Electric Arc Flash Protection


Glossary

electrician with protective clothingElectric Arc - is a sustained electric current passing from one solid electrical conductor, or electrode, to another through air or some other gas. The gas in the space between electrodes becomes an electrical conductor because the current ionizes it, that is, it gives the atoms of the gas an electric charge. An electric arc produces light and heat.

Furnaces that produce heat by means of an electric arc are used in foundries to melt iron. Another common use of the electric arc is in welding, where the heat of the arc fuses metals. An electric arc can be produced by several methods. In one common method, current is applied to two electrodes in contact with each other; when they are slowly pulled apart, an electric arc is formed.

Arc Flash - an arc flash is a dangerous condition associated with the release of energy caused by an electric arc. It occurs when electrical insulation or isolation between conductors is broken or can no longer withstand the applied voltage. Arc flash temperatures can reach or exceed 35000° F at the arc terminals. The massive energy released in the fault rapidly vaporises the metal conductors involved, blasting molten metal and expanding plasma outward with extreme force. A typical arc flash incident can be inconsequential but could conceivably easily produce a catastrophic explosion. The result of the violent event can cause destruction of equipment involved, fire and injury or death, not only to immediate workers but also to persons nearby.

Switch RoomArc Flash Blast - the plasma cloud from an arc flash will conduct as much energy as is available and is only limited by the impedance of the arc. This massive energy discharge burns the bus bars, vaporising the copper and thus causing an explosive volumetric increase, the arc blast, conservatively estimated, as an expansion of 40,000 to 1. This fiery explosion devastates everything in its path, creating deadly shrapnel as it dissipates.

Arc Flash Boundary – a measurement of distance determined during Arc Flash Hazard Analysis to inform workers of the area and boundary distance from energised equipment, which when entered, could result in second-degree burns should an arc flash occur.

Arc Flash Hazard Analysis - the assessment process to determine the potential exposure to the energy released by an arc flash. The analysis determines the arc flash protection boundary (distance) and the Arc Thermal Performance Value which the FR work wear must at least equal.

Arc Rating - a term used to indicate the protection level of a garment.

Arc Thermal Performance Value (ATPV) – the incident energy on a material that results in sufficient heat transfer through the material for a 50% probability of the onset of a second-degree burn injury. The Industrial EngineersATPV is expressed in calories per centimetre squared. The higher the value, the more insulated a person is from second-degree burns.

Calorie – a calorie is the energy required to raise one gram of water one degree Celsius at one atmosphere. Second-degree burns will occur at 1.2 calories per centimetre squared per second, expressed as cal/cm2. For example, a potential arc flash situation might have a calculated incident energy of 8.7 cal/cm2 in which case FR work wear with an ATPV of at least 9 cal/cm2 (or higher) would be applicable.

Energy Break-open Threshold (Ebt) - represents the amount of energy needed to cause the material to break-open, that is, to exhibit failure, hole formation or destruction. The arc rating of a fabric is equal to its ATPV or its Ebt (in cases where the Ebt is lower than the ATPV or no ATPV can be determined).

Heat Attenuation Factor - is the total percentage of the arc flash heat energy, which the textile blocks or prevents from passing through. The higher the HAF percentage, the more heat is blocked by the textile. If the textile breaks open prior to reaching the second-degree burn level, the Ebt is then calculated.

Electrical linesmenIncident Energy - is one of the key terms in understanding any arc flash hazard. NFPA 70E defines incident energy as "‘the amount of energy impressed on a surface, a certain distance from a source, generated during an electrical arc event." The two most important numbers to remember are 1.2 and 40. Incident energy levels greater than 1.2 cal/cm2 can produce second-degree burns. NFPA 70E requires that workers wear PPE when working with 50 volts or more. Arc flash levels above 40 cal/cm2 can be fatal usually resulting in a massive pressurised blast with sound pressure waves and projectiles. PPE is available for 100 cal/cm2 however the force from the pressurised blast can be fatal regardless of the PPE.

