insulator under humid conditions. Though online monitoring and partial discharge measurements are being applied as diagnostic tools, the randomness associated with a fault and insulation breakdown are well recognized, and a breakdown can occur at any time, jeopardizing the safety of a worker, who may be in close proximity of the energized equipment. Arc temperatures are of the order of 35,000°F, about four times the temperature on the surface of the sun. An arc flash can therefore cause serious fatal burns.
Figure 1.1. Treeing phenomena in nonself-restoring insulation, leading to ultimate breakdown of insulation.
1.1 ELECTRICAL ARCS
Electrical arcing signifies the passage of current through what has previously been air. It is initiated by flashover or introduction of some conductive material. The current passage is through ionized air and the vapor of the arc terminal material, which has substantially higher resistance than the solid material. This creates a voltage drop in the arc depending upon the arc length and system voltage. The current path is resistive in nature, yielding unity power factor. Voltage drop in a large solid or stranded conductor is of the order of 0.016–0.033 V/cm, very much lower than the voltage drop in an arc, which can be of the order of the order of 5–10 V/cm of arc length for virtually all arcs in open air (Chapter 3). For low voltage circuits, the arc length consumes a substantial portion of the available voltage. For high voltages, the arc lengths can be considerably greater, before the system impedance tries to regulate or limit the fault current. The arc voltage drop and the source voltage drop are in quadrature. The length of arc in high voltage systems can be greater and readily bridge the gap from energized parts to ground.
Under some circumstances, it is possible to generate a higher energy arc from a low voltage system, as compared with a high voltage system.
In a bolted three-phase short circuit, the arcing resistance is zero, and there is no arcing, and no arc flash hazard. Sometimes, when short circuit occurs, it can be converted into a three-phase bolted short circuit by closing a making switch or circuit breaker, which solidly connects the three-phases. The fault current is then interrupted by appropriate relaying. This method, however, will subject the system to much greater short-circuit stresses and equipment damage, and, is, therefore, not recommended.
1.1.1 Arc as a Heat Source
The electrical arc is recognized as high-level heat source. The temperatures at the metal terminals are high, reliably reported to be 20,000 K (35,000°F). The special types of arcs can reach 50,000 K (about 90,000°F). The only higher temperature source known on earth is the laser, which can produce 100,000 K. The intermediate (plasma) part of the arc, that is, the portion away from the terminals, is reported as having a temperature of 13,000 K.
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
