The demand for continuous supply of power has brought about the need for elec electr tric ical al work worker erss to perf perfor orm m main mainte tena nanc ncee work work on expo expose sed d live live part partss of electrical equipment. Besides the existence of electrical shock hazard that results from direct contact of live conductors with body parts, there also exists a possibility of of electric arcs arcs striking across across live conductors. conductors. We will first take overview of arc flash hazards and briefly describes the various cause causes, s, natur nature, e, result results, s, standa standards rds and and proce procedur dures es assoc associat iated ed with with arc flash flash hazards. n order to deal with the hazard, it is first necessary to develop an understanding of the phenomena.
!n electric arc or an arcing fault is a flashover of electric "urrent through air in electrical equipment from one exposed live conductor to another or to ground.
#lectric arcs produce intense heat, sound blast and pressure waves. They have extremely high temperatures, radiate intense heat, can ignite clothes and cause severe burns that can be fatal.
#$#"T%"!$ !%"& #lectrical arcing signifies the passage of current through what has previously been air. t 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. 'oltage drop in a large solid or stranded conductor is of the order of (.()*+(.( '-cm, very much lower than the voltage drop in an arc, which can be of the order of the order of +)( '-cm of arc length for virtually all arcs in open air. /or low voltage circuits, the arc length consumes a substantial portion of the available voltage. /or high voltages, the arc lengths can be considerably greater, before the system impedance tries to regulate or limit the fault current.
The length of arc in high voltage systems can be greater and readily bridge the gap from energized parts to ground. 0nder some circumstances, it is possible to generate a higher energy arc from a low voltage system, as compared with a high voltage system.
!rc as a 1eat &ource The electrical arc is recognized as high2level heat source. The temperatures at the metal terminals are high, reliably reported to be 3(,((( 4 5,(((6/7. The special types of arcs can reach (,((( 4 5about 8(,(((6/7. The only higher temperature source known on earth is the laser, which can produce )((,((( 4. The intermediate 5plasma7 part of the arc, that is, the portion away from the terminals, is reported as having a temperature of ),((( 4. n a bolted three2phase fault, there is no arc, so little heat will be generated. f 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. "onsequently, less voltage is available to overcome supply impedance and the total current decreases. 1uman body can exist only in a narrow temperature range that is close to normal blood temperature, around 89.96/. &tudies show that at skin temperature as low as ::6" 5))(6/7, the
body temperature equilibrium starts breaking down in about * hours. "ell damage can occur beyond * hours. !t );6/, only a )2second duration is required to cause total cell destruction.
!rcing
T># >?T?@ &T0A#& ?f necessity and for the continuity of processes, maintenance of electrical equipment in energized state has to be allowed for. f all maintenance work could be carried out in deenergized state, short circuits cannot occur and therefore there is no risk of arc flash hazard. /or 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.
This reaction time is too large for a worker to move away and shelter himself from an arc flash hazard situation.
!%" B$!&T !s opposed to arc flash, which is associated with thermal hazard and burns, arc blast is associated with extreme pressure and rapid pressure buildup. "onsider a person positioned directly in front of an event and high pressure impinging upon his chest and close to the heart and the hazard associated with it. The reports of the consequences of arc in air include descriptions of the rearward propulsion of personnel who were close to the arc. n many cases, the affected people do not remember being propelled away from the arc. The heat and molten metal droplet emanation from the arc can cause serious burns to the nearby personnel. ! substance requires a different amount of physical space when it changes state, say from solid to vaporized particles. When the liquid copper evaporates, it expands *9,((( times. This accounts for the expulsion of vaporized droplets of molten metal from an arc, which is propelled up to distance of )( ft. t also generates plasma 5ionized vapor7 outward from the arc for distances proportional to the arc power. ?ne cubic inch of copper vaporizes into ;.; cubic feet of vapor.
