A physics-based approach
Any company that generates, transmits, distributes, or uses electricity at high, medium, or even low voltages will find it difficult to completely protect its workers from arc flash and arc blast hazards. The task is even more difficult for users of direct-current systems that cannot be de-energized. The biggest obstacle to protection is the inability to predict the type and magnitude of arc flash effects in a way that would allow the optimal use of personal protective equipment (PPE) and work practices.
The need to quantify arc flash and blast has led to a call for a physics-based approach capable of accurately describing the phenomena. Given their complexity, any description would likely involve an algorithm containing many equations. A key goal of the IEEE-NFPA joint research and testing initiative, which is just getting under way, is to gather data on all of the expressions of energy during arc flash and arc blast events. This data then could be used to help solve the equations. The data also could serve as the foundation of practical safeguards against arc flash and blast for those who work on or near electrical equipment.
The IEEE/NFPA Arc Flash Collaborative Research Project is a multiyear effort encompassing more than 2,000 test protocols. So far, over $1.8 million of the $6.5 million needed to fund the program has been contributed by power companies, manufacturers, and test and certification organizations. Major donors to date include HydroOne, Bruce Power, Underwriters Laboratories, and Ferraz Shawmut.
The initial phase of the project will explore published and unpublished information on arc flash and blast to build a coherent picture of what is known about these phenomena. This will lead to a research and test plan that seeks to tie the electrical characteristics of equipment to arc-fault hazards. The program will go well beyond what was done in the past. For instance, most arc fault studies to date have involved controlled conditions and stabilized arcs between opposing electrodes. In real life, turbulent arcs often occur between parallel electrodes and vary by several orders of magnitude along their length and with time.
The extreme temperatures in arcing faults make burns the predominant injury they cause. As a result, the program will focus on thermal mechanisms. Testing will build on current methods for measuring how arcing faults transfer thermal energy via convection, conduction, and radiation and how these processes affect human tissue and clothing. This includes understanding how much of the electromagnetic spectrum released in arc faults falls into IR wavelengths, as well as evaluating the thermal energy conducted through PPE when a plasma cloud passes over a face shield or clothing. This can involve a significant amount of heat because copper vapor, which boils at 4,500F, makes up much of this cloud.
The program will survey existing protocols for measuring the thermal effects of arcing faults on a human body and, when needed, create new protocols that encompass such variables as arcing time, working distance, frequency, electrode gap, voltage, current, and the number of phases. It will refine existing test methods and instrumentation, such as measuring heat flux and relating it to burns and upgrading calorimeter specifications.
In addition, it will review a NIOSH electrical injury database to identify measures that could have limited injury and create a template for obtaining future electrical incident information. The program also will investigate low-current (1 kA to 10 kA), long-duration arcs, because the energy transfer in such events is often so great that available PPE cannot protect against it.
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Comments (1)
It may be over conservative, but is this the intent of the definition? There is concern for ventilated switchgear in particular because the path of the arc flash cannot be accurately predicted.