Hazard Analysis

Recognizing the risk to workers, NFPA70E calls for an arc flash risk assessment. For companies electing to use the Incident Energy Analysis Method, the equations of IEEE 1584-2002 Guide for Performing Arc Flash Hazard Calculations has been the predominate method and is incorporated in all major analysis software packages. Based on research and modeling by the IEEE/NFPA Collaboration (Collaboration) for Arc Flash Research, a new revision of the IEEE1584 Guide was released in 2018. The new model addressed the growing knowledge of the arc flash hazard with the addition of several variables and new equations. Along with a new model is a modified 10 step arc flash hazard analysis process that incorporates the new variables and approach for current limiting fuses.

NFPA 70E 130.3 requires that a shock risk assessment and arc flash risk assessment be performed to ensure that workers are properly safeguarded whenever they are working while exposed to electrical hazards from work on or near equipment that is not in an "electrically safe condition" (see NFPA 70E Article 120 for guidance on putting equipment in an electrically safe condition). 

This includes work required to de-energize the equipment and other equipment interactions where there exists a likelihood of injury from an exposure to the arc flash hazard exists (even if conductors are not exposed). For example, it is common practice when an electrical worker closes a switch (such as in the photo to the left) to stand to the right of the disconnect switch and close with their left hand while turning their head to the right. This has been a tacit recognition that it is possible to be exposed to the arc flash hazard (as shown in the right photo) should an arc flash occur due to equipment failure during the equipment interaction (switching).  For more information, see the paper “Exposed to the Arc Flash Hazard.” 
                   
NFPA 70E Section 130.5 Arc Flash Risk Assessment provides guidance on performing an arc flash risk assessment in order to:

• identify arc flash hazards
• estimate the likelihood of occurrence of injury and the potential severity of injury 
• determine if additional protective measures are required, including the use of PPE 

To assist in estimating the likelihood of injury, Table 130.5(C) provides estimates of the likelihood of occurrence of an arc flash event for various tasks.

When using the Incident Energy Analysis Method, Section 130.5 requires the determination of:

• The incident energy at the expected working distance. Article 130.5(G) identifies the working distance as the distance from a prospective arc source to the worker's chest and face areas. 
• The arc flash boundary.  This boundary is defined in Article 130.5(E)(1) as the distance at which the incident energy equals 1.2 cal/cm2. With incident energy levels higher than this value, a person could receive a second degree burn if an electrical arc flash were to occur. 130.7(C)(1) requires that the proscribed arc flash PPE be worn per the arc flash risk assessment.
• The required Arc Flash PPE (personal protective equipment), for workers within the arc flash boundary. This selection is to be based on the incident energy present at the working distance for the task.

When using the Incident Energy Analysis Method discussed in 130.5(G) the incident energy at the expected working distance is determined and can be quite useful for mitigation strategies used to minimize the risk of arc flash injury.  Likewise, the distance to the arc flash boundary can be calculated using the unique details of each power system.

When using the Incident Energy Analysis Method Table 130.5(G) provides the requirements when selecting the proper PPE. Annex H also provides additional information selecting arc-rated PPE.

NFPA 70E also offers a table method for selecting protective clothing and other PPE. But you must be sure that the parameters of your electrical system are covered by these tables, as indicated in the various footnotes of Tables 130.7(C)(15)(a), 130.7(C)(15)(b), and 130.7(C)(16).

For a discussion comparing PPE selection using the table method versus the analytical method, see "A Summary of Arc Flash Energy Calculations" by D.R. Doan and R.A. Sweigart, found in the July/August 2003 issue of IEEE Transaction on Industry Applications, or contact Mersen Technical Services.

Article 130.5(G) identifies the working distance as the distance from a prospective arc source to the worker's chest and face areas and cautions that additional protection may be in order for parts of the body (e.g. hands) that may be closer to the arc source and exposed to higher energies than that calculated for the working distance.

See Section 130.7(C) of NFPA70E for information on measures to protect workers from the other arc flash hazards in addition to incident energy protection.

The Information Note in 130.5(G) refers to Annex D for information on estimating incident energy. It is in Annex D that IEEE 1584 is identified as a method for performing incident energy analysis. Although the 2002 version of 1584 is mentioned in the Annex, it directs readers to consult the latest version of the document.

After years of testing by the Collaboration and work with the IEEE1584 working group, a major revision to Guide for Performing Arc Flash Hazard Calculations was released in November of 2018. This new version to the Guide has major revisions to the predictive model. Good information on the changes is covered in Introduction To IEEE Standard. 1584 Ieee Guide For Performing Arc-Flash Hazard Calculations- 2018 Edition [presented at the 2019 IEEE PCIC-Conference]. Some of the major changes to the model are covered below.