The air in the arc stream expands in warming up from the ambient temperature to that of an arc, about 3(,((( 4. This heating is related to the generation of thunder by passage of lightning current through it. n documented instances a motor terminal box exploded as a result of force created by the pressure build2up, parts flying across the room.
Arc Blast Pressure
n one case, with approximately )((2k! fault level and arc current of :3 k!, on a :;(2' system, an electrician was thrown 3 feet away from the arc. Being forced away from the arc reduces the electriciansD exposure to the heat radiation and molten copper, but can subCect the worker to falls or impact inCuries. The approximate initial impulse force at 3: inches was
calculated to be approximately 3*( lb-ft 3 as determined from the equation below. Pressure =
11.58 * I arc
5.))7
D0.9
where,
The hot air vapor from the arc starts to cool immediately however, it combines with the oxygen of the air, thus becoming the oxide of the metal of the arc. These continue to cool and solidify, and become minute particles in the air, appearing as black smoke for copper and iron and gray smoke for aluminum. These are still hot and cling to any surface these touch, actually melting into many insulating surfaces that these may contact.
The oxide particles are very difflcult to remove because surface rubbing is not effective. !brasive cleaning is necessary on plastic insulation. ! new surface varnish should be applied, or surface current leakage could occur and cause failure within days.
>!H>0> A0%!T?@ ?/ !@ !%" /$!&1 #'#@T !@A !%" /$!&1 B?0@A!%I ! maximum duration of 3 seconds for the total fault clearance time of an arc flash event is considered, though, in some cases, the fault clearance time can be higher t is stated that= Jif the time is longer than 3 seconds, consider how long a person is likely to remain in the location of the arc2flash. t is likely that a person exposed to arc flash will move
away quickly, if it is physically possible and 3 seconds is a reasonable maximum time for calculations. ! person in a bucket truck or a person who has crawled into equipment may need more time to move away.K
Causes of Electric Arcs Llow to arc discharge= M Aust and impurities= Aust and impurities on insulating surfaces can provide a
path for current, allowing it to flashover and create arc discharge across the surface. This can develop into greater arcs. /umes or vapor of chemicals can reduce the breakdown voltage of air and cause arc flash. M "orrosion= "orrosion of equipment parts can provide impurities on insulating surfaces. "orrosion also weakens the contact between conductor terminals, increasing the contact resistance through oxidation or other corrosive contamination. 1eat is generated on the contacts and sparks may be produced, this can lead to arcing faults with nearby exposed conductors of different phase or ground.
Causes of Electric Arcs "ondensation of vapor and water dripping can cause tracking on the surface of insulating materials. This can create a flashover to ground and potential escalation to phase to phase arcing.
Causes of Electric Arcs &park discharge= !ccidental touching= !ccidental contact with live exposed parts can initiate arc faults.
M Aropping tools= !ccidental dropping of tools may cause momentary short circuit, produce sparks and initiate arcs. ?ver2voltages across narrow gaps= When air gap between conductors of different phases is very narrow 5due to poor workmanship or damage of insulating materials7, arcs may strike across during over2voltages. /ailure of insulating materials #lectric arcs are also caused by the following= mproperly designed or utilized equipment. mproper work procedures
The Nature of Electrical Arcs #lectric arcs produce some of the highest temperatures known to occur on earth + up to ,((( degrees /ahrenheit. This is four times the surface temperature of the sun. The intense heat from arc causes the sudden expansion of air. This results in a blast with very strong air pressure 5$ightning is a natural arc7. !ll known materials are vaporized at this temperature. When materials vaporize they expand in volume 5"opper + *9,((( times, Water+)*9( times:7. The air blast can spread molten metal to great distances with force. /or a low voltage system 5:;(-399 '7, a to :2inch arc can become JstabilizedK and persist for an extended period of time.
#nergy released is a function of system voltage, fault current magnitude and fault duration. !rcs in enclosures, such as a >otor "ontrol "enter 5>""7 or switchgear, magnify blast and energy transmitted as the blast is forced to the open side of the enclosure and toward the worker.
