What is an Arc Flash?
Let’s start by explaining the electrical phenomenon of an arc flash. The arc flash is the resulting discharge of energy caused by an arcing fault. An arcing fault is the unintended flow of current through a medium not intended to carry the current. What? That just means that the electricity is flowing through something it shouldn’t be; in most cases that result in injury, the medium was the air. The air becomes like a piece of copper, conducting the electricity, only with the air, you can see the massive discharge of the electrons from the discharging element. This is the arc flash. It’s lightning on a smaller, yet a more deadly, scale.What causes an arc flash? Well, as said above, the cause is an arcing fault. What causes an arcing fault? Many things: The most common causes of an arcing fault are equipment failure, human error (improper placement of tools or improper use of equipment), or the conduction of electricity due to foreign particles in the air (usually metal shavings).
• EN 470-1 is the European standard detailing minimum design and fabric requirements for protective clothing worn in welding and allied processes. Relevant as welding risks - molten metal splash, flame and UV are also found in electrical arcs.
• EN 531 European standard detailing minimum requirements of clothing used to protect against industrial heat and flames. Includes general design requirements and five performance requirements each of which are standards themselves and given code letters:
A: Limited flame spread EN 532
B: Convective Heat EN 367
C: Radiant Heat EN 366
D: Molten aluminium splash EN 373
E: Molten iron splash EN373
Clothing complying with EN 531 must meet code A and level 1 or above of at least one of the other performance requirements (B-E).
• ENV 50354 Also referred to as DD ENV 50354:2001, a European draft standard entitled Electrical arc test method for material and garments, for use by workers at risk from exposure to an electrical arc. It stipulates testing procedure and pass requirements for testing either a garment or piece of fabric to either a class 1 or higher energy class 2 test. Test parameters are as follows:
- voltage = 400V,
- arc duration = 500ms,
- ac frequency = 50 Hz,
- phase = single,
- electrode gap = 3cm,
- distance to receiving surface = 30cm
All PPE are subject to the European Norm EN 340 specifying general requirements for protective clothing. It is the user responsibility to carry out risk analysis and select the appropriate personal protective equipment.
* Standards: performance requirements ** Standards: test methods
|Thermal Hazard ||Typical Exposure Time||Temperatures||Heat Transfer|
|Heat and Flame||Up to 15s||Up to 1,000oc||50% convective heat|
50% radiant heat
|Heat of an electric arc||Up to 1s||Up to 10,000oc||10% convective heat|
90% radiant heat
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"
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.
|Box Test ||Arc Duration ||Arc Current |
|Class 2||0.5 s||7kA|
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).
• OSHA (Occupational Safety and Health Administration ) CFR 1910.269 ‘The employer shall ensure that each employee who is exposed to the hazards of flames or electric arcs does not wear clothing that, when exposed to flames or electric arcs, could increase the extent of injury that would be sustained by the employee.’ (I)(6)(iii)
• Clothing made from acetate, nylon, polyester and rayon either pure or blended should not be worn when working in hazardous environments. Clothing made from 100% cotton or wool must be determined acceptable for the conditions the worker will be exposed to. Clothing made from flame-resistant materials, which meet current ASTM F1506 is acceptable. (ASTM is a standard developed in the USA).
• ASTM F1506 details the specifications of a textile to be used by an electrical worker as a means of electrical arc protection. A garment must include a label, which states the following information: Tracking I.D. Code, Meets ASTM F1506, Manufacturer’s name, Care Instructions & Fibre Content, Size and ‘Arc Rating’ –ATPV or EBT.
Fault current is either 4kA for a class 1 test, or 7kA for a class 2 test. The electrodes are enclosed in a plaster box with one side open to direct the arc imitating a real life situation. This box test method is proposed to become IEC 61482-1-2. There is no documentation stating any equivalents between these classes and a cal/cm2 rating.
*ATPV (Arc Thermal Performance Exposure Value) is defined in the ASTM F1959 standard arc test method for flame resistant (FR) fabrics as the incident energy that would just cause the onset of a second degree burn. EBT (Break open Threshold Energy) is the average of the five highest incident energy exposure values below the Stoll curve where the fabric does not exhibit break open. EBT is reported when ATPV cannot be measured due to FR fabric break open.
Always Perform a Hazard Assessment
The NFPA 70E Standard for Electrical Safety Requirements for Employee Workplaces requires employers to perform an Electrical Arc Hazard Assessment. Each situation is unique and needs to be evaluated on its own merits. ASTM F1959 details the standardised test that must be used to determine the thermal protective value of textiles in an electric arc application.Clothing selected for a particular application shall have an arc protective rating of (EBT or APTV) higher than the potential hazard to prevent the onset of 2nd degree burns.
Factors Affecting the Extent & Seriousness of Worker Injury when Exposed to Arc Flash
1. Electric arc intensity
• Fault current
• System voltage
• Electrode gap
• Number of phases involved
• Open arc configuration or enclosure
2. Electric arc duration
3. Distance of the worker from the electric arc
4. Type and fit of clothing worn
5. Age and health factors
Effects of Electrical Shock
Even relatively low voltages can be fatal. For example, electrical shocks produced from common 50 Hz ac power passing from hand to foot for a duration of one second can have the following effects:
1-3 Milliamps Tingling Sensation
3+ Milliamps Shock (pain)
10+ Milliamps Muscular Contractions (can’t let go)
30+ Milliamps Respiratory Paralysis (may be fatal)
60+ Milliamps Ventricular Fibrillation (usually fatal)
4+ Amps Heart Paralysis (fatal)5+ Amps Tissue Burning (fatal, vital organs destroyed)