The “Other” 70E arc flash analysis table

April 29, 2014

Users of the NFPA 70 (National Electrical Code) know the organizational system all too well.

Users of the NFPA 70 (National Electrical Code) know the organizational system all too well. If the information is in the first 8 chapters it generally is applicable. Chapter 9 is a transitional chapter where only what is referenced in one of the previous 8 chapters may apply. The rest of the information in Chapter 9 and all the annexes is not mandatory and is there for engineers and inspectors to argue some minor point. If it were really important it would be in the main body of the book, right?

This is not an accurate portrayal of the reason why the organizers of this great document placed the information as they did. In the consensus process used to assemble and provide advise on the subject some information is deemed to be more generally accepted or have greater importance than other information and is placed in the main part of the book. Other information that has been proposed or submitted that is either new, does not have a lot of history behind it, is felt to be of lesser application or is merely used as substantiation or clarification of the code requirement is relegated to the later part of the book. Although I do not disagree with this format it causes those less diligent in their research to miss very crucial information. 

The NEC becomes a legal document when adopted by a local jurisdiction and this format of mandatory information in the main body and informational only in the annexes carries with it some weight of law. Two other NFPA documents are just as important for the electrical worker and their safety as the NEC yet both are entirely voluntary in their use. NFPA 70B (Recommended Practice for Equipment Maintenance) and NFPA 70E (Standard for Electrical Safety in the Workplace) as suggested in earlier articles are equally important. Most electrical workers think of the 70B and 70E as they do the NEC, mandatory in the body of the work with discretionary use of the annexes.  Thinking of these very applicable and relevant voluntary documents as mandatory is not a bad thing. After all, what else is there to use that has such wide acceptance by both the regulatory agencies and the workforce as a whole? But in doing so the format of mandatory and discretionary application may miss some very good information in providing worker protection from electrical hazards.

Analyzing for potential arc flash energy and choosing appropriate worker protection is not yet an exact science and generally follows one of two paths. The first and more accurate means is to do an incident energy analysis, which involves obtaining accurate fault current and tripping time data of the system in question. This means results in a calculated cal/cm2 value of expected arc energy upon which PPE can be chosen. This means is totally dependent on maintaining the electrical system to guides such as NETA and NFPA 70B to have any relevance beyond a theoretical number.  The IEEE has an arc flash task force working diligently to better understand and clarify this method. Yet it does not take into consideration the risk of the task to be preformed and treats all tasks as equal. The lack of the risk component in the analysis has made this method less acceptable to the field workers who counter with, “Are you kidding me? There’s a door (or whatever is there) in the way!”

In parallel with this method the NFPA 70E technical committee has assembled a matrix of work tasks based on their extensive and collective experience found in common electrical systems.  In the 2012 NFPA 70E these are tables 130.7(C)(15)(a) for AC systems and table 130.7 (C)(15)(b) fro DC storage batteries. This method is less accurate and oft times results in an analysis that appears a bit over conservative. The main advantage of this method is its simplicity. Workers understand system characteristics like voltage and equipment and like the fact it takes into consideration their perceived abilities. This is expressed in the inclusion of risk-based factors in the overall analysis. These same workers have to be cautioned that the tables were based on maximum fault current and clearing times as listed in the tables. A bit more research on their part is required to ensure these values are not exceeded.

In the 2000 NFPA 70E an annex was added which followed what most large industries with diverse electrical systems were doing. A simplified 2-category system was put in place that categorized all work in one of two categories. Hazard risk category 2 included all tasks from the previous matrix tables up to category 2. This was done as many companies were transitioning everyday work wear for their electrical workers to be of at least a 8 cal/cm2 value. Added to the everyday work wear a worker would then add head, face, and hand protection to complete the HRC 2 ensemble when needed. All other work was categorized as HRC 4 and arc blast suits were then supplied. This resulted in a more acceptable means of protection both to the worker and the management tasked with their protection.

As a way to better clarify when these tables can be applicable, two new tables were added in Informative Annex H of the 2012 NFPA 70E. Table H.4(a). Table H.4(a) is for Low-Voltage systems up to 690 volts and Table H.4(b) for High-Voltage systems up to 15-kV. The voltages seem a bit odd for US use ranging from 690, 600, 480, 400 and 208. This was due to the submitter of these tables having successful use in their international facilities where these are very common voltages. These tables give a wider range of clarity when using the 2-category simplified protection scheme yet do not include a risk-based task analysis most field workers desire. An employer must label their equipment, or in some other way communicate to their employees when certain tasks are acceptable when using these tables.

The bottom line is workers must be protected from both the electrical hazards and themselves. It is the employer’s responsibility to identify all hazards in their workplace, train the workers about the existence of that hazard and how to appropriately interact with it and provide them protection while they are doing assigned work where that hazard may exist. Protection is supplied in 3 ways. First an employer may establish a policy on how and when that hazard may be encountered. A statement in their electrical safety program stating “NO energized repair work” is a common example. If any encounter with the electrical hazard is expected workers must be given the road map, a procedure to follow and while performing the tasks outlined in the procedure appropriate PPE use is directed. 

By following the NFPA format used in the NEC many have missed the substantive and useful information in the back of the book. Employers have many effective tools to use today when attempting to provide their worker’s protection from the electrical hazards. All of these methods are still a best guess at what may occur. Training workers to have fewer encounters with energized circuit conductors and parts is still the most effective way to move towards greater protection. There remains only one way to provide workers with up to 100% protection from the electrical shock and arc flash hazards.  Provide direction in your policies to turn the circuits off!    

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