Hazards of Arcing Faults
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1eat= Fatal burns can occur when the victim is several feet from the arc. &erious burns are common at a distance of )( feet *. &taged tests have shown temperatures greater than :9 °/ on the neck area and hands for a person standing close to an arc blast9.
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?bCects= !rcs spray droplets of molten metal at high2speed pressure. Blast shrapnel can penetrate the body.
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"lothing can be ignited several feet away. "lothed areas can be burned more severely than exposed skin.
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1earing loss from sound blast. The sound can have a magnitude as high as ):( dB at a distance of 3 feet from the arc
The exposure to arc flash depends on the following= •
@umber of times the workers work on exposed live equipment.
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"omplexity of the task performed, need to use force, available space and safety margins, reach, etc.
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Training, skills, mental and physical agility, coordination with helper.
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Tools used.
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"ondition of equipment.
! category : <<# outfit looks like a space suit with face hood, eye shields, cover, and gloves. t restricts the mobility of a worker to perform delicate tasks, for example, maintenance work on terminals and wiring. Thus, not only an accurate calculation of incident energy level, but its reduction in the planning and design stage and selection of appropriate protection and relaying of electrical systems are gaining importance. &uppose that an ; cal- cm3 protection is required this can be achieved by= N ;2cal-cm3 arc2rated pants and shirts N :2cal-cm3 treated pants and shirts and : cal-cm3 arc2rated overall N ;2cal-cm3 arc2rated overall cotton shirts and pants. t is the total level of arc2rated protection that matters
The boundaries are deflned in @/
0nqualified persons, that is, those not specifically trained to carry out the required tasks, are safe when they stay away from the energized part of a certain distance, which is the limited approach boundary. They should not cross the limited approach boundary and arc flash boundary unless escorted by a qualified person. "rossing the restricted approach boundary means that special shock prevention techniques and equipment are required, and an unqualified person is not allowed to cross this boundary. /inally, the prohibited approach boundary establishes the space that can be crossed only,
The limited approach, restricted approach, and prohibited approach boundaries are all defined based upon the system voltage. @o calculations are required for establishing these boundaries. Working Aistance Working distance is defined as the closest distance to a workerDs body excluding hands and arms. ### );: Luide E8F specifles required working distances 5Table ).*7. /or the )2k' switchgear, it is * in, while for a :;(2' >"", it is ); in. ! larger working distance reduces the incident energy and therefore the 1%". The working distance does exclude hands and arms, which will be much closer to the seat of arc. t is the vital organs like eyes, chest, and heart that are at the working distance from the seat of the arc. This assumes that a worker does not stick his head inside the switchgear doorQ !rc /lash $abels The labeling on the equipment contains the following data= voltage
N N N N N N N
<<# category incident energy release in cal-cm3 working distance restricted approach boundary prohibited approach boundary equipment identification the protective device identification that clears the fault.
The labels can be generated on a variety of media, including plastic weatherproof laminates, and most commercial arc flash analysis program will allow custom designing the labels. ! user can choose what goes on the label, including the description of <<#. #ven the type fonts can be user selectable.
Hazard Assessment Methods
availability of resources and quality of arc flash hazard mitigation program. ). Hazard Risk Category Classifications in NFPA 70!"000 provides a simple way to determine the hazard category. This method requires the least time and is suitable when limited information is available on the power system. This is the least accurate method because it is very generalized. These tables do not provide you with the exact <<# rating that are required in cal-cm 3. 3. Hand calculations# Iou can perform hand2calculations using @/
This method is limited to radial single source systems and errors increase with the size of the system. :. Co%%ercial integrated soft&are= This is practical for all systems with multiple power sources and multiple scenarios of interconnections where better accuracy is desired and where the system goes through ongoing changes over time. ?nce the data is entered into the software, carrying out hazard assessment takes very little time. The results are instantly observed.
Whatever method is used, the qualified person performing the assessment should be aware of the limitations of the method
!chieve best results.