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Introduction
Flash protection beyond normal cotton or wool work clothing is not required as long as all parts of the worker’s body stay outside the flash boundary as calculated above. Outside the flash boundary the minimum recommendation is either:
1. Flame-resistant clothing with an ATPV of 4.5 cal/cm2
or higher
2. Natural fabric (cotton or wool) work clothing of 7 oz/yd2or more
If, however, the worker must cross the flash boundary and is qualified to do so, he or she must wear flame-resistant clothing to protect against the arc incident energy levels that may be encountered.
The following sections provide suggestions and direction for determining the arc energy levels to which the worker may be exposed. Note that the research into such methods is an on-going effort. The reader should always refer to the most current version of such research as found in IEEE Standard 1584 and NFPA 70E.
To provide arc protection, the worker must select and wear flame-resistant clothing with an ATPV or EBTequal to or greater than the incident energy level. Note that the incident
3.52 CHAPTER THREE
energy level is calculated based on a given distance between the arc source and the worker, for example 24 inches. The worker will not be adequately protected if he or she gets any closer than the distance used in the calculation for the specific situation.
Flash boundaries can be calculated from the formulas in this section by setting the inci- dent energy level to 1.2 cal/cm2
(5.02 J/cm2
) and then solving the appropriate equation for the distance.
Caution: The following sections present several complex calculation procedures using a variety of mathematical and engineering applications. These calculations should be per- formed or evaluated only by qualified, experienced engineers or other such technical per- sonnel with the requisite knowledge and skills.
The Lee Method
This method is named after its developer Ralph Lee, PE (deceased). Mr. Lee was one of the early, and most successful researchers into the theoretical models for the amount of energy created by an electrical arc and the amount of incident energy received by workers in the vicinity of the arc.
The Lee method is based on two major assumptions:
1. The maximum energy transfer from the power system to an electric arc is equal to one-
half of the bolted short-circuit power available at the point where the arc occurs.
2. All of the arc electrical energy will be converted to the incident heat energy.
Starting with these two assumptions, Mr. Lee and subsequent researchers have refined the Lee method to the form shown in equation 3.4
(3.4) Where E= the incident energy (J/cm2or cal/cm2depending on the value of K)
K= a constant. If K = 2.142 then E is in J/cm2. If K= 0.512 then E is in cal/cm2.
V= the system phase-to-phase voltage (kV) Ibf = the bolted fault current (kA)
t= the arcing time (seconds)
D= the distance from the arcing point to the worker (mm)
The Lee method is a very conservative method and was developed without the aid of the empirical results available today from the extensive research in this field. While it can be used for all situations, its principle application is in those cases that have not been modeled based on actual experimental measurements. It is most often used in systems where the sys- tem voltage is greater than 600 V and/or where fault currents, arc lengths, or other such parameters are beyond the ranges that have been tested using field measurements.
Although the Lee method does not take the focusing effect of equipment into account, the so-called arc-in-a-box, it is very conservative. The use of multipliers should be required only in the most extreme applications.
Methods Outlined in NFPA 70E
In addition to the LEE method, the 2004 Edition of NFPA 70E, Standard for Electrical Safety in the Workplace gives two different methods for calculating incident energy levels.
E K VI t D bf = × 2 × 6 10
SAFETY PROCEDURES AND METHODS 3.53
Method #1. This method is based on a research by Richard L. Doughty, Thomas E. Neal, and H. Landis Floyd.2
They performed research using the thermal manikin approach described in Chap. 1. Their research was bounded by the following conditions:
1. Systems with voltage levels of 600 V and below
2. Systems with maximum available short circuit currents between 16 kA and 50 kA. 3. Working distances of equal to or greater than 18 inches.
After running numerous tests by creating electric arcs in open air, they took their results and performed a curve fit to predict the incident energy. Equation 3.5 is the formula that was developed and which most closely models the energies they measured.