PPE (Arc Flash Protection Kit) - refers to a full coverage arc flash suit or system, with a specified arc rating of between 8 cal/cm2 and 100 cal/cm2, made up of individual components including an arc flash coat and bib overall combination, arc flash hood, hard hat, safety glasses, insulating rubber gloves, leather glove protectors and dielectric footwear. The overall arc rating of the arc flash suit is determined by the uniform arc rating of the individual components. Insulating rubber gloves, rubber sleeves, leather glove protectors and dielectric footwear also constitute PPE however arc ratings for these items are usually determined from other data, such as the class and maximum use voltage of the insulating rubber.


Introduction

When workers cannot work de-energised, it is imperative they have suitable and adequate flame resistant work wear, which matches the potential arc flash hazard they are working with. Work wear, which is not FR will most likely continue to burn after the onset of ignition, resulting in the fabric perhaps dripping and melting into the skin, only increasing the severity of the injuries. FR work wear can under certain circumstances ignite, however, it will self-extinguish after the ignition source, that is, the flame or arc has ceased.

Heat generated from an electric arc is capable of reaching temperatures of around 4000o C (7232o F) thus, producing a significant amount of calories. Arc flash temperatures can reach or exceed 35000o F at the arc terminals.

To match FR work wear to the potential arc flash hazard, corporations must undertake arc flash hazard assessments to determine the potential incident energy and arc flash boundary their workers are exposed to. Corporations must then choose arc rated clothing and head protection that has an ATPV equal to, or greater than, the calculated hazard level. When using layered protection systems, it should be noted that the ATPV ratings of individual layers cannot be simply added together, since in some cases, layering can actually decrease the total protection. The only way to use a layering system to achieve protection is to conduct a multi-layer arc flash hazard assessment. 


NFPA 70E

NFPA logo

The National Fire Protection Association (NFPA) published the latest edition of the NFPA 70E Standard (Standard for Electrical Safety Requirements for Employee Workplaces) in 2015. The next edition of this standard is now open for Public Comment. NFPA 70E states, "employees shall wear FR clothing wherever there is a possible exposure to an electric arc flash." This requires employees 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. Employers are required to perform a flash hazard analysis to determine the potential energy of the hazard and the arc flash protection boundary. A flash hazard analysis can be performed by calculating the potential incident energy of a piece of equipment or using Hazard Risk Category classifications. Protective clothing must meet the calculated incident energy or the corresponding Hazard Risk Category that has an arc rating of at least the value listed in the ‘Protective Clothing Characteristics’ section of the standard (see NFPA 70E Requirements below).


Arc Flash Hazard Analysis

The duration and intensity of electric arcs is never equal and is the reason why flash hazard analysis must be undertaken. The analysis will quantify the potential arc's energy level and identify the appropriate type and textile weight of the work wear that should be worn whilst working in the particular environment.

Two computer software programs are available to assist corporations to determine the incident energy level of a potential arc flash. They are –

  • The Duke Power Heat Flux Calculator - the software calculates the heat flux on a surface resulting from an arc flash with a specified voltage, available current and air gap. The calculator can be useful when determining arc flash boundaries.

  • ARCPRO™ from Kinectrics, Inc., of Toronto, Canada - the software calculates the thermal parameters of electrical arcs and is designed to provide users with data regarding heat exposure, total heat flux and heat energy on a surface at various distances from an electric arc. The software is used to predict the potential arc hazard in a particular work environment.

The programs ask a number of important and company specific questions relating to the potential hazard situation and once answered, they will calculate a calorie per centimetre squared per second value, which is the level of protection (ATPV rating) the work wear must provide to prevent the onset of second-degree burns.

The NEC (National Electrical Code [NFPA]) and more recently the NFPA 70E state that any of the following types of electrical equipment located in manufacturing and commercial establishments (other than dwelling occupancies) must be field marked with a warning label if subject to examination, adjustment, service or maintenance while energised -

  • Switchboards.

  • Panel boards.

  • Industrial control panels.

  • Meter socket enclosures.

  • Motor control centres.

The labeling requirement is the responsibility of the employer, not the manufacturer or installer of the equipment.

The current NFPA 70E edition requires the following three elements to be included on Arc Flash labels -

At Least One of the Following -

  • The available incident energy (discussed in Glossary above) and the corresponding working distance.

  • The minimum arc rating of work wear. Arc rated work wear indicates it has been tested for exposure to an electrical arc.

  • The required level of work wear, that is, the arc rating of the work wear must correspond with the appropriate hazard level and incident energy present.

  • The highest Hazard Risk Category (HRC) for the equipment.

