PROTECTION AGAINST ELECTRIC ARC HAZARDS

What is electric arc?

Arc originates from gas ionization, electrodes at different potentials are the conductive electrical connection between different phases. Electric arc can occur due to a technical or operator error. While galvanic contacts are required to trigger the electric arc in low-voltage areas and in high-voltage areas, live parts may be caused by an inconvenience in the relative distance.

An electric arc explosion is a very short-term electricity flow or discharge in the form of heat and light through air between two conductors that do not touch each other along an unintended path.

IEEE Std 100-1988 defines electric arc as a discharge of electricity through a gas, normally characterized by a voltage drop in the immediate vicinity of the cathode approximately equal to the ionization potential of the gas and flashover as:

1) (general) a disruptive discharge through air around or over the surface of solid or liquid insulation, between parts of different potential or polarity, produced by the application of voltage wherein the breakdown path becomes sufficiently ionized to maintain an electric arc.

2) (high voltage ac cable termination) A disruptive discharge around or over the surface of an insulating member, between parts of different potential or polarity, produced by the application of voltage wherein the breakdown path becomes sufficiently ionized to maintain an electric arc.

3) (high voltage testing) Term used when a disruptive discharge occurs over the surface of a solid dielectric in a gaseous or liquid medium.

Air is not a good conductor, most of the current flows are through post-arc vapor (usually copper and aluminum vapor) and ionized particles in the air. This mixture at very high temperature is called plasma.

Effects of electric arc

Depending on the power and burning time of electric arc, different physical effects can be seen due to the high temperature. Electric arc energy is discharged in multiple ways, such as electrical, thermal, mechanical, photonic or luminous energy. Mechanical energy usually occurs in the form of explosions. Many various factors affect the energy created by electric arc (See; Factors affecting the damage of electric arc).

According to NFPA 921 Sections 14-1 and 14-9 through 14-12.2 Electricity and Fire, an arc is a high-temperature luminous electric discharge across a gap. Temperatures within the arc are in the range of several thousand degrees depending on circumstances including current, voltage drop, and metal involved. For an arc to jump even the smallest gap in air spontaneously, there must be a voltage difference of at least 350 V. In the 120/240-V systems being considered here, arcs do not form spontaneously under normal circumstances. In spite of the very high temperatures in an arc path, arcs may not be competent ignition sources for many fuels. In most cases, the arcing is so brief and localized that solid fuels such as wood structural members cannot be ignited. Fuels with high surface-area-to-mass ratio, such as cotton batting and tissue paper and combustible gases and vapors, may be ignited when in contact with the arc.

Temperature

According to NFPA 921 Sections 14-1 and 14-9 through 14-12.2 Electricity and Fire, for ignition to be from an electrical source, the following must occur:

(a) The electrical wiring, equipment, or component must have been energized from a building's wiring, an emergency system, a battery, or some other source.

(b) Sufficient heat and temperature to ignite a close combustible material must have been produced by electrical energy at the point of origin by the electrical source.

The arcs can exceed over 10,000 °F and electrical fires are likely outcome of these arc flashes. The material in the center of the arc evaporates and causes a conductive connection between the electrodes. The fact that the tip of a burning cigarette is 580 °C when not smoked, 700 °C when smoking, and the surface temperature of the sun 6,000 °C reveals how dangerous arc explosions are. The temperature effect of an arc blast with a diameter of 20 cm at a distance of 50 cm is approximately 1300 °C.

As the condensed heat increases during the arc blast, a plasma is formed between the electrodes. Plasma separates due to the separation and ionization of all other chemical components. After the metal has evaporated, a very high temperature rise occurs and the explosive metal vapor and splashes cause mass expansion towards the center of the arc. As a result of cooling and after reaction with oxygen in the atmosphere, the metal oxide can be found as black or gray smoke. Because the steam or fumes have enough heat, they leave a sticky waste.

The following result comes from the experimental studies carried out by IEEE (Institute of Electrical and Electronics Engineers); Arc-induced heat energy can kill or injure people even from great distances. For example, it can cause 2nd degree burns from a distance of 3.6 m. Second degree burn is called for the increase in heat energy of 1.2 cal/cm² on the skin in 1 second (threshold value).