EMA= 5271DA−1.9593× tA(0.0016I2SC− 0.0076ISC+ 0.8938) (3.5) Where EMA= the maximum open air arc incident energy (cal/cm2
) DA= the distance from the electrodes (in)—note that DA ≥ 18 in
tA= the duration of the arc (seconds)
ISC= the short circuit current (kA)—note that ISCis the actual arc current, not the maximum bolted fault current.
In the second part of their research they created electrical arcs in a cubic box with one open side. The box was 20 inches on a side and the measurements were taken at various dis- tances from the open end. They then performed a curve fit to model the results. Equation 3.6 is the resulting formula.
EMB= 1038.7DB−1.4738× tA(0.0093I2SC− 0.3453ISC+ 5.9675) (3.6) Where EMB= the maximum arc-in-a-box incident energy (cal/cm2)
DB= the distance from the electrodes (in)—note that DB ≥ 18 in All other variables are as described for the open air formula (Eq. 3.5)
Using either Eqs. 3.5 or 3.6, incident energies can be calculated, then protective cloth- ing can be purchased with the necessary ratings.
Method #2. Method number two is actually the method taken from the IEEE Standard 1584-2002.3
This method is described in the next section.
IEEE Standard Std 1584-2002
System Model Limitations. Institute of Electrical & Electronics Engineers (IEEE) work- ing group P1584 has performed research and theory development over several years to expand the work done by Doughty, et al.2As shown in Table 3.18, the working group has
significantly expanded the applicable range for the application of their methods.
Calculation of Arcing Current. One of the principal drawbacks of the Lee method is that the bolted fault current, which is used in that method, is always somewhat greater than the actual current flow when an electrical arc is formed. At voltages below 1000 V, the arcing current is often substantially less than the bolted fault current.
Using both theoretical and empirical data, Standard 1584, develops a formula to calcu- late the arcing current that will present in a system when the bolted fault current is known. Equations 3.7, and 3.8 are used to calculate the logarithm to the base ten of arcing current
(log10Ia). Equation 3.7 is used for systems below 1 kV, and Eq. 3.8 is used for systems that
are equal to or greater than 1 kV.
log10Ia= K + 0.662(log10Ibf)+ 0.0966V + 0.000526G + 0.5588V(log10Ibf) (3.7)
−0.00304G(log10Ibf)
log10Ia= 0.00402 + 0.983(log10Ibf) (3.8)
Where Ia= the arcing current (kA)
K= a constant. K = −0.153 for open air arcs and K = −0.097 for arc-in-a-box. Ibf= the maximum, symmetrical rms, three-phase bolted short circuit current (kA)
V= the system three phase voltage (kV) G= the conductor gap (arc length) (mm)
After log10Iahas been calculated from Eqs. 3.7 or 3.8, equation 3.9 is used to calculate Ia. (3.9)
Calculating incident energy. The calculation of actual incident energy is done in two steps using the empirically derived arc current (Ia). In step one, normalized incident energy (En) is calculated. Enis normalized for an arcing time of 0.2 seconds and a distance from the arc of 610 mm. Equations 3.10 and 3.11 are used for this calculation.
log10En= k1+ k2+ 1.081(log10Ia)+ 0.0011G (3.10)
(3.11) Where En= the normalized incident energy (cal/cm2
)
k1= −0.792 for open air arcs and −0.555 for arcs-in-a-box
k2= 0 for ungrounded and high-resistance ground systems and −0.113 for grounded
systems.
G= the conductor gap (arc length) (mm)
The actual incident energy at any distance D is then calculated using Eq. 3.12. (3.12) Where E= the incident arc energy at distance D in cal/cm2
(kCJ= 1) or J/cm2
(kCJ= 4.184) kCJ= 1 for Enis cal/cm2and 4.184 for Enin J/cm2
Cf= 1 for systems above 1 kV or 1.5 for systems equal to or less than 1 kV En= the normalized incident energy calculated from equation 3.11.
