Understanding Rating of Arc-Rated PPE
Any worker at risk of exposure to an arc flash hazard shall wear specially designed and appropriately rated personal protective equipment (PPE). There are hundreds, if not thousands of commercially available arc-rated product options to choose from in multiple designs, shapes and sizes. The selection process can be overwhelming without first understanding how arc-rated PPE is rated.
The arc rating on all arc flash PPE in North America is established based on ASTM standardized laboratory test methods. Testing is conducted in a high current test laboratory, such as Kinectrics in Toronto, Canada. The three ASTM standards used to determine arc ratings are ASTM F1959 (fabrics), ASTM F2178 (face & head protection) and ASTM F1506 (performance requirements).
- ASTM F1959 uses a flat instrumented panel to test a fabric or multiple layers of fabrics.
- ASTM F2178 uses an instrumented mannequin head that is outfitted with protective product like a face shield or hood).
- ASTM F1506 provides the requirements that product manufacturers shall follow.
Arc ratings are conservative estimates for how the arc-rated product would perform if exposed to an arc flash. Different types of arcs are possible, and the arc rating is only known for the type of arc used in the ASTM test methods; 8kA using vertical electrodes in open air (VOA) with a 30cm gap. It is generally accepted within the electrical safety industry that arc ratings determined using open air type arc testing can be applied to other types of arcs (e.g., horizontal oriented electrodes). During arc flash testing sensors are used to measure the difference between the arc energy (incident energy) and the insulative value of the test specimen(s). There are two different types of arc ratings as defined in ASTM F1959 as:
Arc Thermal Performance Value (ATPV): the incident energy on a material or a multilayer system of materials that results in a 50 % probability that sufficient heat transfer through the tested specimen is predicted to cause the onset of a second-degree skin burn injury based on the Stoll curve, cal/cm2.
Breakopen Threshold Energy (EBT): the incident energy on a material or material system that results in a 50 % probability of breakopen. The test specimen is considered to exhibit breakopen when any hole is at least 1.6 cm2 (0.5 in.2) in area or at least 2.5 cm (1.0 in.) in any dimension.
Arc Ratings are calculated using at least 20 data points to determine the incident energyvlevel of a 50% probability that there is enough heat transfer to cross the Stoll Curve (used to determine ATPV), or for the material to break-open and create exposure (used to determine EBT). Both the ATPV and EBT arc ratings are considered equally acceptable. All types of arc-rated fabrics will break-open when exposed to sufficient arc energy. The type of arc rating that is identified on the label of an arc-rated product is determined by the lowest of the two, or what occurred first during arc testing. This ensures that arc-rated products are labeled with the most conservative of the two arc ratings. Products labeled with an EBT would have exhibited break-open before it reached sufficient heat transfer to cause the onset of a second-degree skin burn. Products labeled with an ATPV did not exhibit breakopen before reaching the onset of a second-degree skin burn. ATPV rated products tend to be constructed with stronger materials (i.e., tensile strength) as compared to EBT products that usually have more insulative properties. Common examples for both types of arc ratings include:
- ATPV: woven fabrics (dress shirts, pants, coveralls, jackets) and denim (jeans, jackets).
- EBT: knit fabrics (long sleeve t-shirts, crewnecks, hoodies) and coated fabrics (rain jackets, wind breaker shells).
Arc flash PPE is selected by matching the arc rating, either ATPV or EBT to the anticipated release of incident energy released from exposure to the arc flash hazard. The arc rating is the protective ability of the product to mitigate the thermal energy that could harm a worker. The protection is reported as calories (a unit that measures energy) per cm2 (square centimeter) of surface area. The thermal energy created by an arc flash hazard, incident energy, is also measured in calories per square centimeter. The bigger the calorie number, the greater the protection provided by the product and the greater the thermal heat energy potential of the arc flash hazard. Matching the arc rating to the hazard would best replicate the predicted performance as determined during laboratory arc flash testing. When the arc rating exceeds the arc flash hazard the probability of a worker receiving a burn injury is further reduced. Over protection should be balanced with maintaining worker comfort and productivity. For example, wearing extra layers of arc-rated PPE could contribute to human error by causing heat stress and diminish a worker’s ability to perform work safely (e.g., dexterity, movement). Another example of what not to do is avoid excessive sweating from over- dressing, which would negatively impact the arc rating of the worker’s arc flash PPE once the fabric has become saturated.
The ASTM standards for electrical specific personal protective equipment are designed to work together. The most important of which is the ASTM F1506; an overarching standard with governance over all arc flash PPE performance requirements including how arc-rated PPE is rated and labeled. To be arc rated as per the ASTM F1506 standard, the fabric must pass several tests including wash testing and the vertical flammability test using ASTM D6413. Employers creating arc flash PPE specifications as part of their Electrical Safety Program should require compliance with ASTM F1506.
Workplace electrical safety Standards that reference ASTM standards for electrical protective equipment include NFPA 70E Standard for Electrical Safety in the Workplace (USA) and CSA Z462 Workplace Electrical Safety Standard (Canada). Other industries not covered by the scope of these standards, most notably Electrical Utilities, also refer to these ASTM standards when evaluating the arc-rating of PPE.
Now that you understand how arc-rated products are rated, hopefully the selection process can be easier. When selecting arc flash PPE, the arc rating must match or exceed the anticipated incident energy of a worker’s exposure to the arc flash hazard. Actual arc flash incidents are focused events where the thermal energy often impacts an area approximately 12 in.2 in size on a worker’s body. Survivability is achieved when we effectively prevent ignition of garments and undergarments when workers wear arc flash PPE that is adequately rated and appropriate for use.