Quantifying The Arc Rating Of A Protective Garment
IEC 61482 Series
The International Electrotechnical Commission (IEC) has developed a series of International Standards for clothing to protect against the thermal hazards of an electric arc. The clothing is intended to be used for electrotechnical work where there may be an electric arc hazard. These standards do not address electric shock hazards, and are not suitable for electric shock protection unless used in combination with other standards. They also do not address the effects of noise, UV emissions, air pressure, shrapnel, hot oil, the consequences of physical and mental shock or toxic influences of an electric arc.
The following test procedures are available for electric arc testing: procedures according to standards:
1) IEC 61482-1 “Open Arc Test Method”
The arc rating is most commonly quantified by the Arc Thermal Performance Value (ATPV) determined by the open arc test method IEC 61482-1. This test method is currently being revised to IEC 61482-1-1/CDV. The ATPV represents the maximum incident thermal energy in units of energy per surface area (e.g. kJ/m2 or cal/cm²) that a fabric can support before the wearer will suffer 2nd degree burns.
The break-open threshold energy (or EBT) is another fabric characteristic. It represents the highest incident energy exposure value on a fabric where the garments do not exhibit break-open. The formation of holes in the fabric caused by break-open would allow heat or flames to pass through. Workers are assumed safe if the arc rating of their clothing (or ATPV value) exceeds the electric arc incident energy as calculated in the worst case scenario of a risk assessment.
Materials or assemblies are given an ATPV value, expressed in kilojoules per square metre (kJ/m2), which can be converted into the familiar cal/cm2.
2) IEC 61482-1-2 "Box Test"
Schematic diagram of the Box Test method
The box test is another way to measure the protective performance of clothing against the thermal effects of an electric arc, and is defined in the IEC 61482-1-2 test method. In this test, the fabric is exposed to an electric arc confined in a specific box with a specific electrode arrangement.
A fabric passes the test:
• If the heat transferred behind the fabric does not cause second degree burn (i.e. is below the Stoll curve)
• If the after flame time is below 5 seconds
• If there is no melting to the inner side of the fabric
• If a hole caused by the arc is not larger than 5 mm in every direction (in the inner most layer)
Test conditions for Class 1 try to simulate typical exposure conditions for a short circuit current of 4 kA protected by devices limiting the duration of the arc to 0.5 seconds in confined space, and of 7 kA respectively for Class 2.
This box test method should be referred to for Low Voltage Systems only.
For instance to replicate potential hazards in;
service entrance boxes, cable distribution, cabinets, distribution substations or comparable installations, where arc is directed to the front of a worker at the height of the breastbone
It is the opinion of UK technical committee PEL/78 that the “box test” does not provide the user with a realistic and reliable test. A premise of this test is that the fault currents will not exceed 4000 A or 7000 A and the worker will not be closer than the specified distance from the arc (in reality this cannot be guaranteed).
The energy on the worker’s body is directly proportional to the length of the arc, the current and the duration of the arc. For example, if a worker positioned at 60cm from the arc and is exposed to 4 cal/cm2 at 30cm from the arc the energy will be approximately16 cal/cm2. Experience in the field has shown that hazards can range in strength starting from 3 or 4 cal/cm2 to levels higher than 40 cal/cm2.
UK technical committee PEL/78 believes that open arc test will provide the best way to determine whether a particular material will provide the best protection for the worker for any given job.
IEC 61482-2/CDV - Performance Requirements For Garments
Flame resistant garments are CE marked to the above standard. This standard contains performance specifications for protective clothing tested according to either “open arc” or “arc in a box”.
The standard requires the following:
- A single layer garment having a minimum of ATPV = 4 cal/cm2 (167.5 kJ/m²) when test to the open arc test method or Class 1 when tested to the box test method;
- Garments must be sewn with Inherently Flame Resistant threads;
- When garments have a higher rating of arc protection on the front than the back, the torso, sleeves and legs must be in the same arc rated material the back can have a lower arc rating;
- Garments must have no exposed external metal (any internal metal or melting parts are to be covered inside to avoid contact with the skin);
- The garment label will show the lowest ATPV or Class if different panels on the garment are used;
- The double triangle symbol will be on the label with the ATPV and/or Class underneath it;
- Upper (ie. torso) garments only will be visually tested to the IEC Open Arc or Box test and the fastenings are to still perform after the arc test;
- Additional User Information will be provided to include that no undergarments which melt under arc exposure are to be worn and that other PPE should be worn for full body protection (e.g. helmets and gloves).