In a bolted three-phase fault, there is no arc, so little heat will be generated. If there is some resistance at the fault point, temperature could rise to the melting and boiling point of the metal, and an arc could be started. The longer the arc becomes, the more of the system voltage it consumes. Consequently, less voltage is available to overcome supply impedance and the total current decreases.
Human body can exist only in a narrow temperature range that is close to normal blood temperature, around 97.7°F. Studies show that at skin temperature as low as 44°C (110°F), the body temperature equilibrium starts breaking down in about 6 hours. Cell damage can occur beyond 6 hours. At 158°F, only a 1-second duration is required to cause total cell destruction.
1.1.2 Arcing Phenomena in a Cubicle
The arc formation in a cubicle may be described in four phases:
Phase 1: Compression. The volume of air is overheated due to release of energy, and the remaining volume of air inside the cubicle heats up due to convection and radiation.
Phase 2: Expansion. A piece of equipment may blow apart to create an opening through which superheated air begins to escape. The pressure reaches its maximum value and then decreases with the release of hot air and arc products.
Phase 3: Emission. The arcing continues and the superheated air is forced out with almost constant overpressure.
Figure 1.2. The various stages of pressure buildup and its release for an arc in a cubicle. A: Compression, pressure rises; B: Expansion, relief of pressure; C: Emission, gases exhausted; D: Thermal, pressure equalizes (not to scale).
Phase 4: Thermal. After the release of air, the temperature inside the switchgear nears that of an electrical arc. This lasts till the arc is quenched. All metals and insulating materials undergo erosion, may melt and expand many times, produce toxic fumes, and spray of molten metal.
Figure 1.2 shows these four phases.
1.2 ARC FLASH HAZARD AND PERSONAL SAFETY
The phenomenal progress made by the electrical and electronic industry since Thomas Edison propounded the principle of incandescent lighting in 1897 has sometimes been achieved at the cost of loss of human lives and disabilities. Although reference to electrical safety can be found as early as about 1888, it was only in 1982 that Ralph Lee [11] correlated arc flash and body burns with short-circuit currents. This article is considered by many as pioneering work on arcing phenomena in the open air. It quantified the potential burn hazards. Lee established the curable burn threshold for the human body as 1.2 cal/cm2, which is currently used to define the arc flash boundary. Lee published a second article in 1987, “Pressure Developed from Arcs” [12].
Doughty et al., published two articles [13, 14], and Jones et al. published an article in 2000 [15]. The IEEE 1584 Guide can be considered a breakthrough for arc flash analyses. The previous methods in NFPA 70E were based upon theoretical concepts or drawn from limited testing. The new testing concentrated on arcing faults in a variety of electrical equipment enclosures, arcs in boxes, which is more typical of actual work locations. Yet some researchers are critical of the methodology of the IEEE 1584 Guide; for example, Stokes and Sweeting in “Electrical Arc Burn Hazards” [5], critique Lee’s models and IEEE 1584 Guide equations and testing setup for arc flash burns. Yet the statistics collected on the prevention of arc flash hazard injuries shows that such injuries were prevented when the workers used the required personal protective equipment (PPE) calculated according to the IEEE Guide; see Chapter 3. Wilkins et al. published an article, “Effect of Insulating Barriers in Arc Flash Testing,” in 2008 [16]. The authors used vertical conductors terminated in insulating barriers for their testing methodology. See Chapter 3 for further discussions and observations on these issues.
The OSHA definition of a recordable injury, TRIR, for 1 year of exposure, is as follows:
(1.1)
Most insurance companies accept this parameter of definition because there is a cost associated with these incidents.
1.3 TIME MOTION STUDIES
Of necessity and for the continuity of processes, maintenance of electrical equipment in energized state has to be allowed for. If all maintenance work could be carried out in deenergized state, short circuits cannot occur and therefore there is no risk of arc flash hazard. For the continuous process plants, where the shutdown of a process can result in colossal amount of loss, downtime and restarting; it becomes necessary to maintain the equipment in the energized state. Prior to the institution of arc flash standards, this has been carried out for many years, jeopardizing worker safety, and there are documented cases of injuries including fatal burns.
The time/motion studies show that human reaction time to sense, judge, and run away from a hazardous situation varies from person-to-person. A typical time is of the order of 0.4 second. This means that 24 cycles is the shortest time in which a person can view a condition and begin to move or act. In all other conditions, it is not possible to see a hazardous situation and move away from it. As will be further demonstrated, this reaction time is too large for a worker to move away and shelter himself from an arc flash hazard situation.
1.4 ARC FLASH HAZARDS
Apart from thermal burns, an arcing phenomenon is associated with other hazards too, namely:
electrical shock
molten metal
projectiles
blast and pressure waves
intense light
intense sound
fire
effect of strong magnetic fields and plasma, of which not much is known
toxic gases and vapors.
Thus, thermal burns due to arc flash are only a part the picture for overall worker safety. Figure F.1a,b in NFPA 70E [17], not reproduced here, provides hazard risk analysis procedure flowchart. It implies that each establishment must perform a number of tasks and establish training and safety procedures that should be implemented for workers’ safety. The numbers of injuries from arc flash accidents are high (see Chapter 2). IEEE 1584 Guide documents many such cases.
This book is confined to the analysis of arc flash thermal damage and calculation of arc flash boundary, subsequently defined, according to IEEE 1584 Guide equations. The book concentrates on the various design, planning, and protection strategies by which the arc flash hazard can be reduced.
1.5 ARC BLAST
As opposed to arc flash, which is associated with thermal