Significant Changes to the Model

• New Equations and Variables. A whole new set of equations have been created for calculation of arc current and incident energy. A complex calculation is used for voltages greater than 600V that requires interpolation between calculations at key voltages of 600V, 2300V and 14,400V. 
  
  The new equations require additional input for new variables including electrode configuration and enclosure size. The new CF (correction factor) for enclosure size and electrode configuration can be used to refine results for real world equipment.
• Additional variables are used for medium voltages calculations of arc current. The equations now use voltage and gap as variables and may result in lower calculations of arc current
  
  The requirement for a second Iarc calculations for Low Voltage (<600V) systems with 85% of Iarc has been replaced with a second calculation with a new calculated Iarc min. This second calculation now applies to all voltages. Clearing times in existing studies could change; incident energy calculations for MV applications might yield higher results with the second calculation.
• There is a new approach for Current Limiting Fuses. If the arcing current is greater than the current limiting threshold of the fuse protecting the circuit, the calculation of incident energy can use the ‘effective’ arc current derived from the fuse’s peak let through curves using the calculated arcing currents as the prospective fault currents.
• The correction factor (Cf) of the 2002 model is removed. To address the spread of incident energy measurements for low voltage tests, the 2002 model multiplied the calculated incident energy by 1.5 for voltages below 1kV. With a significantly greater number of test results, the 2018 model does not have this multiplier. Calculations for the VCB electrode configuration may be lower than previously calculated.
• The ‘penalty’ for ungrounded systems (including HRG systems) was removed. The K2 constant in the 2002 incident energy equation (4) of Clause 5.3 ensured that ungrounded systems had higher 3 phase incident energy calculations. Since the research by the Collaboration did not substantiate the need for this ‘penalty’ it was not included in the new model. 
• There is new guidance for Voltages 240V and below. Clause 4.3 states:
  "Sustainable arcs are possible but are less likely in three-phase systems operating at 240 V nominal or less with an available short-circuit current below 2000 A"
  
  Testing by the Collaboration and in the Mersen lab showed that arcs could be sustained with the VCBB configuration at much lower fault current levels than the VCB configuration used to develop the 2002 model.
  
  Refer to the paper “Investigation Of Factors Affecting The Sustainability Of Arcs Below 250V.” for more information on 208V arc flash hazards.

The following steps are covered in more detail in Clause 6 of IEEE 1584-2018.

Step 1: Collect System and Installation Data
The data needed for an arc flash hazard analysis is similar to that needed for a short circuit and coordination study. It is essential to model the system in detail to get a reasonable assessment of the arc flash hazard. For many facilities, this will mean collecting all the data needed to build an up-to-date one-line diagram.

For facilities with a recent short circuit study, it may mean:

• Extending the existing study to include control equipment. 
• Refining a study that omits impedances to determine worst-case short circuit currents. Worst-case arc flash energies may be achieved when lower fault currents result in considerably longer clearing times for the overcurrent protective device. 

Step 2: Determine System Modes of Operation
The IEEE 1584 guide gives examples of different modes of operation, including operation with more than one utility feed, tie breakers opened or closed, and generators running. This information is important in determining the different short circuit currents that might be available to each location for the different modes. As noted in Step 1, the highest available fault current may not yield the worst-case arc flash energy, since the worst-case energy also depends on the characteristics of the overcurrent protection devices. 

Step 3: Determine Bolted Fault Currents
Calculate the bolted fault currents from the data gathered in Step 1 and Step 2. The typical method is to enter the data into a commercially available software program that allows you to model your system and easily switch between modes of operation. Additional guidance on the calculation is given in the guide. 

Step 4 Determine gap and enclosure size based upon system voltages and classes of equipment 
Factors such as bus gap and enclosure opening size can affect arc energies and are required for IEEE 1584-2018 equations. If gap size is unavailable, the Guide provides a table with typical bus gaps and enclosure sizes for various types of equipment up to 15kV. 

Step 5: Determine the equipment electrode configuration(s) 
The electrode configuration (electrode configuration) can have a dramatic effect on the magnitude of the incident energy calculation. The 5 options are:

• VCB - Vertical Conductors in a Box
• VCBB - Vertical Conductors in a Box with a Barrier
• HCB - Horizontal Conductors in a Box
• VOA - Vertical Conductors in Open Air
• HOA - Horizontal Conductors in Open Air

Table 9 of the Guide provides additional insight into applying the test lab configurations to equipment found in the field.

Step 6: Determine the Working Distances
Typically, this is assumed to be the distance between the potential arc source and the worker's body and face. Incident energy on a worker's hands and arms would likely be higher in the event of an arcing fault because of their closer proximity to the arc source. Typical working distances for various types of equipment are suggested in Table 10 of the Guide. Note that incident energy decreases as the distance from te arc source increases.

