Is it Sufficient to Use NFPA 70E Tables in Place of an Arc Flash Hazard Study?

Under all circumstances, it is necessary to protect the working personal from dangers associated with live electricity installations. For this reason and to ensure compliance with the OSHA regulations, Arc flash hazard and electric shock hazards need to be evaluated and studied for every facility having an electrical installation.

An arc flash hazard can either be estimated using lookup tables given in the NFPA 70E guidelines or it can be calculated with much more accuracy by a calculation procedure outlined in the IEEE Standard 1584.

What is an Arc Flash Hazard, and Why does it Occur?

Arc Flash hazard is the danger of excessive heat exposure and serious burn injury due to arching faults in electrical power systems [1].

 

Electric arcs produce intense heat, sound blast and pressure waves. They have extremely high temperatures, radiate intense heat, can ignite cloths and cause severe burns that can be fatal. These arc flashovers occur due to dust on the conductor surface, condensation of vapor, corrosion of equipment parts and even accidental contact with live conductors.

The need for continuous power is expected from a utility company and demanded by the customers. There is a need to perform electrical and maintenance work on exposed live parts of electrical equipment. It is for this reason that an arc flash study is critical for the protection of all working personnel within the facility.

What are the NFPA 70E Tables?

The 2012 NFPA 70E Article 130 gives tables that specify:

• The safe distances or the arc flash boundaries that are to be followed when working on systems that have a voltage greater than 50 Volts.
• The personal protective equipment or PPEs that must be worn when working on systems that have a voltage greater than 50 Volts.

The tables also give safe working distance values for three distance classifications within the arc flash boundary.

Limited Approach Boundary: An approach limit at a distance from an exposed energized electrical conductor or circuit part within which a shock hazard exists.

Restricted Approach Boundary: An approach limit at a distance from an exposed energized electrical conductor or circuit part within which there is an increased risk of shock, due to electrical arc-over combined with inadvertent movement, for personnel working in close proximity to the energized electrical conductor or circuit part.

Prohibited Approach Boundary: An approach limit at a distance from an exposed energized electrical conductor or circuit part within which work is considered the same as making contact with the electrical conductor or circuit part.

Personal protective equipment that are to be worn while performing different types of work on live electrical systems are also specified according to:

• Hazard risk levels ranging from 0 to 4.
• Estimated incident energy levels in cal/cm2.

When the NFPA 70E Tables or the Arc Flash Hazard Study is Required?

Whenever any maintenance related or otherwise work has to be done on or near a live electrical installation that may not be brought into a safe working state for a number reasons, two things must be determined to ensure safety of employees: [1]

• The approach limit at a distance from the live electrical installation within which a person could receive a second degree burn if an electrical arc flash were to occur this is also called an arc flash boundary.
• The personal protective equipment that must be worn and the work procedure adopted while working in this arc flash boundary.

Both of these may be determined by:

• An arc flash hazard / Incident energy analysis study or
• NFPA 70E guideline tables

The NFPA 70E standard permits the use of tables for the determination of the above mentioned parameters in lieu of an arc flash hazard or incident energy analysis study.

The NFPA 70E table method can be applied to estimate the arc flash hazard in the following steps:

Step 1: Gather Required Information and Identify the Equipment

Identify the task that needs to be performed and the equipment on which the task has to be performed. Gather the data associated with that facility e.g. Up to date one line diagrams with short circuit current ratings. If the one line is not updated or short circuit current is not given it must be evaluated.

Step 2: Find the task to be performed in the NFPA Tables

Look for the identified task in the NFPA 70E table 130.7(C)(15)(a). If the desired task is not listed then the table may not be used.

Step 3: Identify the Hazard Risk Category

Once the task is found in the table determine the hazard risk category from the same table and determine if voltage rated gloves are required or not.

Step 4: Designate the PPEs According to Risk Category

Use the NFPA 70E table 130.7(C)(16) to look for the appropriate PPEs for the hazard risk category identified earlier.

Step 5: Evaluate the Arc Flash Boundaries

Determine the estimated approach boundaries from NFPA 70E table 130.4(C)(b)

Limitations of the NFPA 70E Table Method

Testing NFPA 70E tables is usually the easiest, fastest and most straight forward method to enforce electrical safety standards, however there are some limitations to it [2]:

NFPA 70E tables do not guarantee the safety of the workers since the approach boundaries and PPEs are at best rough estimates.

 

• NFPA 70E tables are based on the short circuit ratings determined for the electrical installation. If short circuit values are not available or system configuration has changed, they will have to be calculated first.

• NFPA 70E tables base their recommendations on a standard MCCB type 0 model for 600V or less systems with a standard trip time of 0.03 seconds for one test case and 0.33 seconds for another case. If the trip time or operating time of breaker changes, which is most likely if the breaker type changes, tables may not be reliable.
• They also assume standard values for arc gap and working distances in these cases. If any of this changes, tables will not give a reliable recommendation.
• For systems rated 1kV or above, their recommendations are also based on standard values, if a variation in these standard values and breaker type is likely to occur, tables may not be relied on.
• Use of tables may subject the workers to overprotection and they may feel uncomfortable by extra PPEs.
• If the desired task to be performed is not listed in tables then table cannot be used and a complete hazard analysis is required.
• System one line diagram should be updated and include all recent changes.
• All system voltage levels may not be covered.

For the reasons stated above NFPA 70E article 130.4 and 130.5 recommend that an arc flash hazard analysis and shock hazard analysis be conducted to accurately determine the arc flash boundaries and PPEs.

The Arc Flash Hazard Analysis

Some of the limitations and inadequacies of the table method can be abridged by carrying out an Arc Flash Hazard Analysis for the facility.

The IEEE Std 1584 recommends a comprehensive calculation methodology that can be used to carry out an arc flash hazard study to uniquely determine the following for each location:

• The three phase arcing fault current.
• The incident energy.
• The arc flash protection boundary.
• PPEs required.

When the above five parameters are uniquely determined by calculations they set themselves free from the limitations arising from the use of table method.

This method may either be applied using:

• Hand calculations.
• A calculation spreadsheet, itself supplied by the standard.
• Integrated system analysis using computer aided tools.

IEEE 1584 Method for Arc Flash Hazard Analysis

The IEEE 1584 method of arc flash hazard analysis dictates that an arc flash study must be done in association with or follows in continuation of a short circuit analysis study and then a relay coordination study as detailed below:

Benefits of Performing an Arc Flash Hazard Analysis

An arc flash study by way of IEEE 1584 method and associated calculations, if done in place of using NFPA 70E tables will bring the following benefits for the facility:

An arc flash study guarantees the safety of workers because PPEs and approach boundaries are carefully determined by accurate calculations.

 

• Separate PPEs will be determined for each task and each installation so that workers are saved from the hassle of overprotecting themselves.
• A documented arc flash study will help the facility to save on the insurance costs.
• A documented arc flash study will make the facility compliant with OSHA codes and NFPA standards.
• Incident energies are calculated considering the individual time-current characteristic of each protective device as opposed to a single protective device model in the NFPA 70E tables resulting in more accurate values.
• Systems of greater complexity and size can be solved, which have multiple voltage levels, tie breakers and parallel sources.
• Allows greater diversity in protective devices.
• Short circuit studies and protective coordination study may be done with the arc flash study as a single entity to save costs.
• Ensures superior level of accuracy of flash protection boundaries.

So, it is evident from the above discussion that although the NFPA 70E tables are a considerable first option, only the Arc Flash Hazard Calculation is able to accurately ascertain the arc flash protection boundaries and guarantee the safety the workers.

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