30 June 2025
How are arc flash ratings calculated?
Arc flashes are among the most dangerous hazards faced by electrical workers. The intense energy released during an arc event can result in severe burns, trauma and even fatal injuries in a fraction of a second. This is why arc-rated protective clothing is essential in high-risk environments. But not all garments offer the same level of protection.
Arc flash ratings help employers and safety professionals determine the right level of arc-rated workwear for each task. Read on to understand how these ratings are calculated, what they actually mean, and why they are key to making informed safety decisions and protecting your workforce.
Learn more: Arc flash PPE: Everything you need to know
what is an arc flash rating?
An arc flash rating tells you how much energy a protective garment can withstand before it risks breaking down or causing injury. It is typically measured in calories per square centimetre (cal/cm²), a unit that expresses how much thermal energy the fabric can absorb. This number is known as the Arc Thermal Performance Value (ATPV), and is commonly referred to as an arc flash cal rating or cal rating for arc flash. A higher ATPV means better protection against more severe arc flash incidents.
Ratings are determined through rigorous laboratory testing, ensuring garments meet the safety thresholds needed for the real-world environments they’re designed for.
how to calculate arc flash rating
There are two main methods used to calculate arc flash ratings: the open arc test and the box test.
Note: These tests and calculations must be conducted by a health & safety expert. It is the responsibility of the employer to provide arc flash PPE and ensure these protective garments are properly tested and arc-rated.
the open arc test (EN 61482-1-1)
The open arc test is designed to measure how much thermal energy a fabric can withstand when exposed to an electrical arc in open air. It replicates the kind of arc flash you might encounter in a switchroom or on overhead lines; environments where the arc is not confined. The test uses high-current equipment to generate a controlled arc, exposing the fabric to a brief, intense burst of energy just as it would be in a real application.
From this, the Arc Thermal Performance Value (ATPV) or Energy Breakopen Threshold (EBT) is calculated. This method gives a clear picture of how the garment behaves in real-world arc conditions, helping safety professionals choose appropriate clothing.
the box test (EN 61482-1-2)
Unlike the open arc test, the box test simulates a more enclosed arc flash scenario, closer to what you might experience in a cabinet or panel board. The arc is generated inside a metal box, creating a concentrated burst of heat, flame, and pressure. The test measures how well a fabric performs under these confined conditions and assigns it a rating of either Class 1 or Class 2.
Class 1 indicates a lower level of protection, suitable for tasks with reduced arc risk, while Class 2 is used where higher protection is needed. It’s a pass/fail test rather than a graduated scale, but it’s still a valuable tool in arc flash workwear selection.
understanding ATPV vs. EBT
Both ATPV and EBT are indicators of a garment’s arc flash protection, but they reflect different performance characteristics. ATPV is the level of energy at which there’s a 50% chance of second-degree burns through the fabric. EBT, on the other hand, is the point at which the fabric breaks open, exposing the skin. A fabric may be more likely to tear than to transfer heat, or vice versa, depending on its construction. In either case, the lower of the two values is used for labelling, as this represents the weakest point in the garment’s performance.
why accurate arc ratings matter
The consequences of underestimating arc energy levels can be catastrophic. If a worker’s clothing isn’t rated for the energy they’re exposed to, the fabric may fail, leading to serious arc flash injuries. Accurate arc ratings ensure that protective clothing corresponds with the actual risks identified in the workplace. Employers should therefore conduct arc flash risk assessments, calculate potential incident energy at various equipment points, use the expertise of health & safety personnel for these assessments and calculations, and provide garments with ratings that meet or exceed the energy levels in the workplace.
regulations and standards for arc flash rated clothing
In the UK, arc flash protective clothing must meet recognised international standards, such as BS EN 61482. These regulations set the criteria for how clothing is tested, labelled and used in the workplace. The Electricity at Work Regulations 1989 and PPE Regulations 2002 require employers to provide suitable protective gear based on identified electrical risks. In addition, standards like ISO 45001 encourage a risk-based approach to managing workplace safety.
trust alsico for arc-rated workwear
Arc flash ratings aren’t just numbers; they’re critical safety benchmarks that inform which protective garments are appropriate for specific tasks and environments. Calculated through controlled testing and grounded in internationally recognised standards, these ratings help employers safeguard their teams against some of the most dangerous electrical hazards in the workplace.
By understanding how arc flash ratings are determined and applied, you can make confident, compliant choices about the workwear your workforce depends on. Explore alsico’s arc flash protective clothing range today, or contact our team for more information about our products and bespoke design service.
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