Open arc and box test – what’s the difference?

Two tests – the box test and open arc – are used to measure a material and garments level of protection. The test methods use different test set-ups, arc configurations, test parameters, test procedures and result parameters. The results of either test method cannot be physically compared or mathematically transformed into each other. The arc rating has to be tested and assessed using either one method or the other.

Box test The box test is carried out in accordance with EN 61482-12. It includes a directed electric arc in an enclosed space APC 1 or APC 2. This test was previously the most common method used to determine a material’s level of protection.

Open arc Open arc measures protection using the 61482-1-1 test method, which is equivalent to the NFPA 70 US standard. The method uses an 8 kA circular arc that strikes the test material from a distance of 30 cm. The test results are presented in calories per square centimetre (cal/cm2), revealing at what calorie level the garment offers a 50% chance of protection against second-degree burns as a ATPV or EBT50 value. The higher the value, the better the protection.

For wearers of protective clothing, it’s crucial to know what level of protection their clothing offers. The foundation of good protection is that the garments provide certified protection against electric arcs and that they do not combust. The clothing must be tested and certified against heat, flames and electric arcs. This applies not only to the outer garment layer, but to underwear, the base layer, mid layer and all other layers as well. IEC 61482-2 is the main standard for electric arc protection, and the aim of this standard is to eliminate the risk of exposure to a second-degree burn.

ATPV or EBT50? Different fabrics may have different properties and characteristics, which means the level of protection needs to be presented in two different ways. Strong fabrics may remain intact after a test procedure, yet allow enough heat to be transferred through the material that a burn still occurs. In these cases, the test results are presented as an Arc Thermal Protection Value (ATPV). The other alternative is that the fabric offers high insulation, but instead ruptures and tears so that a hole opens up in the material, causing a burn. This value is called the EBT50 (Energy Break Open Threshold).

Neither rating can be said to be better than the other, as the risk of a burn in both cases is 50%. In the first case, the fabric remains intact but allows heat to pass through the material. In the second case, the fabric ruptures and there is a hole in the material. The lowest rating is the one that matters and so that’s the one that’s given.

ELIM – eliminate the risk of a second-degree burn Whereas the ATPV or EBT50 value gives an indication of what energy level you run a 50% risk of getting a seconddegree burn at, the incident energy limit (ELIM) defines the energy level at which you avoid the risk of a second-degree burn. Remember, however, that there is still a risk of getting a first-degree burn injury.

The ELIM value is calculated as the average of the three highest incident energy data points without rupture, shrink open and without reaching or exceeding the Stoll curve. The three incident energy data points are taken just below the mix zone in the test report.

The Stoll curve determines the rating of the transfer of heat energy (calories) based on the time of transfer and the level of heat energy produced.

A promise is only as good as the person who makes it.

That’s why a promise backed by scientific tests always rules the day. Especially when your safety – and in certain situations even your life – is at stake. That’s also why we put our high-risk environment (Category III) products through thorough tests, using test methods such as the open arc and box tests. But what do these tests measure? What’s the difference between them? And how are they carried out?

HAF Heat Attenuation Factor (HAF) is a measurement of the percentage of energy that is blocked by the material or material system. Even though a fabric may be 100% flame resistant, this does not mean it will block all of the heat it’s exposed to. A HAF of 85% means that it will block 85% of the heat the fabric is exposed to. This is in the case of a short burst of arc-flash heat – typically less than one second. In the event of prolonged heat exposure, the HAF would be much lower. A pioneer in testing protective wear Snickers Workwear offers great expertise and many years of experience in protective clothing. Our experts have led the development of protective clothing for the energy and power industries for many years. In fact, they were the first to initiate testing for protection against the thermal hazards of an electric arc when developing flame-proof protective wear. It goes without saying that our garments meet current standards and certifications as a minimum.