second cap should only be used when it is realistic that a worker could recognize the event and physically move away.<\/p>\n

Situations where the two-second assumption may be reasonable include standing in front of open, accessible equipment with no physical restraints and a clear escape path.<\/p>\n

Situations where it may not be reasonable include working in a bucket truck, working in enclosed spaces, or other conditions that restrict the ability to rapidly move away.<\/p>\n

If a worker cannot reasonably move away, the actual protective device clearing time should be used, even if it exceeds two seconds.<\/p>\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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Bottom line<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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The 2-second value in IEEE 1584 is not a limit on arc duration, not a guarantee of safety, and not automatic.<\/p>\n

It is a calculation assumption based on human response and should only be applied when justified.<\/p>\n

Used correctly, it helps prevent unrealistic incident energy values. Used incorrectly, it can significantly underestimate arc-flash hazard.<\/p>\n\n\n\n\n\n\n\n\nSources\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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I\u2019ve heard that an arc flash can\u2019t last longer than two seconds. Is that true?<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>","protected":false},"excerpt":{"rendered":"

Question I\u2019ve heard that an arc flash can\u2019t last longer than two seconds. Is that true? Short answer No. An arc flash can absolutely last longer than two seconds. 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