1) What causes an arc flash? An arc faults happens when electric current flows through air gaps between conductors. Insulation failure and accidents caused by touching a test probe to the wrong surface or slipped tool are the most common causes of an arcing fault. The fault current magnetic fields make conductors to separate producing an arc. In other words, arc flash is caused by uncontrolled conduction of electrical current from phase to ground, phase to neutral, and/or phase to phase accompanied by ionization of the surrounding air. Because of the expansive vaporization of conductive metal, a line-to-line or line-to-ground arcing fault can escalate into a three phase arcing fault in less than a 1/1000 of a second. The heat energy and intense light at the point of the arc is called arc flash.
Short circuits and arc faults are extremely dangerous and potentially fatal to personnel. The product of arc fault current and voltage concentrated in one place, results in enormous energy released in several forms. Arc fault generates large amounts of heat that can severely burn human skin and set clothing on fire. Temperatures at the arc can reach four times the temperature of the sun's surface. The high arc temperature vaporizes the conductors in an explosive change in state from solid to vapor. Copper vapor expands to 67,000 times the volume of solid copper. Conductive vapors help sustain the arc and the duration of the arc is primarily determined by the time it takes for overcurrent protective devices to open the circuit. For example, fast acting fuses may open the circuit in 8 ms or faster while other devices may take much longer to operate and open. Metal is blasted and splattered from the fault location. The arcing faults also produce large shock waves that can blow personnel off their feet. The other exposure risks to arcing faults include flying debris, severe sound waves, shock hazard due to touching energized conductors etc.
2) What are the arc flash hazards? Short circuits and arc faults are very dangerous and potentially fatal to personnel. Exposure to an arc flash frequently results in a variety of serious injuries such as severe burns, damaged eyesight, ruptured eardrums, collapsed lungs and in some cases fatal death. Electricians have been injured even when 10 or more feet from the arc location. Equipment can be destroyed causing extensive downtime and requiring expensive replacement and repair. Product of arc fault current and voltage concentrated in one place can be enormous, in some cases resulting in tremendous energy released in several forms.
Arc fault generates large amounts of heat that can badly burn human skin and ignite clothing more than 10 feet away. Temperatures at the arc can reach as high as 20000 degree Celsius or approximate four times the temperature of the Sun's surface. Even curable burns can result in tissue injury that can take months to heal. The high arc temperature vaporizes the conductors in an explosive change in state from solid to vapor. Tools, meters, nuts, bolts in the path of an arc blast may well become projectiles. The arc blast often causes equipment to explode ejecting parts, insulating materials, and supporting structures with life threatening force. The explosive expansion of the surrounding air can create pressure waves able to blow personnel off their feet. Total force on a person standing in front of an open enclosure can well exceed 1,000 lbs. Such forces crush a person's chest breaking bones, puncturing lungs and other organs. Force like this can even propel workers into walls, windows and other equipment exposing to shock hazard due to touching energized conductors and causing additional trauma. Workers can also be injured by falling from ladders or other unstable and non-secure positions. Even if the blast is not powerfull enough to propell the worker, arc fault testings indicate that severe head tossing often occurs. These cause whiplash type injuries with the possibility of brain damage. Intense ultraviolet (UV) light created by arc flash can damage the retina in the eye, cause blurred vision, burning sensations, severe headaches. If proper eye protection is not worn, ejected materials and flying particles can come in contact with the eye and cause further damage. Hot vapours can injure lungs and impair breathing. Hearing can also be lost or damaged by the loud noises and extreme pressure changes created by arc blasts. Test data has shown arc blasts to exceed 140 dB equal to an airplane taking off. Sudden pressure changes exceeding 720 lbs/ft2 for 400 milliseconds can also rupture eardrums. Psychological effects such as depression, job apprehension, family tensions should also be considered.
3) Should I be concerned about arc flash? The primary reason to be concerned about arc flash is for the safety of the personnel. Short circuits and arc faults are very dangerous and potentially fatal to personneL. Exposure to an arc flash frequently results in a variety of serious injuries such as severe burns, damaged eyesight, ruptured eardrums, collapsed lungs, psychological trauma and in some cases - death. Arc flash hazard analysis is required to determine the risk to personnel, warn them of the hazards, and to instruct them as to what kind of personal protective equipment they must wear.
