Understanding Arc Ratings and Calories

We know that the arc rating measures the amount of heat the flame-resistant fabric blocks when exposed to electric arc. The arc rating is the number of calories that the garment is expected to “absorb” if exposed to an electric arc. Arc rating is the level of protection provided to you, the wearer. A calorie is the unit of measure of the heat energy of an arc flash and the protective level of FR clothing. The bigger the calorie number, the greater the heat energy level of arc flash and the greater the protective level of the clothing. You will be protected from an electric arc if your clothing has a higher calorie arc rating than the calories of heat generated by the arc. In basic terms, the higher the calorie arc rating the better protection the garment will provide.

While it does not matter if the fabric has an ebt or ATPV value, it is important to pay attention to the calorie level the fabric can support - This is shown in cal/cm2:

Level Minimum Arc Rating (cal/cm2)
PPE 1 4.0
PPE 2 8.0
PPE 3 25.0
PPE 4 40.0

EN 1149 Protective clothing - Electrostatic properties (Anti-static)

EN 1149 consists of different parts that specify requirements and test methods to measure protection against static electricity or charge decay. EN 1149-5 describes performance requirements. The garment shall be constructed so all outer surfaces is made of electrostatic dissipative material. Conductive components as zippers and buttons shall be hidden by non conductive material. The material shall meet requirements according to EN 1149 -1 or -3. The surface resistance shall be less than or equal to 2,5x109 ? or the discharge time shall be less than 4 seconds. Protective clothing shall always be worn with electrostatic dissipative footwear.

Anti-static properties:

Garments rated as anti-static have the ability to discharge any build-up of electricity instantly; they do not in affect hold an electrical charge. This charge can come from different synthetic fabrics rubbing against each other, badly earthed electrical products and numerous other sources. The main reasons certain industries insist on anti-static gear are firstly that a spark would readily ignite any petro-chemical vapors, causing an explosion, and secondly, some equipment such as computer memory chips and components are able to be rendered useless by static electricity. Natural fibers such as cotton are naturally good at preventing static charge build-up above freezing point, and therefore would be of benefit to someone looking for limited protection against these risks. For a more guaranteed protection against static discharge, it is necessary to either chemically coat the material (which washes out after 12 to 15 washes), or to weave conductive thread into the garment, like we have done with the Arcguard® garments and the Bison® Rigour rainwear.

EN50534 / NFPA 70EElectrical arc flash protection

These standards include specifications on Garment Design and Protection Factor in the event of high voltage arc strike. According to the North American association, the National Fire Protection Association (NFPA) 70E Hazard Risk levels (HRC levels), the minimum requirements for HRC 2 are 8.0 cal/cm².

All of our ArcGuard® garments exceed the 8.0 cal/cm² minimum requirements for HRC 2. These garments range from our top quality FEPVCMW overall, FTPVCMW trouser and FJPVCMW jacket which have an arc rating of 12 cal/cm² ATPV+, to our lightweight CTPCNLW overall which has an arc rating of 11 cal/cm² ATPV+.

Arc Thermal Protection Value Rating (ATPV Rating)

This rating is stated where a fabric gives the wearer protection against the heat generated by an electrical arc from high voltage equipment. The surface temperature at the point of contact of such an arc can reach temperatures greater than 10,000oC, and obviously cause severe if not fatal burns without some protection against this. Our Arcguard® garments have a rating exceeding the minimum requirements of NZ companies for protection against 2nd degree burns. The rating of the materials used are based on the period of time it takes for the required test voltage arc to cause 2nd degree burns to the wearer. The fabric helps prevent serious burns by turning into a fluffy carbonized deposit that sits against the wearer and reduces the heat transmitted directly to the wearer. To meet higher required ratings (where voltages worked around are considerably higher) the wearer of the garment must either use a specialized suit which may include a full hood, or layer other such ATPV rated garments underneath to provide further layers of protective “fluff” in the event of an arc flash. This is similar to comparing thin and thick Pink Bats - the thicker the insulation, the less heat escapes, or in this case, the less the heat penetrates.

Most line companies have adopted the NFPA 70E American Electrical Standard for levels of protection required and designing of the garment. We are using the ATPV 70E testing regime which is the method for determining suitability of a garment to meet this standard.