Depending on the intensity of the electric arc, heat radiation can also ignite flammable materials around it. Molten metal splashes that may occur increase the risk of fire. In addition, workers in hazardous areas exposed to toxic erosion from electric arc can also suffer severe lung damage, apart from burning on the skin.

Arc blast causes burns to the point of death. The causes of accidents that the vast majority of those who go to hospitals due to electrical accidents are exposed to are not electric shock, 80% of them are electric arc accidents. Most of the high-grade burns and deaths resulting from these accidents are caused by the ignition and burning of clothes or underwear that are not resistant to heat and flame.

Mechanical

Another physical reaction during the development of electric arc is the formation of a huge pressure wave of up to 0.3 MPa between the duration of 5-15 ms. This corresponds to a pressure of 20-30 t/m2. The effect of a 22 kA electric arc explosion at a distance of 60 cm will be 225 kg. If continuous pressure wave transmission does not occur, it risks destroying electrical equipment and those around it. Doors and coatings may burst, and pipes and equipment may rupture.

Noise

Noise pressure levels of up to 160 dB can be reached by an electric arc blast. This is a level that can make human deaf. To give an example, the noise of a jet engine at a distance of 30 m is 140 dB.

Flash

It is very dangerous for the eyes as it may cause blindness. If the energy is too high, it may cause ultraviolet burns. For protection, arc flash protective visors (face shields) or hoods with visors that are dark enough not to damage the eyes in the event of a blast and certified according to the relevant standards should be used.

Interior of a medium-voltage cubicle showing the results of an electrical arc and accompanying electrical blast; external view of an aisle and adjacent switchgear for arc flash event shown below. Workers may not assume that they are safe from electric arc flash events even though the access doors and panels are fully secured. Unless it is specially designed arc flash protective clothing, metal-clad equipment will probably not withstand the explosive force of an electric arc blast.

Factors affecting the damage of electric arc

Distance

The magnitude of the damage decreases approximately with the square of the distance to electric arc. In other words, doubling the distance reduces the damage 4 times (may vary depending on the environment).

Temperature

There is a ratio between the difference between the 4th exponent of the arc temperature and the 4th exponent of the body temperature and the energy taken.

Absorption Coefficient

It is the ratio between the energy incoming to the body and the energy received by the body.

Exposure Time

As the exposure time increases, the energy taken increases.

Body Area Exposed to Electric Arc

The larger the body area exposed to electric arc, the more energy absorbed. Calculation of hazard energy level is specified as kJ/m2, J/cm2, cal/cm2.

Direction Angle

The angle of hit and the energy taken are proportional. It is exposed to the most energy when it is hit at an angle of 90 degrees.

Electric arc-related hazards

There are a lot of hazards that may be occured from an arc flash such as thermal burns, burst pressure wave injury, hearing loss, harmful electromagnetic emissions, emission of high toxic gases, shrapnel injury (blast).

An arc flash can cause fatal physical injuries. When the skin is severely burnt, large quantities of liquid are brought to the burnt areas to aid in the healing process. This creates a stress on the renal system and could result in kidney failure. Severe trauma from massive burns can cause a general systemic failure. Burnt internal organs can shut down—causing death. Thus, the more critical the organ that is burnt, the higher the possibility of death. The pressure front from the blast can cause severe injury to the lungs, called blast-lung, resulting in death. Heart failure can result from fibrillation and/or paralysis.

Some PPE standards for protection against electric arc

IEC 61482 1-2 standard has been published by IEC in order to determine the test methods of the materials and heat and flame resistant protective clothings to be used by workers who may expose to electric arc. This standard is a mandatory standard for certification of the product and obtaining the CE mark and includes two different test methods as explained below, it covers the performance values of the fabric and the ergonomic features of the suit:

IEC 61482 1-1 (Protective clothing against the thermal hazards of an electric arc - Part 1-1: Test methods - Method 1: Determination of the arc rating (ELIM, ATPV and/or EBT) of clothing materials and of protective clothing using an open arc): This is a standard specifies test method procedures to determine the arc rating of flame resistant clothing materials and garments or assemblies of garments (layer system eg.) intended for use in clothing for workers if there is an electric arc hazard. An open arc under controlled laboratory conditions is used to determine the values of ELIM, ATPV or EBT of materials, garments or assemblies of garments. The user can classify the arc protective performance into arc rating protection levels based on ELIM, ATPV and/or EBT values which correspond best to the different hazards and risks levels that can result from the user’s risk analysis.