Step 7: Calculation of Arcing Currents
The bolted fault current calculated for each point in the system represents the highest fault current expected to flow to any short circuit. In the case of an arcing fault, the current flow to the fault will be less, due to the added impedance of the arc. It's important to adequately predict these lower levels, especially if the overcurrent protective devices are significantly slower at these reduced levels—such situations have been known to provide worst-case arc fault hazards. 

Because fault current can flow to a fault location from several sources, including running motors, the total arcing current at the point of calculation must be determined (for the incident energy calculation). To properly determine the duration of the arc the portion of the arc fault current passing through each of the upstream protective devices in the circuit must also be determined.

Although not discussed in Step 7 of the Guide, a second minimum arcing current (Iarc_min)must also be calculated as above but using equation (2) in Clause 4.5 to facilitate the second incident energy calculation discussed in Step 9. 

For current limiting fuses, if the arcing current is greater than the current limiting threshold of the fuse protecting the circuit, both arc current calculations can be reduced to their effective arc current using the manufacturer’s peak let through curves. More details are provided under Clause 6.9.2 of the Guide.

Step 8: Determine the arc duration.
IEEE 1584 offers guidance on using the time-current curves of overcurrent protective devices in various scenarios. Additional guidance is provided for current limiting fuses, relay operated-breakers and other relay schemes. It is noted that commercially available software has very extensive collections of time current curves to easily facilitate these calculations.

Discussion on setting a maximum limit for the duration of an arcing event is provided. Although, it is recognized that a worker could get away from an arc event in 2 seconds, caution is given for situations where workers egress could be blocked.

Refer to manufacturer’s information for the clearing time of fuses that are determined to be current limiting.

Step 9: Calculate the incident energy
To account for variability in arc current (Iarc) from the test results used to create the model, the 2018 model requires that two incident energy calculations be done for all equipment; the higher of the two calculation would be used as the final incident energy calculation. The first calculation would be performed with the calculated Iarc and the corresponding clearing time from the overcurrent protective device. The second calculation would be performed with of the new calculated arc Iarc_min and the corresponding clearing time from the overcurrent protective device. Sometimes the second calculation will have such a longer clearing time that it will yield the higher calculation.

For current limiting fuses the reduced effective arc current calculated in Step 7 is used in the incident energy calculations with the manufacturer’s suggested clearing time of ½ cycle or less. Using 0.0083 seconds for Mersen’s current limiting fuses gives results consistent with test lab results.

Because of the complexity and number of manual calculations possible, software is recommended to complete this step. Most software gives you a choice of equations, with selection depending on such factors as type of equipment, voltage levels and protective devices.

Step 10: Determine the Arc Flash-Protection Boundary for All Equipment
Instead of solving for cal/cm² at a given working distance, this equation solves for a distance at which the incident heat energy density would be 1.2 cal/cm² (or 5.0 joules/cm²). Software is also recommended for this calculation, for the same reasons mentioned in Step 3 and 9.

Breaker Cubicle

Panel

Drive

Using the Analytical Method to Select PPE

Section 130.5(F) and 130.5(G) of NFPA70E – 2018 gives direction on using the incident energy method for selection of arc-rated PPE. Refer to Table 130.5(G) for proper selection of arc-rated clothing and other PPE. More guidance on selection of PPE is provided in Annex H.  

Appendix D of NFPA 70E identifies methods for estimating incident energy and identifies IEEE 1584 as an acceptable method. In IEEE Standard 1584, Guide for Performing Arc Flash Hazard Calculations, there is a systematic, ten-step approach for performing a comprehensive arc flash hazard analysis.

The process begins with a short circuit study to determine the available "bolted" fault current at each location in the system. Arcing fault currents are less than the maximum bolted fault current and need to be estimated. All relevant overcurrent protection device data must also be obtained to accurately predict the duration (clearing time) of the arc fault current. Other factors that affect arc flash energies need to be weighed as well.

A choice of formulas for calculating arc flash protection boundaries and incident energy can be found in NFPA 70E and IEEE 1584. If a worker is required to be within the arc flash boundary, then PPE must be selected for the expected incident energy calculated.

Using the Table Method to Select PPE

NFPA 70E also offers a table method for selecting protective clothing and other PPE. But you must be sure that the parameters of your electrical system are covered by these tables, as indicated in the various notes for each type of equipment and the footnotes of Tables 130.7(C)(15)(a), 130.7(C)(15)(b), and 130.7(C)(16).

For a discussion comparing PPE selection using the table method versus the analytical method, see "A Summary of Arc Flash Energy Calculations" by D.R. Doan and R.A. Sweigart, found in the July/August 2003 issue of IEEE Transaction on Industry Applications, or contact Mersen Technical Services.

Refer to the paper below for more information on the new electrode configuration on PPE strategies: