2 af hazard calc myths facts solutions

8/13/2019 2 AF Hazard Calc Myths Facts Solutions

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1077-2618/07/$25.002007 IEEE

MYTHS, FACTS, AND SOLUTIONS

ERFORMING AN ARC FLASH ANALYSIS FOR A NEW OR exist-

ing electrical distribution system can be a daunting task. Efforts have been

made to limit the required scope of the evaluation, but portions of a sys-

tem are often overlooked due to faulty assumptions. Overlooked areas may

have dangerous incident energy levels. The scope of the evaluation may be reduced to a

manageable size, but any assumption must be carefully scrutinized to guarantee an accu-

rate determination of the arc flash hazard. This article discusses a method for generalizing

the arc flash calculations for select areas of a power distribution system. A spe-

cific application of this general method is then presented.

Background

Defining the Scope of Arc Flash Analysis

The NFPA 70E Standard for Electrical Safety in the Workplace, 2004 edi-

tion references the equations from IEEE Standard 15842002 as a method

for quantifying the incident energy available during an arc flash event.

The equations in IEEE Standard 15842002 are applicable for a voltage

range of 20815,000 V, three-phase and for a range of bolted fault cur-

rent magnitudes between 700106,000 A. In addition, IEEE Standard

1584-2002 states that equipment below 240 V need not be considered

unless it involves at least one 125 kVA or larger low-impedance transformer in its

immediate power supply [2].

ARC FLASH

HAZARDCALCULATIONS

P

BY H . W ALLAC E T INSLEY I I I , M IC HAEL HODDER,

& A IDAN M . GRAHAM

ARTVILLE, LLC.


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Equipment locations that are likely to require exami-nation, adjustment, servicing, or maintenance whileenergized should be field marked to warn qualified per-sons of potential electric arc flash hazards [4]. Typically,such equipment includes switchboards, panelboards,industrial control panels, and motor control centers(MCCs). Other work locations such as bus ducts, split-ters, and disconnect switches should be considered if

energized parts above 50 V may be exposed by qualifiedpersonnel. (Unqualified personnel should never be per-mitted to access energized equipment.)

Building a Software Model for Arc Flash Analysis

A commercial software package is usually the most effi-cient and most accurate way to calculate the potentialincident energy throughout a power system. When con-structing a software model for arc flash analysis, it isimportant to include an accurate fault contributionmodel (utility sources and rotating machines), a com-plete impedance model (transformers, cables, and busducts), and a complete overcurrent protection model(fuses, relays, and circuit breakers).

For each location within the power system, the worst-case model will be represented by a unique operating con-figuration. What is often overlooked is that a lowermagnitude of fault current typically results in a longeroperating time of the upstream overcurrent protection,which may then result in a higher level of incident energyand a greater arc flash hazard. Therefore, the power systemincluded in the software model must be represented asaccurately as possible.

Defining the ProblemThe situation just described presents a logistical chal-lenge when attempting to determine the arc flash haz-ard for locations downstream of low-voltage

distribution equipment such as MCCs, panelboards,and bus ducts. If each of the individual circuits is repre-sented, the software model can become extremely cum-bersome and extensive. For example, a model extensiveenough to provide meaningful arc flash calculations fora bus duct requires the modeling of each individual busplug, each section of bus duct between the plugs, andthe cables from the duct to the load. Even when cur-rent-limiting devices protect these locations, there is noguarantee (without knowledge of fault levels) that thesedevices will operate in the current-limiting region ofthe tripping characteristic. Some circuits may be right-ly eliminated from the model, but these circuits cannotbe identified until the available fault current has been

determined for the distribution location (bus duct,MCC, or panelboard.)

Accurate Fault Calculations

Because the arc flash calculations are used to prescribeprotective equipment for qualified workers, great caremust be taken to ensure that the contribution model isable to represent the worst-case scenario in terms of thearc flash hazard.

The available fault currents calculated throughout apower system are determined by the source contributions

and are limited by the system impedances between thesecontributing sources and the location of a fault event.