An electric arc of 8 kA from 3 different points from 120 degree angles is applied to the sample for 167 ms. With the help of the calorimeter behind the material, the temperature increase values are recorded. These values are transferred on a table. A curve is drawn (Stoll curve). The Stoll curve is the thermal energy and time curve used to predict the onset of second-degree burn injuries of human tissue. The Stoll curve determines the rating of the transfer of heat energy based on the time of transfer and the level of heat energy produced. Ideally, protective clothings will delay the transfer by absorbing the heat energy at increased heat fluxes. Standards determine ratings as the amount of energy absorbed in cal/cm2 before the time of transfer to human tissue and the result of a predicted crossing of the Stoll Curve criteria (See; Figure 1).

Samples are exposed to 8 kA energy from a distance of 300 mm. If the arc thermal performance value (ATPV) determined within the framework of the test is exceeded, the thermal energy value that the PPE cannot protect the user from second degree burn is shown in cal/cm2. These ATPV values obtained according to the NFPA 70E standard are divided into different risk groups (HRC) (See; Classification table according to NFPA 70E).

Ei : (Incident Energy) It is the amount of energy that causes a 2nd degree burn. It is evaluated according to the Stoll curve. As seen in the Stoll curve above, the Ei curve must remain below the Stoll curve (See; Figure 2).

ATPV : (Arc Thermal Performance Value) This value is measured in calories per square centimeter and represents the maximum performance capability for arc flash protection of a particular suit or fabric. It is the highest thermal accident energy that will prevent the user from being exposed to a second degree burn at 50% rate. It expresses the maximum incident thermal energy per surface area, in units cal/cm2, that the protective suit can withstand the fabric before second degree burn occurs.

ATPV is found by plotting the probability of a 2nd degree burn versus the energy of the incident causing the burn (Ei). The energy that causes a second degree burn at 50% gives the ATPV value. The ATPV of the protective suit or fabric tested according to the above table is 9.9 cal / cm2. The red points at bottom of the table are non-burn cases (cases where there is a 0% chance of getting burned), the red points at top of the table are cases where there is a 100% burn. The total number of analyzed points is 30, and the number of points remaining on the stoll curve is 21. According to the Stoll curve, the incident energy (Ei) corresponding to 50% of the probability of having a 2nd degree burn is 9.9 (See; Figure 3).

The higher the ATPV value of the product, the more energy will be required for a 2nd degree burn to occur. So the case energy resistance of the material will be high. In other words, the higher the ATPV value of a protective suit in cal/cm2, the higher the level of protection. What ATPV protection level of a protective equipment is needed in a job should be determined as a result of an arc flash risk assessment.

EBT : (Energy Break-open Threshold) The incident energy on a material that results in a 50% probability of breakopen, in units cal/cm2. It represents the highest incident energy exposed in the fabric of the protective suit, causing 50% probability to break open the fabric. Holes in the fabric caused by breaking open cause heat or flame to enter inside of the suit. Breakopen is defined as any open area at least 1.6 cm². The fabric did not overheat to the point that caused the burn reading on the sensor and there is only a very small hole in it (See; Figure 4).

As seen in the Figure 4, Energy Breakopen Threshold (EBT) causing breakage in the fabric with 50% probability is determined as 22 cal/cm².

NOTE: The arc rating of a protective suit’s fabric is equal to ATPV or EBT. ATPV is 50% probability of second degree burn in the 8kA arc test on a flat panel. EBT is the 50% probability of a one inch crack in the material.

