Arc flash risk assessment and labels

Before anyone works on or near exposed energized parts, the employer must assess the arc flash risk. The assessment answers three questions: is there an arc flash hazard, how likely is it to occur and how severe would it be, and what controls, including PPE, are needed. This lesson compares the two methods the standards allow for selecting arc flash PPE, shows how incident energy falls with distance and how the arc flash boundary is set, and explains how to read and check an arc flash label.

The arc flash risk assessment

CSA Z462 requires an arc flash risk assessment to identify arc flash hazards, estimate the likelihood of occurrence of injury or damage to health and its potential severity, and determine whether additional protective measures, including PPE, are required. It must consider the design of the equipment and its overcurrent protective devices and their operating times, and the condition of maintenance. The assessment must be reviewed periodically, at intervals not exceeding five years, and updated whenever a major modification or renovation takes place, to account for changes in the distribution system that could affect the results.

The assessment produces, for each piece of equipment and task, the arc flash boundary, the PPE needed inside it and the other controls required. Those results are recorded on the arc flash label, in the energized work permit and in the job briefing.

Two methods for selecting arc flash PPE

The standards allow either of two methods, but not a mixture of them on the same piece of equipment:

• The incident energy analysis method calculates the incident energy at the working distance for each piece of equipment, usually using IEEE 1584-2018 for AC systems from 208 V to 15 kV. PPE is then selected with an arc rating at least equal to the calculated incident energy. This is the more precise method and is the preferred approach for most facilities.
• The arc flash PPE category method uses tables in the standard. For listed equipment types, provided that the available fault current and the clearing time of the upstream device are within the limits in the table, the table gives a PPE category (1 to 4) and an arc flash boundary. If the equipment falls outside the table's limits, the table cannot be used and an incident energy analysis is required.

The PPE category method is useful for small facilities without an engineering study, but its limits are strict. CSA Z462 and NFPA 70E both prohibit showing both the incident energy and a PPE category on the same label, because the two methods must not be mixed.

The PPE categories correspond to minimum arc ratings of clothing: category 1 requires at least 4 cal/cm², category 2 at least 8 cal/cm², category 3 at least 25 cal/cm² and category 4 at least 40 cal/cm². Lesson 8 describes the full clothing and equipment for each category.

How incident energy falls with distance

Incident energy decreases rapidly as distance from the arc increases. At awareness level, the relationship can be described by \( E \propto 1/D^{x} \), where \( E \) is incident energy, \( D \) is distance and \( x \) is a distance exponent that depends on the equipment configuration. In open air, energy spreads in all directions and \( x \) is about 2, the familiar inverse square law. In an enclosure, the box focuses energy out of the opening and \( x \) is lower, typically between about 1.4 and 1.7 for low voltage equipment in the older IEEE 1584-2002 model. The 2018 model is more complex and does not use a single exponent, but the same principle holds.

Arc flash labels

Rule 2-306 of the Canadian Electrical Code, Part I (CSA C22.1:24) requires switchboards, panelboards, industrial control panels, meter socket enclosures and motor control centres in other than dwelling units, which are likely to require examination, adjustment, servicing or maintenance while energized, to be field marked to warn persons of potential electrical shock and arc flash hazards, with the marking clearly visible before work begins. The rule sets the requirement to mark; CSA Z462 sets out what the label should contain. Under CSA Z462, the label must show the nominal system voltage, the arc flash boundary and at least one of the following: the available incident energy and the corresponding working distance, or the arc flash PPE category (but not both); the minimum arc rating of clothing; or a site-specific level of PPE. Most labels also show the shock boundaries, the date of the assessment and the source of the study.

When you read a label, check it rather than simply obeying it:

• Does the equipment name on the label match the equipment in front of you?
• Is the date within the last five years, and have there been changes since, such as a new transformer, a utility upgrade, a new generator or changed breaker settings?
• Does it show either incident energy or a PPE category, but not both?
• Does the working distance on the label match the way you will actually do the task? If you must lean in closer, the exposure will be higher.
• Is the equipment in normal operating condition and properly maintained, as the study assumed?

If the answer to any of these is no, stop and ask for the label to be verified. A label is only as good as the study and the maintenance behind it.

Shock risk assessment

The arc flash assessment is half of the picture. A shock risk assessment determines the voltage to which workers will be exposed, the boundary requirements and the PPE needed to minimize the possibility of shock. It also considers the likelihood of contact, which depends on the task and the equipment condition, just as for arc flash. Lessons 3 and 8 cover the boundaries and the shock protective equipment. Both assessments feed into the energized work permit described in the next lesson.

Who performs the study, and what the worker should ask

An incident energy analysis is an engineering task. In Canada it is normally performed or supervised by a licensed professional engineer, using a current single-line diagram, utility fault data, cable lengths and sizes, transformer data and protective device settings collected in the field. The quality of the study depends entirely on the quality of that data. A study built on an outdated single-line diagram, or on assumed breaker settings rather than the ones actually dialled into the trip units, can produce labels that look precise and are wrong.

A qualified worker does not need to repeat the calculation, but can ask useful questions. Which operating scenarios were studied, including generator operation and tie breakers closed? Were the protective device settings verified in the field? Has anything changed since the study date? Does the working distance on the label reflect how the task is really done? A worker who asks these questions helps keep the labels honest, and an employer who welcomes them has a stronger program.

Some facilities supplement labels with a site-specific PPE matrix, which lists tasks and equipment together with the required PPE. Where such a matrix exists, it must be derived from the same study and kept consistent with the labels.