The primary source of fault current in a typical short-circuit model is the utility or the primary generationsupplying power to a facility. When a facility is poweredby a utility source, the available fault current informa-tion is often obtained as the maximum available current.IEEE Standard 15842002 requires the consideration of

the maximum and the minimum available short-circuitcurrents [2].When constructing a model for the purpose of short-

circuit analysis and protective device coordination, thefault contribution model is often based on conservativelyhigh calculations of current magnitudes such as utilitydesign values. For example, the utility contribution maybe determined by the maximum let-through current ofthe largest (kVA) transformer with the lowest impedancethat the utility company is permitted to install. Thisinformation may provide conservative results for evalua-tion of interrupting ratings, but it does not provide com-plete or adequate results for the arc flash calculations.

Other contributing sources in a power system include

local generators and motors (rotating machines.) In the arcflash model, these contributions must be considered in theappropriate configurations to represent the maximum andminimum available fault current magnitude for each loca-tion in the system model. For example, a total maximumcontribution may be represented by the maximum utility(source) contribution and the system motors running atfull-load current prior to the simulation of the boltedfault. Similarly, the total minimum contribution may berepresented by the minimum utility or generator contri-bution with the minimum number of motors (rotatingloads) in operation. Additionally, rotating machine contri-butions, unlike the utility contribution, will decay rapidlyand may not be present for the entire duration of the arc

flash event. The short-circuit contribution of an inductionmotor should only be considered for the first three to fivecycles (0.050.0833 s) of the event duration. Should thearc flash event last for a time period greater than0.050.0833 s, the modeling software should be able totake this into consideration.

Obtained from a complete contribution model, thethree-phase bolted fault current values provide the startingpoint for accurate arc flash calculations at each systemlocation. Bolted fault current magnitudes are calculated ateach bus or node location and represent the sum total of allcontributions. In the event of a low-voltage (


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where

lg = the log10Ia = arcing current (kA)K = 0.153 for open configurations and

= 0.097 for box configurationsIbf = bolted fault current for three-phase faults

(symmetrical rms) (kA)

V = is system voltage (kV)G = is the gap between conductors (mm)

For applications with system voltages from 1,000 V upto and including 15,000 V, solve the equation

lgIa = 0.00402 + 0.983lgIbf. (2)

For system voltages greater than 1,000 V, there is nodistinction between open and box configurations. Also, forsystem voltages above 15,000 V, the arcing fault current isconsidered equal to the bolted fault current.

Table 1 shows that the calculated bolted fault currentwill be significantly reduced by the impedance of the arc

when the system voltage is less than 1,000 V.

IEEE Standard 15842002 also requires that the arcingfault magnitude be further reduced to 85% of the calcu-lated value [2]. A corresponding clearing time is selectedfor both the 85% value and the 100% value. This is donebecause it is very difficult to accurately predict the arcingcurrent, and a small change in current could result in asignificant change in clearing time.

To illustrate this fact, the time-current curve of a

typical 480-V, 200-A thermal-magnetic molded-casecircuit breaker (MCCB) is shown in Figure 1. Noticethe change in breaker clearing time when transitioningfrom the instantaneous (magnetic) portion of the curveto the long time (thermal) portion of the curve. Thisthreshold point is defined as the value of currentwhere the protective device transitions from fast toslow clearing time. For MCCBs or motor circuit pro-tectors (MCPs), the threshold point is where instanta-neous tripping may not occur. For current-limitingfuses (CLFs), the threshold point is the minimum cur-rent that will cause current-limiting action.

As illustrated in Figure 1, when the fault current isreduced to a value just below this threshold point, the

maximum clearing time is increased from roughly0.021 to 2.1 s. One can easily see that decreasing thefault current will typically increase the clearing time ofthe upstream protective device, which often results inhigher calculated incident energy values. This makesaccurate fault current calculations essential when per-forming an arc flash hazard analysis.

Instantaneous pickup point of a typical 200-A MCCB.

5 10 100 1,000 10K 100K0.01

0.10

1

10

100

1,000Current (A)

Time(s)

0.021s

2.10s

Threshold Point1

TABLE 1. SAMPLE OF BOLTED VERSUS ARCING FAULT

CURRENTS, BY VOLTAGE LEVEL.

