An arc flash, which is distinctly different from the arc blast, is part of an arc fault,
a type of electrical explosion that results from a low-impedance connection to ground
or another voltage phase in an electrical system.
Definition
An arc flash is the light and heat produced from an electric arc supplied with sufficient
electrical energy to cause substantial damage, harm, fire, 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 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, 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
shrapnel, temperatures far greater than the Sun's surface, and intense, high-energy radiation
capable of vaporizing nearby materials. Arc flash temperatures can reach or exceed
35,000 °F 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. 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.
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.
Examples One of the most common examples of an arc
flash occurs when an incandescent light bulb burns out. When the filament breaks, an arc
is sustained across the filament, enveloping it in plasma with a bright, blue flash. Most
household lightbulbs have a built-in fuse, to prevent a sustained arc-flash from forming
and blowing fuses in the circuit panel. Most 480 V electrical services have sufficient
capacity to cause an arc flash hazard. Medium-voltage equipment 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 defense 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 downline 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. Thus, this fault energy is
equivalent to 380 grams of TNT. The character of an arc flash blast is quite different from
a chemical explosion, 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. There are four different arc flash type events
to be assessed when designing safety programs: Open Air Arc Flashes
Ejected Arc Flashes Equipment Focused Arc Flashes
Tracking Arc Flashes Protecting personnel
There are many methods of protecting personnel from arc flash hazards. This can include personnel
wearing arc flash personal protective equipment or modifying the design and configuration
of electrical equipment. The best way to remove the hazards of an arc flash is to de-energize
electrical equipment when interacting with it, however de-energizing electrical equipment
is in and of itself an arc flash hazard. In this case, one of the newest solutions is
to allow the operator to stand far back from the electrical equipment by operating equipment
remotely, this is called remote racking. Arc flash protection equipment
With recent increased awareness of the dangers of arc flash, there have been many companies
that offer arc flash personal protective equipment. The materials are tested for their arc rating.
The arc rating is the maximum incident energy resistance demonstrated by a material prior
to breakopen or necessary to pass through and cause with 50% probability a second or
third degree burn. Arc rating is normally expressed in cal/cm². The tests for determining
arc rating are defined in ASTM F1506 Standard Performance Specification for Flame Resistant
Textile Materials for Wearing Apparel for Use by Electrical Workers Exposed to Momentary
Electric Arc and Related Thermal Hazards. Among the best fabrics for protection against
electric arc flash are the Modacrylic-cotton blends.
Selection of appropriate PPE, given a certain task to be performed, is normally handled
in one of two possible ways. The first method is to consult a hazard category classification
table, like that found in NFPA 70E. Table 130.7(C)(15)(a) lists a number of typical
electrical tasks by various voltage levels and recommends the category of PPE that should
be worn. For example when working on 600 V switchgear and performing a removal of bolted
covers to expose bare, energized parts, the table recommends a Category 3 Protective Clothing
System. This Category 3 system corresponds to an ensemble of PPE that together offers
protection up to 25 cal/cm². The minimum rating of PPE necessary for any category is
the maximum available energy for that category. For example, a Category 3 arc-flash hazard
requires PPE rated for no less than 25 cal/cm². The second method of selecting PPE is to perform
an arc flash hazard calculation to determine the available incident arc energy. IEEE 1584
provides a guide to perform these calculations given that the maximum fault current, duration
of faults, and other general equipment information is known. Once the incident energy is calculated
the appropriate ensemble of PPE that offers protection greater than the energy available
can be selected. PPE provides protection after an arc flash
incident has occurred and should be viewed as the last line of protection. Reducing the
frequency and severity of incidents should be the first option and this can be achieved
through a complete arc flash hazard assessment and through the application of technology
such as high-resistance grounding which has been proven to reduce the frequency and severity
of incidents. Reducing hazard by design
Three key factors determine the intensity of an arc flash on personnel. These factors
are the quantity of fault current available in a system, the time until an arc flash fault
is cleared, and the distance an individual is from a fault arc. Various design and equipment
configuration choices can be made to affect these factors and in turn reduce the arc flash
hazard. Fault current
Fault current can be limited by using current limiting devices such as grounding resistors
or fuses. If the fault current is limited to 5 amperes or less, then many ground faults
self-extinguish and do not propagate into phase-to-phase faults.
