IEEE 1584, NFPA 70E and the “2-Second Arc Duration” Myth

IEEE 1584, NFPA 70E and the “2-Second Arc Duration” Myth

Question:
I’ve heard that an arc flash can’t last longer than two seconds. Is that true?

Short answer:
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.

This misunderstanding comes from a statement in IEEE 1584-2018, which says that “2 seconds usually is a reasonable assumption for the arc duration.” That sentence gets quoted a lot, but it’s often taken out of context.

What IEEE 1584 is really addressing is how long to assume a worker is exposed to the arc, not how long the arc itself can physically exist.

What the 2-Second Assumption Actually Means

In an arc-flash study, arc duration is normally based on protective device clearing time—how long it takes a breaker, relay, or fuse to detect the fault and open the circuit. In some systems, especially where protective devices are slow or coordination is poor, that clearing time can exceed two seconds.

IEEE 1584 permits the arc duration to be capped 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. In other words:

• The arc may still be burning
• The equipment may still be energized
• But the worker is assumed to no longer be in front of it

Two seconds represents human reaction and escape time, not a physical limit on arc duration. If there is a “boom”, we go into the fight or flight response and in this case, flight is the normal response, jumping out of the way.

Arc Flashes Can Last Much Longer

The actual arc duration is generally unpredictable and is a function of many factors such as arcing current, electrode gap, voltage and much more.  That is why the protective device clearing time is used – a known value. 

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

This is especially true in systems with:

• Failed or miscoordinated protection
• Low arcing fault current resulting in protection having a long response time
• Utility-fed equipment without fast local protection – only the upstream utility device.

You don’t have to dig very deep on YouTube to see videos of some very long duration arc flash events.

That’s why it’s incorrect—and dangerous—to think of two seconds as a hard limit.

How NFPA 70E Uses the Same Concept

NFPA 70E also references the two-second arc duration assumption in specific locations.

As an example, NFPA 70E Table 130.7(C)(15)(b), which provides arc-flash PPE categories for dc systems.

Informational Note 2 to this table states that a two-second arc duration is assumed if there is no overcurrent protective device to limit the duration of the arc.

This language is often misunderstood. It does not mean:

• The arc will self-extinguish after two seconds
• dc arc flashes are limited to two seconds
• Two seconds is inherently safe

What it means is that, for PPE category guidance, NFPA 70E assumes a two-second exposure duration when no overcurrent protective device exists to clear the fault. Often dc systems involve battery systems such as in uninterruptable power supplies (UPS) which often will not have overcurrent protection –

Caution! – technical humor follows.  I’m sure this lack of protection has something to do with the “U” in the title – Uninterruptable.

NFPA 70E’s two-second assumption in Table 130.7(C)(15)(b) is a conservative exposure assumption, not a statement about arc behavior. Similar to ac arcs, a dc arc can potentially continue well beyond two seconds.  

Engineering Judgment Is Required

The 2-second cap should only be used when it is realistic that a worker could recognize the event and physically move away.

Examples where the 2-second assumption may be reasonable:

• Standing in front of open, accessible equipment
• No physical restraints or confined space
• Clear escape path

Examples where it may not be reasonable:

• Working in a bucket truck
• Working on electrical equipment in enclosed spaces
• Similar situations that may restrict the worker’s ability to rapidly move away

If a worker cannot reasonably move away, then the actual protective device clearing time should be used, even if it exceeds two seconds.

Bottom Line

The 2-second value in IEEE 1584:

• Is not a limit on arc duration
• Is not a guarantee of safety
• Is not automatic

It is a calculation assumption based on human response, and it should only be applied when justified. Used correctly, it helps prevent unrealistic incident energy values. Used incorrectly, it can significantly underestimate arc-flash hazard