We conduct arc flash protection studies for mining industries, utilities, small scale industries, factories and establishments where there is risk of arc flash from switching operations or maintenance activities.The voltage levels can be from low voltage to HV. The studies are conducted using ETAP software. We have recently completed an Arc Flash study for Cheetham Salt Mines in Bajool, Queensland. We can do the studies for any power supply system in Brisbane, Melbourne, Sydney, Perth, Darwin or anywhere in Australia. The power system is modelled in Etap and Load flow, short circuit studies are conducted and protection settings are calculated for the switchboard/ breakers. Then the arc flash studies are conducted to ensure the incident energy and arc flash levels are within limits for the arc flash protection PPE. Protection setting levels and trip times may have to be adjusted to bring the incident energy levels to within limits. A few terms are introduced here to understand the arc flash terminology.


Arc Flash

      An arc flash is the light and heat produced from an electric arc supplied with sufficient electrical energy to cause substantial damage, harm, fire,burns or injury. Electrical arcs experience negative resistance, which causes the electrical resistance to decrease as the arc temperature increases. Therefore, as the arc develops and gets hotter the resistance drops, drawing more and more current (runaway) until some part of the system melts, trips, or evaporates, providing enough distance to break the circuit and extinguish the arc. Electrical arcs, when well controlled and fed by limited energy, produce very bright light, and are used in arc lamps(enclosed, or with open electrodes), for welding, plasma cutting, and other industrial applications. Welding arcs can easily turn steel into a liquid with an average of only 24 DC volts. When an uncontrolled arc forms at high voltages, arc flashes can produce deafening noises, supersonic concussive-forces, super-heated sharpnel, temperatures far greater than the Sun's, and intense, high-energy radiation capable of vaporizing nearby materials.

Arc flash temperatures can reach or exceed 35,000 °F (19,400 °C) at the arc terminals. The massive energy released in the fault rapidly vaporizes the metal conductors involved, blasting molten metal and expanding plasma outward with extraordinary force. A typical arc flash incident can be inconsequential but could conceivably easily produce a more severe explosion (see calculation below). The result of the violent event can cause destruction of equipment involved, fire, and injury not only to an electrical worker but also to bystanders. During the arc flash, electrical energy vaporizes the metal, which changes from solid state to gas vapor, expanding it with explosive force. For example, when copper vaporizes it suddenly expands by a factor of 67,000 times in volume.


Arc Blast
In addition to the explosive blast, called the arc blast of such a fault, destruction also arises from the intense radiant heat produced by the arc. The metal plasma arc produces tremendous amounts of light energy from far infrared to ultraviolet. Surfaces of nearby objects, including people absorb this energy and are instantly heated to vaporizing temperatures. The effects of this can be seen on adjacent walls and equipment - they are often ablated and eroded from the radiant effects.

Most 415 V electrical services have sufficient capacity to cause an arc flash hazard. Medium-voltage equipment (above 600 V) is higher potential and therefore a higher risk for an arc flash hazard. Higher voltages can cause a spark to jump, initiating an arc flash without the need for physical contact, and can sustain an arc across longer gaps. Most power-lines use voltages exceeding 1000 volts, and can be an arc-flash hazard to birds, squirrels, people, or equipment such as vehicles or ladders. Arc flashes are often witnessed from lines or transformers just before a power outage, creating bright flashes like lightning that can be seen for long distances.

High-tension power-lines often operate in the range of tens to hundreds of kilovolts. Care must usually be taken to ensure that the lines are insulated with a proper "flashover rating" and sufficiently spaced from each other, or an arc flash can spontaneously develop. If the high-tension lines become too close, either to each other or ground, a corona discharge may form between the conductors. This is typically a blue or reddish light caused by ionization of the air, accompanied by a hissing or frying sound. The corona discharge can easily lead to an arc flash, by creating a conductive pathway between the lines. This ionization can be enhanced during electrical storms, causing spontaneous arc-flashes and leading to power outages.