Voltage Bolted Fault Arcing Fault % of Bolted

(kV) Current (kA) Current* (kA) Fault Current

0.208 10.00 4.35 43%

20.00 7.08 35%

30.00 9.41 31%

40.00 11.51 29%

50.00 13.47 27%

0.480 10.00 6.56 66%

20.00 11.85 59%

30.00 16.76 56%

40.00 21.43 54%

50.00 25.93 52%

0.600 10.00 7.86 79%

20.00 14.88 74%

30.00 21.62 72%

40.00 28.19 70%

50.00 34.62 69%

1.00 10.00 9.71 97%

up to 20.00 19.18 96%

15.00 30.00 28.58 95%

40.00 37.92 95%

50.00 47.22 94%

*Arcing fault current was calculated based on the equations given in IEEEStandard 15842004a. For the low-voltage calculations, a box configuration wasassumed with a gap between conductors of 25 mm; the system is consideredsolidly grounded.


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A similar characteristic is illustratedfor a typical 480-V, 200-A class J fusein Figure 2. As the fault current isreduced below the threshold point ofthe fuse, the maximum clearing timeincreases to a value above 0.01 s.

As illustrated in Figure 2, the clear-ing time and resulting incident energy

are highly dependent on the calculatedarcing fault current. Therefore, thepower system engineer must evaluatenot only the major equipment loca-tions (switchgear, switchboards, panelboards, and MCCs),but all locations downstream where energized work maybe performed; this often leads to a very extensive computermodel. To decrease the size of the required model (and theassociated amount of data collection), some creative gener-alizations can be made without reducing the accuracy ofthe resulting calculations.

Demonstration of the Problem

Old Paradigm for AssumptionsBefore considering the applicable generalizations andassumptions, it is instructive to consider the paradigm that ishistorically associated with a short-circuit analysis and a pro-tective device coordination study. This traditional approachis no longer valid when performing an arc flash analysis.

The arc flash calculations are based on several constantvalues defined by the physical parameters of the equip-ment location for which an analysis is performed. The twomost important system variables, however, will changeaccording to the system configuration, as represented inthe software model. These two variables are the calculatedfault current magnitude and the clearing time of the pro-tective device responsible for the isolation of the fault loca-

tion. When an arc flash incident occurs, the magnitude ofthe fault current flowing through the arc will determinethe clearing time of the overcurrent protection.

Prior to beginning an arc flash analysis, accuratesystem data must be gathered for incorporation intothe software model. When building a short-circuitmodel for evaluation of equipment interrupting andwithstand ratings, assumptions are easily accepted todetermine the worst-case scenario. For example, if thelength of a conductor is unknown, a conservativelyshort assumption will yield the highest fault currentduty calculation for the device to be evaluated. For arcflash analysis, however, the greatest fault current mag-nitude may provide the worst-case calculation, or it

may result in a dangerously erroneous result becausethe high magnitude of fault current will lead to theconclusion that a rapid clearing time of the upstreamovercurrent protection will occur.

For coordination purposes, the overcurrent protectionmodel may only include the larger adjustable devices asso-ciated with a given system location, since most devices200 A or less have fixed trip curves. However, for the arcflash model, every overcurrent protective device responsi-ble for the reduction of the arc flash hazardadjustable ornotmust be accurately modeled in the system.

Goals of Arc Flash Analysis

When performing an arc flash hazardanalysis, it is important to considerseveral goals of the study as well asresults. The primary goal is to deter-mine the hazard risk category (HRC)associated with all locations in the dis-tribution system where energized

work may be performed. The HRC isrequired to define the personal protec-tive equipment (PPE) that should beworn when working on or near ener-

gized electrical equipment. An arc flash hazard is quanti-fied according to the incident energy (cal/cm2) calculatedat a specific working distance for each system location.As summarized in Table 2, NFPA 70E-2004 Table

TABLE 2. HRCS.Hazard Risk Incident Energy Incident Energy

Category (HRC) from (cal/cm2) to (cal/cm2)

0 >0


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130.7(C)(11) defines five HRCs,which correspond to ranges of calcu-lated energy (cal/cm2 ) [1]. Each ofthese HRCs is associated with specificrequirements for PPE.

The second goal of an arc flashanalysis is to determine the ideal set-tings and current ratings for the over-

current protective devices in thesystem. The operating times of thephase overcurrent protective devicesare based on arcing current flow andare used to determine the HRC foreach location analyzed.

When performing an arc flash haz-ard analysis, the settings of theseovercurrent devices define the magni-tude of the downstream hazards. Forthe maximum reduction of the arcflash hazard, all devices should beconfigured to provide the most rapidclearing time. This, of course, elimi-

nates the coordination intervalrequired for selective fault clearing,and such a configuration should notbe recommended. Considering all ofthe factors involved, the power sys-tems engineer is required to deter-mine overcurrent protective devicesettings that provide overcurrent coordination andequipment protection as well as the lowest possible inci-dent energy at all work locations.