t= the arcing time in seconds D= the distance from the arc (mm)
X= a distance exponent taken from Table 3.19
E k C E t D CJ f n X = 0 2. 610 En= 10(log10En) Ia= 10(log10Ia) 3.54 CHAPTER THREE
TABLE 3.18 System Limits for Calculation of Incident Energy Using IEEE Standard 1584–2002
System phase-to-phase voltage 0.208 to 15 kV
System frequency 50 to 60 Hz
Short-circuit current range 700 to 106 kA
SAFETY PROCEDURES AND METHODS 3.55
In Eq. 3.12 the arcing time (t) will be determined by how long the protective devices require to completely interrupt the short circuit. If the arcing current decreases, the arcing time will generally increase. Because of this, NFPA 70E suggests recalculating Eqs. 3.7 through 3.12 using 0.85 Iaand the resulting arcing time. The standard then suggests using the larger incident energy from the two calculations.
Software Solutions
There are several software products on the market that allow the calculation of incident energy and/or flash boundaries. For example, IEEE Standard 1584-2002 comes with a complete set of Microsoft Excel spreadsheet applications. The user only needs to enter the values for his/her par- ticular system to determine the incident energy and the flash boundary using the IEEE method.
At least one freeware software product (MSDOS based) is available for calculating the incident energy for single phase short-circuits. This product was developed by Alan Privette, PE and is available on the internet at a variety of locations. The Cadick Corporation website (http:// www.cadickcorp.com) and the Oberon Company website (http://www.oberoncompany.com) are two locations where this product may be found.
In addition to these virtually all of the commercially available engineering software packages such as SKM Systems Analysis, Inc.—PowerTools for Windows, have added arc- flash calculation packages to their short-circuit analysis and coordination study packages.
Required PPE for Crossing the Flash Hazard Boundary
To select the level of flame-resistant protection required, the following procedure may be used:
1. Calculate the incident arc energy value as shown previously: 2. Select clothing that provides an ATPV*or E
BT†that is less than the incident energy value
previously calculated. Double layers of protective clothing may be required at the higher energy levels. Refer to the manufacturers for recommendations.
TABLE 3.19 Distance Factors (X) Used in Eq. 3.19
System voltage Type of Typical conductor
(kV) equipment gap (mm) 0.208 to 1 Open-air 10–40 2.000 switchgear 32 1.473 MCCs and panelboards 25 1.641 cables 13 2.000 >1 to 5 Open-air 102 2.000 switchgear 13–102 0.973 cables 13 2.000 >5 to 15 Open-air 13–153 2.000 switchgear 153 0.973 cables 13 2.000
*Arc Thermal Performance Value—material rating provided by the manufacturer. This is the amount of heat energy that will just cause the onset of a second-degree burn (see Chap. 1).
†The average of the five highest incident energy values that did not cause the fabric to break open. Test value sup-
3.56 CHAPTER THREE
A Simplified Approach to the Selection of Protective Clothing
The National Fire Protection Association provides a simplified approach to the selection of pro- tective clothing. While this method is convenient and economical, it should be used with extreme caution. The author has found specific situations when the simplified method does not yield the most conservative values, thus putting workers at risk.
Additionally, this method assumes certain short circuit and operating time values. The user must be absolutely certain that his or her power system falls within the assumed values.
1. Identify the Hazard/Risk category from Table 3.20. Note that this is based on the type
of work that will be performed. Note also that Table 3.20 identifies whether insulating gloves and/or tools are required.
2. Use Table 3.21 to select the various types of PPE required for the hazard determined in
step 1.
3. Use Table 3.22 to select the weight of flame-resistant clothing required for the task.
Although this procedure is quite simple and straightforward, it should be used carefully for at least two reasons:
● Because it is conservative, it tends to result in substantial amounts of clothing for the
employee. Workers may tend to disregard necessary equipment out of frustration.
● The standard is task-based as opposed to location-based. Using the quantitative methods
described previously will provide, ultimately, a more easily applied set of rules.
● In some rare cases, the simplified approach may give values that do not provide adequate
protection. This is especially critical if the limits imposed by the method are ignored.