At Nominal System Voltage

This identifies the voltage by which the electrical system is designated, to which certain operating characteristics are related and near the voltage level at which the system operates. Generally, it is about five to ten percent below the maximum system voltage for which system components are designed. Nominal system voltage is measured in volts.

Arc Flash Boundary

The Arc Flash Boundary identifies systems of 50 volts and greater where the distance at which the incident energy level equals 1.2 cal/cm2. This must be identified on a label because this incident energy distance can result in a second-degree burn if skin is unprotected.


NFPA 70E Requirements

Arc Flash Clothing Defined by NFPA 70E - OSHA 29 CFR 1910.269 (1)(6)(iii) states: "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. Clothing selected for a particular application shall have an arc thermal performance value of (Ebt or ATPV) higher than the potential hazard to prevent the onset of second-degree burns."

Different types of arc flash clothing have different ratings. For example, gloves have a voltage rating to protect from electrical shock. Cotton and flame resistant clothing all have thermal ratings. The following table shows the level of arc flash clothing required for each level of hazard.

explosion signWARNING – NFPA 70E does not have a Hazard Risk Category (HRC) above 40 cal/cm2. Working in environments above 40 cal/cm2 should be avoided because of the blast hazards caused by electric arc flash. Arc flash levels above 40 cal/cm2 can be fatal and usually result in a massive pressurised blast with sound waves and projectiles. PPE is available for 100 cal/cm2 however the force from the pressurised blast can be fatal regardless of the PPE.

 

Table 3 NFPA 70E

Standard for Electrical Safety Requirements for Employee Workplaces

Personal Protective Clothing (PPC) Characteristics 

Hazard Risk Category

(HRC)

Incident Energy

Minimum cal/cm2

Required FR Work Wear and PPE

0

2

Non-melting Clothing

1

4

FR Shirt and FR Trousers (or FR Coveralls) and PPE

2

8

FR Shirt and FR Trousers (or FR Coveralls), Cotton Underwear and PPE

3

25

FR Shirt and FR Trousers, FR Coveralls (in addition to FR Shirt and FR Trousers), Cotton Underwear and PPE

4

40

FR Shirt and FR Trousers, FR Coveralls (in addition to FR Shirt and FR Trousers), Cotton Underwear, Full Coverage Arc Flash Suit¹ and PPE

 

¹Arc flash suits cover the entire body and are of heavier weight (gsm/oz) than normal FR work wear. This type of PPE usually comprises an arc flash coat and bib overall combination, arc flash hood, hard hat and safety glasses. Full coverage arc flash suits have arc ratings of between 8 cal/cm2 and 100 cal/cm2, with the overall arc rating determined by the uniform arc rating of the individual components. Insulating rubber gloves, rubber sleeves, leather protector gloves and dielectric footwear also constitute PPE however arc ratings for these items are usually determined from other data, such as the class and maximum use voltage of the insulating rubber. Although not considered PPE, rubber blankets, plastic cover-ups and insulated tools have many similarities and their use helps to enhance arc flash protection.  

 

Some corporations have decided to simplify compliance to NFPA 70E by implementing everyday uniform programs that meet the requirements of NFPA 70E HRC 0, 1 and 2 as a single-layer (see table below). This can alleviate employer concerns about leaving the difficult decision of determining whether a specific routine electrical task is HRC 0, 1 or 2 in the hands of the employee. To supplement everyday uniforms, arc flash suits and hoods in double-layer UltraSoft®, UltraSoft AC® and Indura® combinations are available for higher energy HRC 3 and 4 level tasks.

 

NFPA 70E: Annex H, Two-Category, Flame Resistant (FR) Clothing System

Table H.1 Simplified, Two-Category, Flame Resistant Clothing System

FR Clothing

Applicable Tests

Everyday Work Clothing

FR Shirt with FR Trousers (minimum arc rating of 8) or FR Coveralls (minimum arc rating of 8) and Cotton Underwear

 

All Hazard/Risk category 1, 2 and 2* tasks listed in Table 130.7(C)(9).

Arc Flash Suit

A total clothing system consisting of FR Shirt and FR Trousers, FR Coveralls (in addition to FR Shirt and FR Trousers), Cotton Underwear and Full Coverage Arc Flash Suit (minimum arc rating of 40)

 

All Hazard/Risk category 3 and 4 tasks listed in Table 130.7(C)(9).