Upon the determination of the optimum settings forthe overcurrent protection and analysis of the resulting arcflash hazard at each location, the power systems engineeris able to investigate possible solutions to reduce the arc

flash hazards. These solutions may include the recommen-dation of additional or replacement overcurrent protectivedevices, the implementation of procedures to provide saferaccess to energized equipment, and/or the installation ofsolutions that either increase worker distance or decreaseprotective device clearing time.

Upon the completion of an arc flash analysis, theresults of the calculations must be clearly presented.Warning labels, applied to the outside of equipmentenclosures, are often considered to be the most effectivemeans of displaying the results of the arc flash analysisfor qualified persons. Article 110.16 of the NationalElectric Code (NEC) 2005 requires industrial and com-mercial electrical equipment to be field marked to

warn qualified persons of potential electric arc flashhazards [4]. Specific quantities, ranges, boundaries,and HRCs are not expressly required by the NEC.Many facilities have found that presenting the arc flashhazard warning and listing the HRC on a prominentlydisplayed label enables qualified persons to easily deter-mine the appropriate PPE requirements.

Current-Limiting Devices and the Arc Hazard

Current-limiting devices are very important for equip-ment protection when they operate to prevent equip-

ment damage under a high magnitudefault condition.

Current-limiting protective devices(fuses or circuit breakers) are often rec-ommended as a means of reducing thearc flash hazard. However, care shouldbe taken when such devices are consid-ered as solutions to a hazardous situa-

tion. If the arcing fault currentmagnitude will always be great enoughto cause operation in the current-limit-ing range, then rapid clearing will dra-matically reduce the potential exposureto a dangerous amount of incidentenergy. If, however, under any circum-stances, the arcing fault current is nothigh enough to drive the overcurrentdevice into the current-limiting range,such devices should not be recom-mended as a solution to an arc flashhazard. When a fast clearing time isdetermined on the basis of a calculated

arcing fault current magnitude at thephysical location of a current-limitingdevice, it is a common mistake toassume that the device will operate justas quickly when a fault occurs at a pro-tected location further downstream.Instead, the fault current flowing

through the current-limiting device will be significantlyreduced by the impedance of the cable (often very small incomparison with the ampacity of the equipment bus) sup-plying individual loads or smaller pieces of distributionequipment. To accurately determine the incident energyand corresponding HRC for the downstream locations, theimpedance of the feeder circuit must be considered.

Solution to the Problem

To reduce the overall scope of the arc flash hazard analy-sis, one must limit the required number of calculations.Below is a description of a generalization method devel-oped by the authors to reduce the effort required to pro-vide a comprehensive analysis at a specific location.Similar methods may be applied at other industrial orcommercial locations.

Making Accurate Generalizations

Using accurate generalizations, an arc flash study caninclude all equipment locations that are likely to requireexamination, adjustment, servicing, or maintenance while

energized. Using the following steps, this analysis can beperformed without the need to model every location:

Step 1. Conduct a detailed analysis from the utilityfeeder connections down to MCCs, bus ducts, split-ters, or panels (600 V or less) fed by overcurrentdevices (fuse or trip rating) that are sized at 200 A orlarger. Future enhancements of the generalizationmethod may allow an increase above 200 A. Detailedanalysis can be accomplished using a commercialsoftware package to develop a model that providesthe following for each location studied:

OBTAINED FROM

A COMPLETE

CONTRIBUTION

MODEL, THE

THREE-PHASE

BOLTED FAULT

CURRENT VALUES

PROVIDE THE

STARTING POINT

FOR ACCURATE

ARC FLASH

CALCULATIONS

AT EACH SYSTEM

LOCATION.


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" bolted fault magnitude andX/R ratio

" arcing fault magnitude" protective device clearing time

and duration of the arc flash" flash protection boundary" incident energy calculated at a

working distance (cal/cm2)"

HRC based on NFPA 70E-2004, Table 130.7(C)(11), (seeTable 2) [1].