The number two reason to be concerned about arc flash hazards is liability and government regulations. In the United States, OSHA regulations apply to every worker that may approach or be exposed to energized electrical equipment. Failure for an employer to conform and follow OSHA and NEC requirements can lead to employee injuries, fines, penalties, and expensive law suits. There are several regulations that address arc flash hazards in US. They are:
* National Fire Protection Association (NFPA) Standard 70 known as The National Electric Code. The NEC 2002 addresses the arc flash hazard in Article 110.16 NFPA 70B 2002 Recommended Practice for Electrical Equipment Maintenance.
* NFPA 70E 2000 Standard for Electrical Safety Requirements for Employee Workplaces
* OSHA Standards 29-CFR, Part 1910 Sub part S (electrical) Standard number 1910.333.
In Canada, arc flash is addressed legislatively at both the provincial and federal levels [ more...]. . All provincial occupational health and safety acts have a general duty clause requiring employers to take reasonable precautions to ensure their employees' health and safety. Federally, as of 31 March 2004, Bill C-45 established a duty under the Criminal Code of Canada for employers, managers and supervisors to ensure workplace health and safety. Under the code as amended by Bill C-45, there is no specific limit on fines against a corporation that's found guilty, and individual representatives of a corporation can receive a maximum sentence of life imprisonment if convicted of criminal negligence causing death. The Canadian Standards Association (CSA) does not currently have a standard equivalent to NFPA 70E. Nonetheless, when hearing a case, the court decides on the standard of care by asking what a reasonable person with same background or expertise would have done in the same circumstances. In the absence of such a document, many companies in Canada adhere to the NFPA 70E standard. Also, this standard for electrical safety protection is being considered for adoption in Canada. Adopting the safe work practices found in NFPA 70E is certainly a reasonable measure for an employer to follow to protect his employees.
4) What is Incident Energy? The incident energy is a measure of thermal energy at a working distance from an arc fault. The unit of incident energy is cal/cm2. The working distance is the distance from where the worker stands to the flash location. The most common distance for which incident energy has been determined in tests is 18 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. Dr. Ralph Lee developed formulas for calculating incident energy and determining approach boundaries. Incident energy is inversely proportional to the working distance squared. It is directly proportional to the time duration of the arc and to the available bolted fault current. NFPA 70E 2000 includes Dr. Lee�s calculation formulas, hazards risk assessment, standards for PPE, and similar information. The threshold value of incident energy for 2nd degree burn of human skin is about 1.2 cal/cm2 (5 Joules/cm2). One cal/cm2 is equivalent to the amount of energy produced by a cigarette lighter in one second. It is assumed that a second-degree burn will be curable and will not result in death.
After determining the incident energy, the value can be used to select the appropriate personal protective equipment. There are various types of PPE with distinct levels of thermal protection capabilities termed Arc Thermal Performance Exposure Values (ATPV) rated in cal/cm2.
5) What are the shock approach and flash protection boundaries? NFPA 70E has developed requirements to reduce the risk of injury to workers due to shock and arc flash hazards. There are three shock approach boundaries (limited, restricted and prohibited) required to be observed in NFPA 70E 2000. The limited, restricted and prohibited approach boundaries are based on the voltage of the energized equipment. Also, NFPA 70E 2000 requires that before a worker approaches exposed electric conductors or circuit parts that have not been placed in a safe work condition, a flash hazard assessment must be performed. Until equipment is placed in a safe work condition (NFPA 70E 2000 Part II 2-1.1.3), it is considered live. It is important to note that conductors and equipment are considered live when checking for voltage while putting equipment in a safe work condition. The flash hazard analysis should determine the flash protection boundary (FPB) and level of personal protective equipment (PPE) that the worker must wear. The flash protection boundary is based on voltage, the available fault current and the time it takes for the upstream protective device to operate and clear the fault. The boundaries are summarized below:
Limited Approach Boundary
NFPA 70 defines Limited Approach Boundary as: A shock protection boundary to be crossed by only qualified persons (at a distance from a live part) which is not to be crossed by unqualified persons unless escorted by a qualified person. The limited approach boundary is the minimum distance from the energized item where unqualified personnel may safely stand. No untrained personnel may approach any closer to the energized item than this boundary. The boundary is determined by NFPA 70E Table 2-1.3.4 and is based on the voltage of the equipment (2000 edition). A qualified person must use the appropriate PPE and be trained to perform the required work to cross the limited approach boundary and enter the limited space.