One of the most common causes of arc flash injuries happens when switching-on electrical circuits and, especially, tripped circuit-breakers. A tripped circuit-breaker often indicates a fault has occurred somewhere down the line from the panel. The fault must usually be isolated before switching the power on, or an arc flash can easily be generated. Small arcs usually form in switches when the contacts first touch, and can provide a place for an arc flash to develop. If the voltage is high enough, and the wires leading to the fault are large enough to allow a substantial amount of current, an arc flash can form within the panel when the switch is turned on. Generally, either an electric motor with shorted windings or a shorted power-transformer are the culprits, being capable of drawing the energy needed to sustain a dangerous arc-flash. Circuit breakers are often the primary defence against current runaway, especially if there are no secondary fuses, so if an arc flash develops in a breaker there may be nothing to stop a flash from going out of control. Once an arc flash begins in a breaker, it can quickly migrate from a single circuit to the phases of the panel itself, allowing very high energies to flow. Precautions must usually be used when switching circuit breakers, such as standing off to the side while switching to keep the body out of the way, wearing protective clothing, or turning-off equipment, circuits and panels down line prior to switching. Very large switchgear is often able to handle very high energies, and, thus, many places require the use of full protective equipment before turning it on.

As an example of the energy released in an arc flash incident, a single phase-to-phase fault on a 480 V system with 20,000 amps of fault current. The resulting power is 9.6 MW. If the fault lasts for 10 cycles at 60 Hz, the resulting energy would be 1600 kilojoules. For comparison, TNT releases 2175 J/g or more when detonated (a conventional value of 4,184 J/g is used for TNT Equivalent). Thus, this fault energy is equivalent to 380 grams (approximately 0.8 pounds) of TNT. The character of an arc flash blast is quite different from a chemical explosion (more heat and light, less mechanical shock), but the resulting devastation is comparable. The rapidly expanding superheated vapor produced by the arc can cause serious injury or damage, and the intense UV, visible, and IR light produced by the arc can temporarily and sometimes even permanently blind or cause eye damage to people.

Arc Flash Boundary

Arc Flash Boundary is an approach limit at a distance from exposed live parts within which a person could receive a second degree burn if an electric arc flash were to occur.


Incident Energy

Incident Energy is a measure of thermal energy at a working distance from an arc fault (measured in cal/cm2). The working distance is the distance from where the worker stands to the flash location (commonly18 inches). The incident energy is a function of system voltage, available short-circuit current, arc current, and the time required for circuit protective devices to open.

Hazard Risk Category Level

The Hazard Risk Category level is determined by ATPV (Arc Thermal Performance Value). ATPV is the measure (in cal/cm2) of how much heat can be exposed to a flame resistant garment before a second degree burn injury is expected to occur. HRC is based on specific job tasks and ranges from HRC 0 (which is low risk and allows for 100% untreated cotton), up to HRC 4 (which is high risk and requires Arc-Rated clothing with a minimum arc rating of 40).

Safety Glove Class

Electrical safety gloves are categorized by the level of voltage protection they provide and whether or not they're resistant to ozone. Voltage protection is broken down into six classes. Class 00 is the least protective, while class 4 provides the most protection.

                           
Shock Hazard

Shock Hazard, measured in VAC, is a dangerous electrical condition associated with the possible release of energy caused by contact or approach to energized parts.

Limited Approach Boundary

The limited approach boundary should be entered only by qualified persons or unqualified persons that have been advised and are escorted by a qualified person.

Restricted Approach Boundary

The restricted approach boundary should be entered only by qualified persons. Requires the use of shock protection techniques and PPE.

Prohibited Approach Boundary

The prohibited approach boundary should be entered only by qualified persons. Requires same protection as if in direct contact with live part.

 

 

Arc Flash Studies

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Date published: 02/17/2016
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