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 Post subject: Arc Flash Assessment Requirements
PostPosted: Thu Jan 26, 2012 1:21 am 
Plasma Level
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1528
Location: Scottsdale, Arizona
An arc flash can occur when energized conductors touch each other or when one conductor makes contact with a grounded surface such as an equipment enclosure. The result can be a potentially dangerous and deadly arc that ionizes the air producing an explosion.

Whoosh! What was that? it went fl ying right on by some time ago. Whether you caught it, missed it or ignored it, now it is far behind us in our rear view mirror fading well off into the distance. What was it? it’s the national Electrical safety codes’ (nEsc) January 1, 2009 deadline for arc fl ash assessments.

According to the 2007 edition of the NESC:

“Article 410.A.3 Effective as of January 1, 2009, the employer shall ensure that an assessment is performed to determine potential exposure to an electric arc for employees who work on or near energized parts or equipment. If the assessment determines a potential employee exposure greater than 2 cal/cm2 exists, the employer shall require employees to wear clothing or a clothing system that has an effective arc rating not less than the anticipated level of arc energy.”

Perform an assessment? arc rated clothing? Exposure greater than 2 cal/cm2? January 1? that was so long ago! What does this all mean?

ARC FLASH

Let’s start by taking a look at the problem. an arc fl ash can occur when energized conductors touch each other or when one conductor makes contact with a grounded surface such as an equipment enclosure. the result can be a potentially dangerous and deadly arc that ionizes the air producing an explosion. the worker can be engulfed by a large fi reball; blast pressure can reach thousands of pounds and cause blunt force trauma injury; pieces of exploding equipment can be launched as shrapnel; and the ultraviolet light that is the “fl ash” can cause eye injury and blindness. Each year thousands of people are admitted to hospitals for arc fl ash related injuries. Many of them will ultimately die.

INCIDENT ENERGY

The severity of an arc fl ash is defi ned in terms of its incidetnt (thermal) energy, which is measured as calories/centimeter2 (cal/cm2). this is the amount of energy that can reach the worker standing a specific distance away from the arc, known as the“working distance.” typically, the working distance is 18 to 36 inches, depending on the task being performed as well as the type of equipment being worked on. however, equipment such as hot sticks can greatly increase the working distance and reduce the amount of incident energy that reaches the worker.

The amount of incident energy from an arc fl ash is based on many variables. however, it is the magnitude of short circuit current and the protective device clearing time that are the two most important components. the short circuit current is directly related to how much energy per time is released and the protective device clearing time defi nes how long the duration of the arc fl ash will be.

PROTECTIVE CLOTHING
It is widely accepted that just 1.2 cal/cm2 can produce the onset of a second degree burn, and this value is used as the benchmark for personal protection. according to the nEsc, anytime the potential incident energy exposure to a worker can exceed 2 cal/cm2, protective clothing or a clothing system must be worn that has an arc rating equal to or greater than the available incident energy.

The arc rating refers to the maximum incident energy the garment can withstand while providing a thermal barrier to limit the amount of energy that can reach the worker to no more than 1.2 cal/cm2. Protecting a worker from the arc fl ash usually requires wearing clothing made from fl ame resistant (Fr) fabric as well as using additional personal protective equipment (PPE) such as a hard hat. Fr clothing comes in various arc ratings such as 4 cal/cm2, 8 cal/cm2, 25 cal/cm2, 40 cal/cm2.

THE ARC FlASH ASSESSMENT

Determining how much incident energy could be available at a specifi c piece of equipment or location on a line is something you do not want to discover from a fi eld test (accidental or intentional). this means the only alternative is to predict it from calculations. the most commonly used calculation method is based on iEEE 1584 iEEE Guide for Performing arc Flash hazard calculations. the iEEE equations were empirically derived from tests and are valid for systems operating from 208 volts up through 15 kV with short circuit currents ranging from 700a up through 106ka.

The iEEE equations are based on the incident energy from a three-phase arc fl ash. Even though the majority of arc fl ash events may begin with contact from one phase to ground, it is assumed the conducting plasma that is produced will quickly engulf the other phases and escalate it into a larger three-phase arc fl ash.