HAF: (Heat Attenuation Factor) This is the amount of heat blocked by the fabric. In other words, it is the percentage of arc flash heat energy blocked by the fabric or material. The average of HAF's in electric arc shots in all these tests is taken. For example, it has been determined as 89% in the table below. The standard deviation range is found according to the largest and smallest values (See; Figure 5).

IEC 61482 1-2 Protective clothing against the thermal hazards of an electric arc - Part 1-2: Test methods - Method 2: Determination of arc protection class of material and clothing by using a constrained and directed arc (box test): This is a standard specifies procedures to test material and garments intended for use in heat and flame resistant clothing for workers if there is an electric arc hazard. A directed and constrained electric arc in a test circuit is used to classify material and clothing in two defined arc protection classes. This international standard is not dedicated toward measuring the arc rating values (ATPV, ELIM, or EBT). Procedures determining these arc rating values are prescribed in IEC 61482-1-1, using an open arc for testing. Other effects than the thermal effects of an electric arc like noise, light emissions, pressure rise, hot oil, electric shock, the consequences of physical and mental shock or toxic influences are not covered by this standard.

It has been published to determine the safety requirements for the low and high protection classes specified in the standard for the whole clothing and the fabric layer system, and to determine whether protection is provided against the heat where the electric arc occurs. An electric arc for a duration of 500 ms is applied on the test sample from a distance of 30 mm. With the help of the calorimeter placed behind the protective clothing and/or fabric layer system, the curve formed by combining the points on a graph of the temperature increases occurring after the arc flash is drawn.

It is widely used in Europe and the arc rating is not specified as ATPV as we mentioned above. Instead, products are classified as Class 1 or Class 2. Samples are exposed to 4 kA for Class 1 (158 kJ) and 7 kA for Class 2 (318 kJ) for 0.5 seconds from a distance of 300 mm. These kilojoule values define the intensity of the electric arc flash created in the laboratory. It is proved that a PPE that has passed the box test successfully will prevent second degree burns if these kilojoules values are not exceeded.

As a result of the test, melting is not allowed, burning should be 5 seconds or less. The hole can be seen in the outer layer, but is allowed for a maximum of 0.5 cm in the inner layer. Heat conduction values: All measured values must fall under the Stoll curve.

GS-ET-29 (Supplementary requirements for the testing and certification of face shields for electrical works): The standard was published in 2010 firstly by DGUV. This is arc flash box testing with parameters of 400 V AC; 50 or 60 Hz for 500ms and has 2 classes:

Class 1: 4 kA, 135kJ/m³

Class 2: 7 kA, 423kJ/m³

The main difference to EN 166 is that each visor needs to be tested for electric arc flash. The temperature behind the visor at eye, mouth and chin level of the test head is measured and maximum safe temperatures are given, to ensure that users will not be injured.

Of course the arc flash protection level is the most important criteria, so the arc rating (ATPV) as per IEC 61482 1-1 or the arc protection class as per GS-ET-29. However, typically the higher the protection level, the lower the luminous transmittance also called Visible Light Transmission (VLT). Considering the usually poor illumination inside switching cabinets and several other typical electrician workplaces also this VLT is a deciding safety factor. Due to bad visual conditions mistakes may happen, finally even causing an arc flash accident. Note that products with a VLT between 50% and 75% (VLT Class 1) already may need additional illumination. Products with a VLT of less than 50% are supposed to be very dark. Those products only should be used, when extreme high protection is required and provided by a particular product. For the same reason the field of view, the peripheral area one can see without moving the head is important, for example when a switching hood is required.

NFPA 70E Standard (Standard for Electrical Safety Requirements for Employee Workplaces): The National Fire Protection Association (NFPA) published the latest edition of the NFPA 70E Standard in 2021 which has been originally developed at OSHA's request, NFPA 70E helps companies and workers avoid workplace injuries and fatalities due to shock, electrocution, arc flash, arc blast and assists in complying with OSHA 1910 Subpart S and OSHA 1926 Subpart K. NFPA 70E states, "employees shall wear FR clothing wherever there is a possible exposure to an electric arc flash." This requires workers working on or near energised parts and equipment to wear flame resistant clothing that meets the requirements of ASTM F1506 and is appropriate to the potential energy of the hazard.

