ARC FLASH ANALYSIS

(Arc Flash Analysis)

A Life-or-Death Issue When Maintaining Energized Electrical Panels

Arc flash analysis is a mandatory engineering process under IEEE 1584 and NFPA 70E for assessing the thermal energy released when an electrical fault occurs through the air. An arc flash can generate temperatures of up to 19,000°C (approximately three times the surface temperature of the sun), as well as blast pressure and molten metal, all of which can cause life-threatening injuries.

The core objective of this analysis is to answer two questions: How much PPE must a worker wear? and How far away must non-involved personnel stand?

1. Incident Energy (E)

Incident energy is the amount of thermal energy impacting a unit area of a person's exposed surface (typically the chest and face) at a specified working distance. The standard unit of measurement is cal/cm².

Biological safety threshold: 1.2 cal/cm² is the maximum energy level that human skin can withstand for approximately 1 second before suffering a second-degree burn (blistering and irreversible skin damage).

Physical basis: Incident energy is proportional to the arcing current (Iarc) and the arc duration (t), but inversely proportional to the square of the distance (D):

E ~ (Iarc × t) / D²

2. Safety Boundaries (Approach Boundaries)

NFPA 70E establishes three spatial boundaries around electrical equipment to protect personnel. When an MSB is energized and its door must be opened, the area is divided as follows:

Arc Flash Boundary (AFB): the point at which incident energy decreases to exactly 1.2 cal/cm². Anyone entering this area is required to wear arc-rated PPE.

Limited Approach: a boundary intended to limit the risk of electric shock at a greater distance. It applies to an unqualified person, who may enter only under the supervision of a qualified person.

Restricted Approach: a boundary intended to limit the risk of an arc-over caused by inadvertent movement of the hands or tools. It applies to a qualified person wearing appropriate insulating gloves and using insulated tools.

3. Arc Flash Calculation Procedure

Step 1 — Determine the Bolted Fault Current: based on short-circuit analysis data, calculate the maximum bolted fault current (Ibf) at the MSB busbar, taking into account the transformer rating and the impedance of the feeder/system.

Step 2 — Calculate the Arcing Current: the arcing current (Iarc) is always lower than the bolted fault current because the air path has electrical impedance. Accurately determining Iarc is critical — if it is calculated incorrectly (for example, by assuming Iarc is equal to Ibf), the circuit breaker may respond more slowly on the TCC curve, potentially leading to a dangerously inaccurate analysis result.

Step 3 — Determine the Clearing Time: refer to the Time-Current Curve (TCC) of the protective circuit breaker (ACB/MCCB) to determine how many seconds (t) it takes for the breaker to completely interrupt the fault at the calculated Iarc level.

Step 4 — Calculate Incident Energy and Boundary Distance: use the empirical equations defined by IEEE 1584, taking into account factors such as enclosure dimensions and the distance between phases, to determine the incident energy at the standard working distance (typically 455 mm for low-voltage switchboards) and establish the Arc Flash Boundary.

4. Practical Example: 400V MSB Arc Flash Analysis

Scenario: an MEP engineer needs to open the busbar cover of an energized 400V MSB to measure the temperature of electrical contacts using an infrared thermometer.

Maximum Bolted Fault Current (Ibf): 40 kA

Working Distance: 455 mm (18 inches)

Protection Setting (Relay/ACB): to ensure selective coordination with downstream distribution boards, the main ACB is configured with a time delay of 0.3 seconds.

The system analysis produces the following parameters:

Arcing Current (Iarc): ETAP software calculates the arcing current through the air at only 22 kA (lower than 40 kA).

Clearing Time (t): at an arcing current of 22 kA, the relay maintains the configured time delay, resulting in a total clearing time of 0.35 seconds for the mechanical ACB to fully open.

Incident Energy (E): at a working distance of 455 mm, the calculated incident energy reaches 15 cal/cm².

Arc Flash Boundary (AFB): the point at which incident energy decreases to 1.2 cal/cm² is calculated to be 1,400 mm.

Interpretation for the Maintenance Engineer:

Regarding Personal Protective Equipment (PPE): with an incident energy level of 15 cal/cm², the engineer is required to wear PPE Category 3 arc-rated protective clothing (rated for at least 25 cal/cm²), including an arc flash hood with face shield, rubber insulating gloves with leather protectors, and full-body flame-resistant clothing. If ordinary cotton clothing is worn, the engineer could suffer severe burns or fatal injuries in the event of an arc flash.

Regarding site safety: the Arc Flash Boundary is 1.4 meters. If an assistant worker is standing 1 meter away to record measurement data, that worker is also within the Arc Flash Boundary and must wear the same PPE Category 3 as the primary engineer, or move back to a distance of at least 1.5 meters.

The Protection Engineer's Paradox (Protection vs. Safety): protection engineers typically configure a time delay (0.3 s) to prevent unnecessary tripping of the main circuit breaker when a fault occurs downstream, thereby maintaining selective coordination. However, this same time delay allows the arc to persist long enough to generate a significant amount of incident energy (15 cal/cm²). This is precisely the trade-off that protection system designers must carefully consider between system operational reliability and human life safety.