The iEEE method works well for distribution equipment with close conductor spacing. however, where there is larger spacing between the phases, such as on overhead transmission and distribution lines, it is less likely that a single-phase arc fl ash could escalate into a three phase event.

At the present time, there are limited calculation methods available for a single-phase arc fl ash or for systems operating above 15 kV. commercially available software using proprietary physics-based models is available for these special conditions.

NO CALCULATIONS?

As an alternative to performing arc fl ash calculations, the nEsc also allows the use of tables based on:
“The effective arc rating of the clothing or a clothing system to be worn at voltages 1000 V and above shall be
determined using Tables 410-1 and 410-2 or performing an arc hazard analysis.”

NESC table 410-1 and 410-2 list the arc ratings of Fr clothing based on various levels of short circuit current and protective device clearing times. if the available short circuit current and clearing times are known at a given location, the user can look up the minimum clothing rating from the tables.

HELP! WHERE DO I BEGIN?

Remember one word—Backwards.
Begin by determining what answer you would like to see! yes, you read this correctly. the easiest way to begin the study is to determine what level of Fr clothing you think might be reasonable for workers to wear. Often this is clothing rated 8 cal/cm2 or something similar. Once this has been determined, then begin analyzing the smallest substation with the simplest protection scheme so you can get started more easily and gain confi dence in your efforts. you can begin by using the nEsc tables to see if the rating you selected will work based on the short circuit current and clearing time.

After this fi rst step has been completed, move on and begin performing actual calculations to analyze other scenarios. these could include cases such as what happens when a transformer is out of service, a line is open, or if the arc fl ash occurs further out on the line resulting in a lower short circuit and longer device clearing times. Once you have fi nished the fi rst substation you can move on to other locations.

Resources such as [url='http://www.ArcFlashForum.com']www.ArcFlashForum.com[/url] provide a place where people share information about how to perform the arc fl ash study. this can help you move in the proper direction and get in front of the arcflash assessment requirement instead of watching it fade in the rear view mirror.

_________________
Jim Phillips, P.E.
Brainfiller.com


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PostPosted: Mon Feb 20, 2012 10:31 am 
Sparks Level

Joined: Sun Sep 23, 2007 5:00 pm
Posts: 122
Location: Calgary, Alberta, Canada
brainfiller wrote:
An arc flash can occur when energized conductors touch each other or when one conductor makes contact with a grounded surface such as an equipment enclosure. The result can be a potentially dangerous and deadly arc that ionizes the air producing an explosion.

Whoosh! What was that? it went fl ying right on by some time ago. Whether you caught it, missed it or ignored it, now it is far behind us in our rear view mirror fading well off into the distance. What was it? it’s the national Electrical safety codes’ (nEsc) January 1, 2009 deadline for arc fl ash assessments.

According to the 2007 edition of the NESC:

“Article 410.A.3 Effective as of January 1, 2009, the employer shall ensure that an assessment is performed to determine potential exposure to an electric arc for employees who work on or near energized parts or equipment. If the assessment determines a potential employee exposure greater than 2 cal/cm2 exists, the employer shall require employees to wear clothing or a clothing system that has an effective arc rating not less than the anticipated level of arc energy.”

Perform an assessment? arc rated clothing? Exposure greater than 2 cal/cm2? January 1? that was so long ago! What does this all mean?

ARC FLASH

Let’s start by taking a look at the problem. an arc fl ash can occur when energized conductors touch each other or when one conductor makes contact with a grounded surface such as an equipment enclosure. the result can be a potentially dangerous and deadly arc that ionizes the air producing an explosion. the worker can be engulfed by a large fi reball; blast pressure can reach thousands of pounds and cause blunt force trauma injury; pieces of exploding equipment can be launched as shrapnel; and the ultraviolet light that is the “fl ash” can cause eye injury and blindness. Each year thousands of people are admitted to hospitals for arc fl ash related injuries. Many of them will ultimately die.

INCIDENT ENERGY

The severity of an arc fl ash is defi ned in terms of its incidetnt (thermal) energy, which is measured as calories/centimeter2 (cal/cm2). this is the amount of energy that can reach the worker standing a specific distance away from the arc, known as the“working distance.” typically, the working distance is 18 to 36 inches, depending on the task being performed as well as the type of equipment being worked on. however, equipment such as hot sticks can greatly increase the working distance and reduce the amount of incident energy that reaches the worker.