*With Face Protection.


In Summary

NFPA 70E requires workers to wear flame resistant work wear, which meets the requirements of ASTM F1506, wherever workers are exposed to an electric arc flash. As discussed throughout this section, corporations must conduct flash hazard analysis to determine the required level of FR work wear and the appropriate boundary distance from the flash. ASTM F1506 sets out the requirements to protect workers working inside the flash protection boundary by specifying the FR work wear, which suits each hazard risk category having an ATPV rating at least equal to those listed in NFPA 70E, ‘Protective Clothing Characteristics’, discussed above. OSHA (Occupational Safety & Health Administration) has confirmed that FR work wear, which satisfies the requirements of ASTM F1506 are in compliance with OSHA 29 CFR 1910.269 (Electrical Power Generation, Transmission and Distribution, with regard to garments not contributing to burn severity).

The Arc Thermal Performance Value (ATPV) and the Energy Break-open Threshold (Ebt) are the most commonly used parameters to evaluate the level of protection of arc rated work wear. Under the extreme conditions of an electric arc flash, no garment can be expected to fully protect a worker. However, careful garment selection can help ensure additional hazards are not introduced due to the behaviour of work wear in an arc flash incident. Careful assessment will lead to the percentage of probable body burn being reduced.

ASTM logo

American Society for Testing and Materials (ASTM)

Standards Summary (Electric Arc Flash Protection)

  • Flame resistance is the characteristic of a fabric that causes it to self-extinguish when the source of ignition is removed. The most commonly used test method is ASTM D6413 - Standard Test Method for Flame Resistance of Textiles (Vertical Test). The vertical flame test is a test method with no pass/fail requirements. Industry established standards range from 4" to 6" (100 mm to 150 mm) maximum char lengths. It is very important for flame resistant fabrics to self-extinguish. Fabrics that self-extinguish after the source of ignition is removed can dramatically reduce body burn percentage and increase the chance for survival.

  • F955-07 - Standard Test Method for Evaluating Heat Transfer through Materials for Protective Clothing upon Contact with Molten Substances.

  • F1506-02a - Standard Performance Specification for Flame Resistant Textile Materials for Wearing Apparel for Use by Electrical Workers Exposed to Momentary Electric Arc and Related Thermal Hazards.

  • F1891-06 - Standard Specification for Arc and Flame Resistant Rainwear.

  • F1930-00 - Standard Test Method for Evaluation of Flame Resistant Clothing for Protection Against Flash Fire Simulations Using an Instrumented Mannequin.

  • F1958/F1958M-99 - Standard Test Method for Determining the Ignitability of Non-Flame-Resistant Materials for Clothing by Electric Arc Exposure Method Using Mannequins.

  • F1959/F1959M-99 - Standard Test Method for Determining the Arc Thermal Performance Value of Materials for Clothing.

  • F2178-02 - Test Method for Determining the Arc Rating of Face Protective Products.

  • F2733-09 - Standard Specification for Flame Resistant Rainwear for Protection Against Flame Hazards.

Australian and New Zealand Standards

Australian and New Zealand standards relating to flame and fire resistant clothing for use in protection against heat and fire are summarised as -

  • AS 2374-1980 - Guide to the selection, care and use of clothing for protection against heat and fire.

  • AS 2755.1-1985 - Textile fabrics - Burning behaviour - Determination of ease of ignition of vertically oriented specimens.

  • AS 2755.2-1985 - Textile fabrics - Burning behaviour - Measurement of flame spread properties of vertically oriented specimens.

  • AS 2755.3-1985 - Textile fabrics - Determination of burning behaviour - Determination of surface burning time.

  • AS/NZS 4502.1:1997 - Methods for evaluating clothing against heat and fire - Evaluation of thermal behaviour of materials and material assemblies when exposed to a source of radiant heat.

  • AS/NZS 4502.2:1997 - Methods for evaluating clothing against heat and fire - Evaluation of heat transmission of materials and material assemblies when exposed to flame.

  • AS/NZS 4502.5:1997 - Methods for evaluating clothing for protection against heat and fire - Evaluation of the contact heat transmission through material and material assemblies.

Standards Organisations


Website References

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