Arc-flash incident energy — the IEEE 1584 calculation method

The guide on arc flash covers the risk and PPE consequences in general terms, and the guide on the 50AF optical detection relay covers a specific, fast countermeasure. Both articles assume a number is already known — the incident energy, expressed in cal/cm², on a panel's arc-flash label. This article covers where that number comes from: the calculation method of IEEE 1584, the industry-standard method for quantifying arc-flash risk.

What the method calculates

IEEE 1584 produces two results for a given point in an installation:

The incident energy (E, in cal/cm² or J/cm²) at a specified working distance — the amount of thermal energy a person at that distance would receive from an arc flash at that point.
The arc flash boundary (AFB): the distance at which E = 1.2 cal/cm² — the conventional threshold for a second-degree burn on unprotected exposure.

The key inputs

The calculation requires, for each assessed point in the installation:

the available (bolted) short-circuit current,
the system voltage,
the working distance,
the electrode configuration — five standardised forms (VCB, VCBB, HCB, VOA, HOA: vertical/horizontal, in an enclosure or in open air, with or without a grounded enclosure),
the enclosure dimensions,
the clearing time of the protective device at the calculated arcing current.

From bolted current to arcing current

IEEE 1584 first calculates an estimated arcing current (Iarc) from the available bolted short-circuit current, using an empirical model derived from more than 1,800 laboratory tests. The arcing current is almost always lower than the bolted current, because the arc itself introduces significant impedance into the circuit. This arcing current, not the bolted current, then determines — via the protective device's time-current characteristic (fuse, breaker, relay) — how long the arc keeps burning.

Why a lower available short-circuit current can sometimes give a higher incident energy

Note: this is one of the less intuitive, but practically important, consequences of the method. Because incident energy is roughly proportional to the product of (arcing current)² and arc duration, and because most overcurrent protective devices have an inverse-time characteristic (the lower the current, the longer the clearing time), reducing the available short-circuit current can lengthen the clearing time so much that the resulting incident energy increases, despite the lower current itself. This means the worst-case scenario for incident energy does not automatically coincide with the maximum available short-circuit current: an arc flash study must therefore be performed at both the maximum and the minimum realistically expected short-circuit current to find the true worst case.

Validity range of the model

IEEE 1584-2018 is based on empirical, statistically derived regression equations, not a purely physical formula, and is validated for system voltages from 208V to 15kV across the five electrode configurations mentioned. Applying it outside this validated range (for example at a higher voltage, or to a configuration that does not match one of the five tested forms) produces a result whose reliability has not been demonstrated.

Practical relevance

The incident-energy value from an IEEE 1584 calculation directly determines the required PPE category (see the guide on PPE) and underpins the business case for measures that shorten arc duration, such as a 50AF optical detection relay or zone-selective interlocking (ZSI): both specifically act on the time factor of the energy calculation, not on the current itself, and are therefore most effective precisely in installations where clearing time — not current — dominates the incident energy.

Common mistakes

Assuming the maximum available short-circuit current always gives the worst-case incident energy — with an inverse-time protective characteristic, a lower current can actually give a higher incident energy due to a longer clearing time.
Changing protective device settings without re-running the arc-flash study — clearing time is a direct input to the calculation; any setting change can invalidate the label.
Applying the IEEE 1584 model outside its validated voltage/configuration range without noting or recognising this.
Confusing the arc flash boundary (AFB) with a fixed PPE category table — the simplified category tables (such as in NFPA 70E) are an alternative, coarser approximation, not a substitute for an actual IEEE 1584 calculation on the specific installation.