Arc flash

Arc flash

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An arc flash is a type of electrical
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

explosion
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...

that results from a low impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

connection to ground or another voltage phase in an electrical system.

Definition

An arc flash is an electrical breakdown
Electrical breakdown
The term electrical breakdown or electric breakdown has several similar but distinctly different meanings. For example, the term can apply to the failure of an electric circuit....

of the resistance of air resulting in an electric arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

which can occur where there is sufficient voltage in an electrical system and a path to ground or lower voltage. An arc flash with 1000 ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...

s or more can cause substantial damage, fire or injury. Temperatures can reach or exceed 35,000 degrees fahrenheit at the arc terminals. The massive energy released in the fault rapidly vaporizes the metal conductors involved, blasting molten metal and expanding plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

outward with extreme 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 the worker but also to nearby people.

In addition to the explosive 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
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

to ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

. Surfaces of nearby people and objects 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

In general, arc flash incidents which ignite clothing are highly improbable on systems operating at less than 208 volts phase to phase (120 V
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...

to ground) when fed by less than a 125 kVA
Volt-ampere
A volt-ampere is the unit used for the apparent power in an electrical circuit, equal to the product of root-mean-square voltage and RMS current. In direct current circuits, this product is equal to the real power in watts...

transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

, as 120 volts does not provide sufficient energy to cause an arc flash hazard. Most 480 V electrical services have sufficient capacity to cause an arc flash hazard. Medium-voltage equipment (above 600 V) is higher energy and therefore a higher potential for an arc flash hazard.

As an example of the energy released in an arc flash incident, consider 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
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

. If the fault lasts for 10 cycles at 60 Hz, the resulting energy would be 1.6 megajoules
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

. For comparison, TNT releases 2175 J/g or more when detonated (a conventional value of 4,184 J/g is used for TNT equivalent
TNT equivalent
TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...

). 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
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

, and IR
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

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 (Arc-in-a-box)
• 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
Personal protective equipment
Personal protective equipment refers to protective clothing, helmets, goggles, or other garment or equipment designed to protect the wearer's body from injury by blunt impacts, electrical hazards, heat, chemicals, and infection, for job-related occupational safety and health purposes, and in...

(PPE) 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 then one of the newest solutions is to allow the operator to stand far back from the electrical equipment by operating equipment remotely.

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 (PPE). The materials are tested for their arc rating. The arc rating is the maximum incident energy resistance demonstrated by a material prior to breakopen
Breakopen
In electric arc testing, breakopen is a material response evidenced by the formation of one or more holes in the material, which may allow thermal energy to pass through the material....

or at the onset of a second-degree skin burn. Arc rating is normally expressed in cal
Calorie
The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...

/cm2 (or small calories of heat energy per square centimeter). The tests for determining arc rating is 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.

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
NFPA 70E
NFPA 70E, titled Standard for Electrical Safety in the Workplace, is a standard of the National Fire Protection Association . The document covers electrical safety requirements for employees. The NFPA is best known for its sponsorship of the National Electrical Code .-Purpose:NFPA 70E addresses...

. Table 130.7(C)(9)(a) lists a number of typical electrical tasks are 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 Category 3 Protective Clothing System. This Category 3 system corresponds to an ensemble of PPE that together offers protection up to 25 cal/cm2 (105 J
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

/cm2 or 1.05 MJ/m2). 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 (1.05 MJ/m2).

The second method of selecting PPE is to perform an arc flash hazard calculation to determine the available incident arc energy. IEEE 1584
IEEE 1584
IEEE Std 1584-2002 is a standard of the Institute of Electrical and Electronics Engineers that provides a method of calculating the incident energy of Arc Flash event.-Purpose:...

provides a guide to perform these calculations given that the bolted 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 fault until an arc flash is cleared, and the distance an individual is from an 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 (ZSIP).

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 relay
Protective relay
In electrical engineering, a protective relay is a complex electromechanical apparatus, often with more than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault is detected...

s equipped with 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 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". 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 are high risk for arc flash incidents, such as inserting draw-out circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...

s on a live electrical bus. Remote racking system
Remote racking system
A remote racking system is a system that allows an operator to operate a racking system from a remote location. It offers a safe alternative to manually racking circuit breakers, which reduces the requirement for service personnel to wear a full-body arc flash hazard suit for...

s are available which keep the operator outside the arc flash hazard zone.

Research

Both the Institute of Electrical and Electronics Engineers (IEEE) and the National Fire Protection Association (NFPA) have joined forces on 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.

Loss prevention program

An AFLP - Arc Flash Loss Prevention program is designed to help businesses and organizations comply with Occupational Safety and Health Administration
Occupational Safety and Health Administration
The United States Occupational Safety and Health Administration is an agency of the United States Department of Labor. It was created by Congress of the United States under the Occupational Safety and Health Act, signed by President Richard M. Nixon, on December 29, 1970...

(OSHA) and National Fire Protection Association
National Fire Protection Association
The National Fire Protection Association is a United States trade association that creates and maintains private, copywrited, standards and codes for usage and adoption by local governments...

(NFPA) requirements for protecting employees against arc flash injuries.

Standards

• OSHA
Occupational Safety and Health Administration
The United States Occupational Safety and Health Administration is an agency of the United States Department of Labor. It was created by Congress of the United States under the Occupational Safety and Health Act, signed by President Richard M. Nixon, on December 29, 1970...

Standards 29-CFR, Part 1910. Occupational Safety and Health Standards. 1910 sub part S (electrical) Standard number 1910.333 specifically addresses Standards for Work Practices and references NFPA 70E.
• The National Fire Protection Association
National Fire Protection Association
The National Fire Protection Association is a United States trade association that creates and maintains private, copywrited, standards and codes for usage and adoption by local governments...

(NFPA) Standard 70 - 2011 “The National Electrical Code” (NEC) contains requirements for warning labels. See NEC Article 110.16.
• NFPA 70E
NFPA 70E
NFPA 70E, titled Standard for Electrical Safety in the Workplace, is a standard of the National Fire Protection Association . The document covers electrical safety requirements for employees. The NFPA is best known for its sponsorship of the National Electrical Code .-Purpose:NFPA 70E addresses...

2009 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
Canadian Standards Association
The Canadian Standards Association, also known as the CSA, is a not-for-profit Standards organization with the stated aim of developing standards for use in 57 different areas of specialisation...

CSA Z462
CSA Z462
CSA Z462, Workplace Electrical Safety StandardCSA Z462 is a standard of the Canadian Standards Association. CSA Z462 is based on the U.S. National Fire Protection Association standard NFPA 70E, Standard for Electrical Safety in the workplace. In fact, CSA Z462 was developed in parallel with the...

Arc Flash Standard is Canada's version of NFPA70E. Released in 2008.
• The Institute of Electronics and Electrical Engineers IEEE 1584
IEEE 1584
IEEE Std 1584-2002 is a standard of the Institute of Electrical and Electronics Engineers that provides a method of calculating the incident energy of Arc Flash event.-Purpose:...

– 2002 Guide to Performing Arc-Flash Hazard Calculations.

Arc flash hazard software exists that allows businesses to comply with the myriad of 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.