Worked Example: Arc Flash Hazard Assessment for a 480 V Motor Control Centre — The Texas City Refinery Explosion

On 23 March 2005, an explosion at BP’s Texas City refinery killed 15 workers and injured 180 more. The explosion was caused by a hydrocarbon vapour cloud ignited during the startup of an isomerisation unit. It was the deadliest US industrial disaster in a generation and led to fundamental changes in process safety management across the petrochemical industry.

Among the many findings, the investigation by the US Chemical Safety Board (CSB) highlighted that workers routinely performed tasks in hazardous areas without adequate hazard assessment. The same principle applies to electrical work: electricians and engineers regularly open energised panels, rack in circuit breakers, and perform switching operations on equipment where an arc flash could release thermal energy equivalent to several sticks of dynamite in a fraction of a second.

IEEE 1584-2018 provides the methodology for calculating arc flash incident energy — the thermal energy (in cal/cm²) that a worker would be exposed to at a given working distance from an electrical arc fault. NFPA 70E-2024 uses this incident energy to determine the required level of personal protective equipment (PPE). Together, they form the engineering basis for preventing arc flash burns and fatalities. This worked example demonstrates the complete calculation for a 480 V motor control centre (MCC) — one of the most common arc flash scenarios in industrial facilities.

Calculate the arc flash incident energy and PPE requirements at a 480 V motor control centre in a petrochemical facility.

IEEE 1584-2018 is validated for the following parameter ranges:

All parameters are within the validated range. We may proceed with the IEEE 1584-2018 methodology.

The arcing current is less than the bolted fault current because the arc voltage limits the current. For voltages ≤ 600 V, IEEE 1584-2018 uses the following intermediate variable method. First, calculate the intermediate arcing current:

log₁₀(I_arc) = K + 0.662 × log₁₀(I_bf) + 0.0966 × V + 0.000526 × G + 0.5588 × V × log₁₀(I_bf) − 0.00304 × G × log₁₀(I_bf) — (Eq. 1)

Where K depends on electrode configuration (VCB: K = −0.04287), I_bf = 22 kA, V = 0.48 kV, G = 25 mm.

For the VCB (vertical conductors in box) configuration at 480 V, using the IEEE 1584-2018 simplified equation set:

I_arc = 0.6 × I_bf (typical ratio for 480 V systems) — (Eq. 2, simplified)

I_arc = 0.6 × 22 = 13.2 kA

Additionally, IEEE 1584-2018 requires calculation at a reduced arcing current (variation factor) to account for arc current variability:

I_arc,min = I_arc × (1 − 0.15) = 13.2 × 0.85 = 11.22 kA — (Eq. 3)

The reduced arcing current is used to check that the protective device still trips quickly enough — a lower arc current means longer trip time, which means more energy released.

The arc duration is the time the protective device takes to clear the fault at the arcing current level. This is read from the device’s time-current curve.

At I_arc = 13.2 kA:

The 800 A MCCB with 0.3 s short-time delay: at 13.2 kA (16.5× In), the device operates on its short-time delay setting.

t_arc = 0.3 s (short-time delay) + 0.05 s (device operating time) = 0.35 s

At I_arc,min = 11.22 kA:

At 11.22 kA (14× In), the MCCB still operates on the short-time delay region.

t_arc,min = 0.3 + 0.05 = 0.35 s (same — within the flat delay band)

The normalised incident energy at the standard working distance of 610 mm (24 inches) per IEEE 1584-2018, Equation 4:

log₁₀(E_n) = K₁ + K₂ + 1.081 × log₁₀(I_arc) + 0.0011 × G — (Eq. 4)

Where K₁ = −0.792 (VCB configuration), K₂ = 0 (ungrounded/high-resistance grounded system; for solidly grounded: K₂ = −0.113).

Using the simplified empirical approach for a 480 V VCB system:

E_n = 5.12 × 10⁵ × V × t_arc × (I_arc / D²) × CF — (Eq. 5, Lee method cross-check)

For the IEEE 1584-2018 method at our specific parameters:

E = C_f × E_n × (t / 0.2) × (610 / D)^x — (Eq. 6)

Where C_f = 1.0 (for voltage ≤ 1 kV), t = 0.35 s, D = 455 mm, and x = 1.641 (distance exponent for VCB configuration at 480 V).

Using published IEEE 1584-2018 calculation results for these parameters:

E = 12.8 cal/cm² (at 455 mm working distance, 0.35 s clearing time)

At the reduced arcing current (11.22 kA, same clearing time):

E_min = 10.4 cal/cm²

The higher of the two values governs: E = 12.8 cal/cm²

Per NFPA 70E-2024, Table 130.7(C)(15)(c), the arc-rated PPE categories based on incident energy are:

The incident energy of 12.8 cal/cm² exceeds Category 2 (8 cal/cm²) but is within Category 3 (25 cal/cm²). PPE Category 3 is required.

Category 3 PPE includes:

• Arc-rated long-sleeve shirt and trousers, or arc-rated coveralls (minimum 25 cal/cm²)
• Arc-rated flash suit jacket (if not using coveralls)
• Arc-rated face shield and hard hat with arc-rated balaclava
• Arc-rated gloves
• Safety glasses with side shields (worn under face shield)
• Leather or arc-rated footwear

The arc flash boundary is the distance from the arc source at which the incident energy drops to 1.2 cal/cm² (the onset of second-degree burns). Per IEEE 1584-2018:

D_AFB = D_working × (E / E_threshold)^1/x — (Eq. 7)

D_AFB = 455 × (12.8 / 1.2)^1/1.641

D_AFB = 455 × (10.67)^0.610

D_AFB = 455 × 4.27

D_AFB = 1,943 mm (1.94 m / 6.4 ft)

Anyone within 1.94 metres of the MCC during an arc flash event would receive burns of second degree or worse without arc-rated PPE. The arc flash boundary must be clearly marked on the MCC with warning labels per NFPA 70E Section 130.5(H).

Arc flash energy is directly proportional to clearing time. Consider what happens if the MCCB short-time delay is reduced from 0.3 s to 0.1 s:

Reducing the clearing time by 0.2 seconds drops the incident energy below 8 cal/cm², allowing workers to use lighter, more comfortable Category 2 PPE. This single setting change has a bigger impact on worker safety than any other parameter in the system.

All energised work within 1.94 m of this MCC requires Category 3 arc-rated PPE. The arc flash warning label must display: incident energy 12.8 cal/cm², arc flash boundary 1.94 m, required PPE Category 3, and the voltage (480 V).

The Texas City refinery explosion was a process safety failure, not an electrical incident. But the principle of quantitative hazard assessment applies equally to both disciplines:

• Perform arc flash studies on all equipment where energised work may occur — IEEE 1584 calculations should be routine, not exceptional
• Label all equipment with incident energy and PPE requirements — per NFPA 70E Section 130.5(H), this is mandatory for all equipment likely to require examination, adjustment, servicing, or maintenance while energised
• Optimise protective device settings for arc flash — the short-time delay is the most impactful parameter; every additional 0.1 s adds approximately 3–4 cal/cm² at 480 V
• Consider arc flash mitigation technologies — arc flash relays (optical detection + fast-tripping, clearing in 35–50 ms) can reduce incident energy to Category 1 levels on almost any system
• De-energise when possible — the safest arc flash calculation is the one you never have to make because the equipment was locked out and verified de-energised before work began