Arcing time Arcing time can be reduced by temporarily
setting upstream protective devices to lower setpoints during maintenance periods, or by
employing zone-selective interlocking protection. With zone-selective interlocking, a downstream
breaker that detects a fault communicates with an upstream breaker to delay its instantaneous
tripping function. In this way "selectivity" will be preserved, in other words faults in
the circuit are cleared by the breaker nearest to the fault, minimizing the effect on the
entire system. A fault on a branch circuit will be detected by all breakers upstream
of the fault. The circuit breaker closest to the downstream fault will send a restraining
signal to prevent upstream breakers from tripping instantaneously. The presence of the fault
will nevertheless activate the preset trip delay timer(s) of the upstream circuit breaker(s);
this will allow an upstream circuit breaker to interrupt the fault, if still necessary
after the preset time has elapsed. The ZSIP system allows faster instantaneous trip settings
to be used, without loss of selectivity. The faster trip times reduce the total energy
in an arc fault discharge. Arcing time can significantly be reduced by
protection based on detection of arc-flash light. Optical detection is often combined
with overcurrent information. Light and current based protection can be set up with dedicated
arc-flash protective relays, or by using normal protective relays equipped with an add-on
arc-flash option. The most efficient means to reduce arcing
time is to use an arc eliminator that will extinguish the arc within a few milliseconds.
Distance The radiant energy released by an electric
arc is capable of permanently injuring or killing a human being at distances of up to
20 feet. The distance from an arc flash source within which an unprotected person has a 50%
chance of receiving a second degree burn is referred to as the "flash protection boundary".
The incident energy of 1.2 cal/cm^2 on a bare skin was selected in solving the equation
for the arc flash boundary in IEEE 1584. The IEEE 1584 arc flash boundary equations can
also be used to calculate the arc flash boundaries with boundary energy other than 1.2 cal/cm^2
such as onset to 2nd degree burn energy. Those conducting flash hazard analyses must consider
this boundary, and then must determine what PPE should be worn within the flash protection
boundary. Remote operators or robots can be used to perform activities that have a high
risk for arc flash incidents, such as inserting draw-out circuit breakers on a live electrical
bus. Remote racking systems are available which keep the operator outside the arc flash
hazard zone. Research
Both the Institute of Electrical and Electronics Engineers and the National Fire Protection
Association have joined forces in an initiative to fund and support research and testing to
increase the understanding of arc flash. The results of this collaborative project will
provide information that will be used to improve electrical safety standards, predict the hazards
associated with arcing faults and accompanying arc blasts, and provide practical safeguards
for employees in the workplace. Standards
OSHA Standards 29 CFR, Parts 1910 and 1926. Occupational Safety and Health Standards.
Part 1910, subpart S §§ 1910.332 through 1910.335 contain generally applicable requirements
for safety-related work practices. On April 11, 2014, OSHA adopted revised standards for
electric power generation, transmission, and distribution work at part 1910, § 1910.269
and part 1926, subpart V, which contain requirements for arc flash protection and guidelines for
assessing arc-flash hazards, making reasonable estimates of incident heat energy from electric
arcs, and selecting appropriate protective equipment. All of these OSHA standards reference
NFPA 70E. The National Fire Protection Association Standard
70 - 2011 “The National Electrical Code” contains requirements for warning labels.
See NEC Article 110.16. NFPA 70E 2012 provides guidance on implementing
appropriate work practices that are required to safeguard workers from injury while working
on or near exposed electrical conductors or circuit parts that could become energized.
The Canadian Standards Association's CSA Z462 Arc Flash Standard is Canada's version of
NFPA70E. Released in 2008. The Underwriters Laboratories of Canada’s
Standard on Electric Utility Workplace Electrical Safety for Generation, Transmission, and Distribution
CAN/ULC_S801 The Institute of Electronics and Electrical
Engineers IEEE 1584 – 2002 Guide to Performing Arc-Flash Hazard Calculations.
Arc flash hazard software exists that allows businesses to comply with the myriad government
regulations while providing their workforce with an optimally safe environment. Many software
companies now offer arc flash hazard solutions. Few power services companies calculate safe
flash boundaries. References
External links Information, Statistic and Video Resource
on Arc Flash Fast Facts on Arc Flash.
Arc Flash Resource Center Arc Flash Awareness video available on YouTube
or from NIOSH Free Online Arc Flash Calculator and Label
Maker. How to implement NFPA 70E to reduce Arc Flash
hazards. How to choose a vendor to complete an Arc
Flash Analysis in your facility. Video of 46 cycle, 10,000 A Staged Arc Flash.
myElectrical online Arc Flash Calculator Free Arc Flash Calculator
Arc Flash Guide