The amount of incident energy from an arc fl ash is based on many variables. however, it is the magnitude of short circuit current and the protective device clearing time that are the two most important components. the short circuit current is directly related to how much energy per time is released and the protective device clearing time defi nes how long the duration of the arc fl ash will be.

PROTECTIVE CLOTHING
It is widely accepted that just 1.2 cal/cm2 can produce the onset of a second degree burn, and this value is used as the benchmark for personal protection. according to the nEsc, anytime the potential incident energy exposure to a worker can exceed 2 cal/cm2, protective clothing or a clothing system must be worn that has an arc rating equal to or greater than the available incident energy.

The arc rating refers to the maximum incident energy the garment can withstand while providing a thermal barrier to limit the amount of energy that can reach the worker to no more than 1.2 cal/cm2. Protecting a worker from the arc fl ash usually requires wearing clothing made from fl ame resistant (Fr) fabric as well as using additional personal protective equipment (PPE) such as a hard hat. Fr clothing comes in various arc ratings such as 4 cal/cm2, 8 cal/cm2, 25 cal/cm2, 40 cal/cm2.

THE ARC FlASH ASSESSMENT

Determining how much incident energy could be available at a specifi c piece of equipment or location on a line is something you do not want to discover from a fi eld test (accidental or intentional). this means the only alternative is to predict it from calculations. the most commonly used calculation method is based on iEEE 1584 iEEE Guide for Performing arc Flash hazard calculations. the iEEE equations were empirically derived from tests and are valid for systems operating from 208 volts up through 15 kV with short circuit currents ranging from 700a up through 106ka.

The iEEE equations are based on the incident energy from a three-phase arc fl ash. Even though the majority of arc fl ash events may begin with contact from one phase to ground, it is assumed the conducting plasma that is produced will quickly engulf the other phases and escalate it into a larger three-phase arc fl ash.

The iEEE method works well for distribution equipment with close conductor spacing. however, where there is larger spacing between the phases, such as on overhead transmission and distribution lines, it is less likely that a single-phase arc fl ash could escalate into a three phase event.

At the present time, there are limited calculation methods available for a single-phase arc fl ash or for systems operating above 15 kV. commercially available software using proprietary physics-based models is available for these special conditions.

NO CALCULATIONS?

As an alternative to performing arc fl ash calculations, the nEsc also allows the use of tables based on:
“The effective arc rating of the clothing or a clothing system to be worn at voltages 1000 V and above shall be
determined using Tables 410-1 and 410-2 or performing an arc hazard analysis.”

NESC table 410-1 and 410-2 list the arc ratings of Fr clothing based on various levels of short circuit current and protective device clearing times. if the available short circuit current and clearing times are known at a given location, the user can look up the minimum clothing rating from the tables.

HELP! WHERE DO I BEGIN?

Remember one word—Backwards.
Begin by determining what answer you would like to see! yes, you read this correctly. the easiest way to begin the study is to determine what level of Fr clothing you think might be reasonable for workers to wear. Often this is clothing rated 8 cal/cm2 or something similar. Once this has been determined, then begin analyzing the smallest substation with the simplest protection scheme so you can get started more easily and gain confi dence in your efforts. you can begin by using the nEsc tables to see if the rating you selected will work based on the short circuit current and clearing time.

After this fi rst step has been completed, move on and begin performing actual calculations to analyze other scenarios. these could include cases such as what happens when a transformer is out of service, a line is open, or if the arc fl ash occurs further out on the line resulting in a lower short circuit and longer device clearing times. Once you have fi nished the fi rst substation you can move on to other locations.

Resources such as [url='http://www.arcflashforum.com/']www.ArcFlashForum.com[/url] provide a place where people share information about how to perform the arc fl ash study. this can help you move in the proper direction and get in front of the arcflash assessment requirement instead of watching it fade in the rear view mirror.


Jim:

The NESC only applies to Utility Companies and they should take note that the Tables in the back that provide arc flash data are wrong.

Regards;
Terry Becker


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