In NFPA 70E, it does not matter the fabric’s EBT or ATPV. The important thing is the cal/cm2 that the fabric can support. According to NFPA 70E, HRC classifications are as follows;

Category 0 – 2 cal/cm2 (Required minimum ATPV)

Category 1 (HRC1)  – 4 cal/cm2 (The required minimum ATPV – arc rating)

Category 2 (HRC2) – 8 cal/cm2 (The required minimum ATPV – arc rating)

Category 3 (HRC3) – 25 cal/cm2 (The required minimum ATPV – arc rating)

Category 4 (HRC 4) – 40 cal/cm2 (The required minimum ATPV – arc rating)

In environments with an electric arc hazard, arc flash protective equipments that comply with the standards must be used.

ASTM F1506 (Standard performance specification for flame resistant and arc rated textile materials for wearing apparel for use by electrical workers exposed to momentary electric arc and related thermal hazards): This specification provides performance requirements for clothing worn by electric utility workers and other personnel working around energised parts. In addition to non-thermal requirements, the standard requires the fabric to be flame retardant (FR). FR here is measured using ASTM D6413 vertical flame test (maximum 2.0 seconds afterflame and 6.0 inch char length). The arc rating is either Arc Thermal Performance Value (ATPV) or Energy Break-open Threshold (EBT) as measured by the ASTM F1959-06ae1 arc thermal performance test.

The standard has a general requirement that thread, findings and closures do not contribute to the user’s injuries in an electric arc exposure. Knitted or woven fabrics may not melt and drip or have more than 2.0 seconds afterflame or 6.0 inches char length. Arc ratings must appear on garment labels.

Arc flash protective clothings

Preventing clothing and underwear from ignition often ensures survival. Limiting burns to a small surface area provides better results. Prevention of all burns is of course the best option, however it is also very important to survive from an accident with non-fatal burns of various sizes. Protective equipments must be used for this reason.

Clothings made of acetate, nylon, polyester, silk or their mixture should never be worn in hazardous workplaces. In environments where there is a risk of burning, clothings and/or underwears made of flame retardant fabrics should be used. Flame retardant fabrics are divided into two according to the production technique.

Finished fabrics with chemical FR treatment

Fabrics, such as cotton, cotton / polyester blends, cotton / polyamide blends, whose flame is delayed by various chemical treatments, do not lose their flame retardant properties until a certain wash. When washing in accordance with the manufacturer's instructions, there are flame resistant fabrics up to 30-50-100-150 washes, depending on the process applied.

Inherently flame retardant fabrics

Fabrics produced from various fibers such as metaaramid or paraaramid, viscose FR are inherently flame retardant. Even if washed many times, they never lose their flame-resistant properties. However, they are technical fabrics with higher cost compared to finished fabrics.

All of our ELECTPRO® series electric arc flash protective suits produced by our company, IST Safety Ltd are manufactured in accordance with the latest (EU) 2016/425 PPE regulation and have EU type examination certificates and passed the relevant tests of IEC 61482 standard.

ELECTPRO® G2L ULTRASOFT 900 High Energy Electric Arc Flash Protective Suit which has been launched by our company protects the user's body against the negative effects of electric arc such as heat, flame, molten particles. The protective hood is certified according to GS ET 29 standard and carries the CE mark. The visor of the hood is dark enough to protect the eyes from flash. Protective suit consists of three parts as jacket, bib trousers and hood with visor and all of them are made double-layered. The suit is Class 2 (7 kA) according to IEC 61482 1-2 standard. According to the IEC 61482 1-1 standard, the ATPV value of the suit and thw whole layer system is 63 cal / cm2.

References:

Electrical Safety Handbook by Dennis K. Neitzel, Mary Capelli-Schellpfeffer, Al Winfield

IEEE Std 100-1988 Standard Dictionary of Electrical and Electronics Terms

NFPA 921 Sections 14-1 and 14-9 through 14-12.2 Electricity and Fire

IEC International Electrotechnical Commission Website

ASTM International Website

Doruk TURKUCAR

IST Safety Ltd

Export Manager

HSE Professional

https://www.ist.com.tr