Restricted Approach Boundary
A shock protection boundary to be crossed by only qualified persons (at a distance from a live part) which, due to its proximity to a shock hazard, requires the use of shock protection techniques and equipment when crossed. To cross the Restricted Approach Boundary into the Restricted Space, the qualified person, who has completed required training, must wear appropriate personal protective equipment (PPE). Also, he must have a written approved plan for the work that they will perform and plan the work to keep all parts of the body out of the Prohibited Space. This boundary is determined from NFPA Table 2-1.3.4 (2000 Edition) and is based on the voltage of the equipment.
Prohibited Approach Boundary
A shock protection boundary to be crossed by only qualified persons (at a distance from a live part) which, when crossed by a body part or object, requires the same protection as if direct contact is made with a live part. Only qualified personnel wearing appropriate personal protective equipment (PPE), having specified training to work on energized conductors or components, and a documented plan justifying the need to perform this work may cross the boundary and enter the Prohibited Space. Therefore, personnel must obtain a risk assessment before the prohibited boundary is crossed. This boundary is determined by NFPA 70E Table 2-1.3.4 (2000 Addition) and is based upon the voltage of the equipment.
Flash Protection Boundary (FPB)
The FPB is a safe approach distance from energized equipment or parts. NFPA 70E establishes the default flash protection boundary at 4 feet for low voltage ( < 600V ) systems where the total fault exposure is less than 5000 amperes-seconds (fault current in amperes multiplied by the upstream device clearing time in seconds. NFPA 70E also allows the FPB to be calculated. In some instances, calculations may decrease the boundary distance. Persons crossing into the flash protection boundary are required to wear the appropriate PPE as determined by calculating methods contained in NFPA 70E. In addition, a qualified person must accompany unqualified persons. The boundary is defined as the distance at which the worker is exposed to 1.2 cal/cm2 for 0.1 second. IEEE Std 1584 - 2002 details the procedure and needed equations for arc flash calculations. The equations are used to calculate the incident energy and flash boundary. The IEEE procedure is valid for voltages ranging from 208V volts to 15kV with gap ranges between 3 mm. and 153 mm.
6) What is PPE?
Personnel can be protected from some of the shock and flash hazards by wearing personal protective equipment (PPE). Until equipment is placed in a safe work condition per NFPA 70E 2000 Part II 2-1.1.3, it is considered live. It is important to note that conductors and equipment are considered live when checking for voltage while putting equipment in a safe work condition. PPE includes clothing, gloves, tools, face protection, and glasses. The purpose of PPE is to prevent burns to the body that could cause death. The head and chest areas are the most critical and must be protected. Although burns on the person's limbs are serious, they usually are not likely to cause death. Different types of clothing have different ratings. For example, gloves have a voltage rating to protect from electrical shock. Cotton and fire retardant (FR) clothing all have thermal ratings. Table below is a PPE Rating Table. This is used to determine the minimum rating of Personal Protective Equipment in calories per centimeter squared, with the intent to protect the worker from the thermal effects of the arc flash. It has the personal protective equipment (PPE) requirements divided into six risk categories.
7) Arc blast pressureElectric arcs produce some of the highest temperatures known to occur on earth - up to 35000 degrees Fahrenheit, 19500 degree Celsius. This temperature is four times higher than the temperature on the surface of the Sun. The intense heat from an arc causes sudden expansion of air resulting in a blast. Copper expands during an arc flash event at a factor of 67000 times within a few milliseconds.
Because the chest and face area for most workers consists of nearly half square meter of surface area, the worker could easily be blown off his feets or somersaulted during intense arc flash explosion. As an example, 50 kA arc would provide enough pressure to propel a person standing 2 feet away from the arc source and weighting 75kg (170 lbs) with acceleration of approx 100 meters (330 feet) per second.
The arc flash blast energy or pressure is not currently addressed in IEEE 1584 or NFPA 70E. Ralph Lee's IEEE paper Pressures Developed by Arcs addresses arc blast phenomenon and provides the formula for calculating initial impulse force:
P = 11.58 x Iarc / D0.9
where,
P is pressure in lbs/ft2
Iarc is arcing current in kA
D is distance from arc in feet
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