Step 2. Extend the analysis tolocations fed from MCCs, busducts, splitters, or panels. Thiscan be accomplished by" using the results from Step 1

in a spreadsheet to calculatethe maximum cable lengthfor each feeder size fromMCCs, bus ducts, splitters,or panels,

" conducting a site survey to find all feeders that are

longer than the maximum cable length in thespreadsheet,

" collecting data for the feeders that exceed the max-imum length (including protective device andconductor data), and

" adding these feeders and locations to the modeldeveloped in Step 1.

The spreadsheet calculates a conductor size based onthe feeder trip or fuse amp rating, which is converted to aconductor size based on

80% rated protective equipment (molded casebreaker or fuse)

copper conductor, 75C insulation in magneticconduit.

From the conductor size, the spreadsheet calculatesthe maximum circuit length, which equates to animpedance. The impedance is used to calculate a faultlevel at the end of the feeder. The spreadsheet deter-mines a circuit length that will cause the HRC at theMCCs, bus ducts, splitters, or panels to increase at theend of a feeder to the next category or HRC. This isaccomplished by comparing the calculated arcing faultat the end of the feeder to the threshold point. Thethreshold point was previously defined when referringto Figures 1 and 2.

The authors designed a spreadsheet that compares thecalculated arcing fault current to the threshold point ofthe protective device at the line end of the feeder.

If the fault current at the load endof the feeder is higher than thethreshold point, then the HRC atthe load end bus location doesnot increase to the next category.

If the fault current at the load endof the feeder is lower than thethreshold point, then the HRC at

the load end bus location mayincrease to the next category andfurther analysis is required.

The spreadsheet model uses conserv-ative values for threshold point:

50 X trip rating for MCCB tripratings from 15225 A

20 X trip rating for MCCB tripratings from 250600 A

20 X trip for MCP trip ratingsfrom 31,200 A

25 X fuse amp rating for CLFfrom 15600 A.

Applying the Method at an Industrial FacilityWe will illustrate this method using an example froman arc flash hazard analysis study at an industrial loca-tion. Step 1 was performed from the utility connectionat 27.6 kV down to a total of 280 locations at 600 V orbelow (MCCs and panels sized at 200 A or larger). InStep 2, a spreadsheet was developed for these 280 loca-tions for protective device types MCCB, MCP, and CLF.The spreadsheet included calculations of the maximumcable length from each of the 280 locations out to a dis-tance where the HRC will not increase to the next cate-gory. The maximum cable length (ft) is reported on thespreadsheet for each location called the bus name, andwas provided for standard ratings of protective devices.

Table 3 shows an example of one of the rows of thespreadsheet. Note that Table 3 shows only a few of thetrip ratings included, while the actual spreadsheetincluded all of the standard trip ratings that would befound in this type of equipment. The table shows max-imum cable length for MCCBs with trip units ratedfrom 1560 A. For the bus name 200 A WIRE TR,the HRC level will not rise above HRC = 0 as long asa 20-A feeder breakers cable distance is less than 160ft. If there is a 60-A feeder breaker at 200 A WIRETR, then the downstream HRC = 0 as long as thecable distance is less than 100 ft.

A site survey was performed to identify the numberof subfeeders that exceeded the distances in the

TABLE 3. EXAMPLE OF MAXIMUM CABLE LENGTH TO REMAIN AT HRC 0.

Bus Voltage BoltedName (V) Fault

(kA) X/R HRC 15 20 30 40 50 60

200 A 600 13.18 1.798 0 140 ft 160 ft 160 ft 110 ft 130 ft 100 ft

WIRE TR

TO REDUCE THE

OVERALL SCOPE

OF THE ARC

FLASH HAZARD

ANALYSIS, ONE

MUST LIMIT THE

REQUIRED

NUMBER OF

CALCULATIONS.

Trip Rating (A)


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spreadsheet. A conservative estimate of the total num-ber of subfeeders was obtained by assuming that therewere at least five at each of the 280 locations. An esti-mated total of 1,400 (280 5) locations requiredinvestigation on site to determine if the circuit lengthexceeded the values in the spreadsheet. The results of

the site survey indicated that there were only 85 feed-ers where the circuit length was exceeded. For each ofthe 85 additional feeders, additional data gathering atthe site was required to enable the power system engi-neer to add these feeders to the commercial softwarepackage model that was created in Step 1. This addi-tional information is summarized below:

1) circuit name2) feeder protective device manufacturer3) feeder trip unit or fuse ampere rating4) feeder length5) feeder configuration, which is one of the following:" conduit (magnetic or nonmagnetic)" free air spacing.

