I have been looking at the question of ATPV, following the CEN Consultant's evaluation of IEC 61482-2. I wondered if you could shed some light on my question.
If the ATPV for a fabric or system is calculated at the level where 2nd degree burns occur, is this taken into account in the Arc Flash calculations or in the selection of clothing systems? In other words, does the protection worn usually have a higher ATPV than the equipment is rated for, in order to provide a safety margin and to ensure there will not be 2nd degree burns?
This seems to be the CEN Consultant's concern, that if the ATPV is at the 50% probability of burns, then there is still a risk of workers being burned when wearing the correct PPE. I have read through NFPA 70E and various other articles but I can't see anything on this subject.
There are several things that play into this.
1. All testing is done at 8KA for the IEC -1-1 testing and ASTM F1959. We have done substantial work which shows that as the fault current increases clothing is 2X or more protective. Most faults are >8KA (we chose this current to make the testing more conservative).
2. The calculations in IEEE 1884 are approximately 80% worst case so matching the energy.
3. A study of 40 accidents has shown that the arc ratings are very conservative. IEEE paper by Dan Doan, Tom Neal and Hugh Hoagland (see IEEE Electrical Safety Workshop for the paper).
4. The testing is done on a flat panel which is the worst case. We see better results on garments. Garments touch the skin on approximately 40% of the body or less in most cases.
5. The 50% probability of burns is most cases is near the 10% probability of burns. We chose the 50% because it is what we have statistically adequate points to state. To do 10% or less would require approximately 100 points statistically. One can use those numbers but they will could be unreliable.
6. The flash fire standards allow 50% body burn which can be 2nd degree or 3rd degree burn. The arc flash standard only allows 2nd degree burn on the 80% worst case portion of the arc flash. This is lightyears beyond flash fire protection because we can predict arc flash energies better.
7. Arc flashes are very focused events. So only a small portion of the body will get the maximum energy in the arc and only that small portion will receive ANY burn if the clothing protection is overcome. Out of over 100 accidents I have helped investigate, only 3 have recieved more than 25% body burn IF clothing didn't ignite (one was 26%, two were about 80% but they were oil filled transformers or cables and ~10 cal protective systems). Most were UNDER protected by NFPA 70E standards yet the percentage body burn was less than 25%.
8. Substantial research is ongoing in the Box method 1-2 and in the Arc Rating method 1-1. I believe a hybrid method will be developed with a plasma rating / IR rating that will combine the ejected arc of ASTM F2676 and the indirect IR arc of ASTM F1959 (In ASTM F1959 an arc front starts hitting the panel/hood at about 20 cal/cm2 so higher levels in F1959 are really exponentially more protective).
9. Second degree burns was chosen because they are curable burns. The normally do not leave scarring and if limited to less than 25% of the body or less, they rarely lead to death. What of first degree burns. The goal of any company is to eliminate fatalities and worker injuries.
In worst case senarios workers can get limited body burn in arc flash events no matter what the test method or the calculation method. The arc flash standards, either ASTM or IEC provide information for specifiers to provide reasonable parameters to their clothing specs for arc flash. They also allow product to product comparison in a lab arc flash. The higher the level of protection chosen the less risk of all burns but one must weigh this with compliance by the workers, other hazards, worker acceptance and other concerns.
The ASTM F1959, IEC 61482-1-1, and IEC 61482-1-2 standards are designed to protect workers from arc flash in ways that work clothing have never protected before. Using arc flash calculations and these standards will do a much better job than NOT doing calculations and only using the IEC 61882-1-2. The 1-2 standard is hazard assessment and testing built into one BUT it has two levels which do not take into account all arc flash events. It is really built around a meter box at a home service and is very adequate for those exposures. The 1-1 or the ASTM F1959 test is not dependent on the exposure because we measure the incident energy on each test. The IEC method 1-2 measures at the start of testing but not during the test except under the cloth. The round robin testing for ASTM F1959 and precision testing has shown this test to be very reliable intra and between the few labs which have done this test. Most PPE in the world has been tested to this method allowing direct comparison. Some lab differences remain in the 1-2 testing but this method also eliminates ignitable, melting materials so I'm really a fan of using either test but none of the current tests mean workers will never be burned in arc flashes. Most lives are saved by stopping ignition and melting. Extra protection saves a few more lives but results in better quality of life by reducing burn percentage and severity. The best practice today is using NFPA 70E or CSA Z462 and IEEE 1584 for choosing PPE by calculations and risk assessment. Choosing arc rated clothing is the most important decision a company can make for preventing arc flash deaths.