The result of the additional analysis in the commercialsoftware package revealed that out of the 85 new subfeed-ers that were modeled, there were only 17 small-panellocations with HRC downstream that was greater than theHRC at the upstream MCC or panel location. Based onthis analysis, it was concluded that all other small-panellocations in the power system that were fed from panelsand MCCs would have the same HRC as the upstreamlocation that feeds them.

The electrical workers at this site were instructedthat there were only 17 small-panel locations where the

HRC level was higher than the location from wherethey were fed. This information was recorded in the arcflash report (see Table 4). If a small-panel locationdownstream of a panel or MCC did not have an arc flashhazard label, and the small panel was not on the list oflocations in Table 4, then it would have the same HRCrating as the panel or MCC from which it was fed.

Summary of Savings in Industrial ExampleFor the example presented in this article, applyingaccurate generalizations reduced the total number oflocations that needed to be added to the computermodel from 1,400 to 85. Thus, a reduction in thesmall-panel locations that required detailed analysiswas (1,40085/1400) 100% = 94%.

There were also savings in additional onsite datagathering and entry of information into the computermodel. Savings were also realized in the reduction ofadditional arc flash hazard warning labels and the asso-ciated cost of installing these labels. In this example,only 17 additional arc flash hazard warning labels wererequired for the small panels.

ConclusionsPerforming an arc flash hazard analysis for all equip-ment locations likely to require examination, adjust-ment, servicing, or maintenance while energized canprove to be an enormous task. Developing innovatemethods to reduce the scope of work without sacrific-ing the accuracy of the results is the goal of every engi-neer. This article presented a method that can be usedto help reduce the number of equipment locations thatmust be analyzed using a commercial software pack-age. In an industrial example, this method reduced thenumber of small feeder locations that required adetailed analysis by 94%. This resulted in a large

reduction of onsite data collection, detailed modeling,and arc flash warning labels.

AcknowledgementThe authors thank W.S. Vilcheck for his sponsorship,advice, and encouragement.

References[1] NFPA 70E-2004, Electrical Safety Requirements for Employee Workplaces ,

2004.[2] IEEE Standard 15842002, IEEE Guide for Performing Arc-Flash Haz-

ard Calculations, 2002.[3] IEEE Standard 2422001 (IEEE Buff Book), IEEE Recommended Practice

for Protection and Coordination, 2001.[4] NEC 2005 NFPA 70,National Electric Code, 2005.

[5] IEEE Standard 1412001 (IEEE Red Book), IEEE Recommended Practicefor Electrica Power Distribution for Industrial Plants, 2001.

H. Wallace Tinsley III is with Eaton Electrical in Warren-dale, Pennsylvania. Michael Hodder is with Eaton Electricalin Mississauga, Oregon. Aidan M. Graham is with EatonElectrical in Bellevue, Washington. Tinsley and Hodder are

Members of the IEEE. Graham is a Senior Member of theIEEE. This article first appeared as Arc Flash Hazard Cal-culations: Myths, Facts, and Solutions at the 2006IEEE/IAS Pulp and Paper Industry Committee Conference.

TABLE 4. SMALL-PANEL LOCATIONS WHERE HRC WAS

HIGHER THAN THE UPSTREAM LOCATION.

Label # Bus Name Protective Device Name

255 *57EP104 PD-57EP906-104

256 *57EP105 PD-57EP906-133

265 *57EP106 PD-57EP906-105

266 *57EP107 PD-57EP906-107

267 *57EP112 PD-57EP905-112

268 *57EP113 PD-57EP905-113

269 *57EP133 PD-57EP904-133

270 *LP11&LP13 66 PD7/9/11/13

271 *LP12&LP14 66 PD8/10/12/14

272 *LP7&LP9 66 PD7/9/11/13

273 *LP8&LP10 66 PD8/10/12/14

274 *PANEL-RP-3A PD-RDP3A-RP3A

275 *PANEL-RP-3B PD-RDP3A-RP3B

276 *PANEL-RP-3C PD-RDP3A-RP3C277 *RDP-3C PD-RDP3A-RDP3C

278 *RP-4A PD-RDP3A-RP4A

279 *WELDRECEPT PD-PDP12B-WELD

2 af hazard calc myths facts solutions

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