The importance of arc flash hazard analysis
Many employers are one fatality away from compliance with current standards. The question is “Which employee do you want to sacrifice?” Treatment costs for burn victims can approach $500,000/mo. A workplace fatality can cost an estimated $8.5 million. Cost should never be a consideration when it comes to workplace safety — especially for the hazards that electrical workers face. Electrical workers rarely return to their previous job assignment following an arc flash catastrophe — they are either killed, maimed, or their quality of life is affected significantly.
Potential electrical hazards include shock or electrocution, arc flash, and arc blast. An electrical arc occurs when insulating materials can no longer contain the applied voltage. A short circuit or insulation breakdown creates a bypass (which can be from phase-to-phase, phase-to-ground, or a combination thereof) around a circuit. The heat generated by the high current flow melts or vaporizes the conducting material, creating an arc. The resulting arc flash produces a brilliant flash, intense heat, and a fast-moving pressure wave that propels the arcing materials.
As electrical demand increases, transformers at utility and industrial levels are upgraded or replaced with those having higher kVA ratings and lower impedances. Transformers are operated in parallel to satisfy requirements for higher system reliability. These modifications can cause dramatic increases in available fault current, resulting in more electrical energy available.
The downside to these system changes is an increase in the electrical current that could feed a fault within existing equipment. This can wreak havoc on underrated or improperly maintained equipment.
Most employers and employees understand electrical shock hazard; few understand electrical arc flash hazard — let alone how to perform an analysis.
An arc flash hazard analysis is an extension of a power system analysis (see “Benefits of performing a detailed arc flash hazard analysis”). Companies used to have electrical facilities engineers that were dedicated to keeping the electrical system up-to-date. But with budget cuts and downsizing, this function has virtually vanished.
Necessity of arc flash hazard analysis
An electrical arc could happen at any time — when no one is around, when someone is walking in proximity, or when someone is working on the equipment. The most hazardous situation is when someone is working on or near energized equipment. The equipment doors may be open, placing workers close to electrical components, conductors, and connections.
An electrical arc can form when an electrical worker makes contact between phases or from phase to ground with a conductive object such as a screwdriver, pliers, or body parts while working inside an energized electrical panel. The temperature of the arc is intense enough to produce radiation burns, which could result in long-term internal bodily damage. The explosive energy released by this electrical arc creates a pressure wave. When this wave comes into contact with a surface — which could be a person — it is called incident energy.
Organizations such as OSHA, NFPA, ASTM, and IEEE aspire to protect electrical workers from electrical hazards through training, proper equipment maintenance, proper use of tools and protective equipment, and sound engineering methods for design and analysis of electrical systems (see “Electrical standards organizations focus on arc flash hazard protection”).
While NPFA 70E provides equations and a quick table to determine the level of personal protective equipment (PPE) necessary to keep a worker safe when performing a task, it is based on a specific system configuration. Every electrical distribution path in every plant is different. Each component in each of these paths is a variable that must be considered when evaluating potential arc flash hazards. An arc flash hazard analysis is more than recommended — it is urged because of the likelihood that your specific system configuration is quite different from the configuration used to develop the table, which should be used as a guideline — not a safety net.
The equations in the recently released IEEE Standard 1584 are based on significant high-current testing to determine incident energy. From this incident energy, one can accurately determine the proper PPE for the task to be performed. Refer to the standard for the equations and how to use them. A revision to IEEE Standard 1584 is due to be released in April 2004.
For starters, one can calculate the 3-phase bolted fault current on the low side of a transformer feeding a switchgear line, for example. First, use a worst-case scenario by assuming an infinite bus (which assumes that the impedance ahead of a device is essentially zero) to provide maximum fault current. Second, assume a fault-clearing time of approximately 0.2 sec and a working distance of 18 in. Based on these data, one may think they are calculating the worst-case scenario for incident energy, which typically is expressed in calories per centimeter squared (cal/cm2) (Fig. 1).
Calculations using 3-phase bolted fault current values based on an infinite bus may indicate a faster time/current response from protective devices. However, the results are lower calculated incident energy and arc flash boundary values, which could give an electrical worker a false sense of protection, when actually he or she is not adequately protected.
When using current-limiting fuses, it is important to calculate a realistic fault-current value based on actual system impedance. A current-limiting fuse interrupts all available current above its threshold current and below its maximum interrupting rating, and limits the clearing time to equal to or less than 1/2 cycle at rated voltage. Proper maintenance and coordination of protective devices is imperative when doing an arc flash hazard analysis.
For example, a 1000 kVA 13.8/0.480 kV (which steps 13.8 kV down to 480 V) transformer with 6% impedance should supply approximately 20 kA bolted fault current at 480 V, assuming an infinite bus. With a clearing time of 0.11 sec, the incident energy at 18 in. is approximately 4.4 cal/cm2.
However, after a thorough system analysis, the fault current is closer to 10 kA because of system impedance. The clearing time is actually 2.5 sec because this lower fault current falls into the long-time pickup range of the upstream protective device. At a distance of 18 in., the actual incident energy is 53.2 cal/cm2. A person working on this switchgear would most likely receive life-threatening external and internal burns as well as broken bones from the arc blast. A very serious burn can occur without ever making physical contact with the energized equipment.
How to calculate incident energy accurately
Arc flash hazard analysis requires more than just calculating bolted fault current based on an infinite bus using a generalized table or a calculator on the internet. The analysis starts with gathering up-to-date equipment information, then performing a detailed analysis comprised of load flow, short circuit, and protective device coordination studies as well as equipment evaluations to determine if the current-withstand rating is acceptable. For facilities with generators and large motors (100 hp or larger) a motor starting and fault-contribution analysis should be performed also.
Although experienced inhouse personnel using proper procedures and methodology can perform an arc flash hazard analysis, the process described in the following paragraphs assumes that an independent electrical consultant will carry out these tasks.
Site assessment and data gathering — Data gathered during the initial site visit and the site overview are critical elements in performing a safe and realistic arc flash hazard analysis. The system one-line diagram, supporting electrical system schematics, and pertinent documents should be checked and updated during this visit (Fig. 2). Data gathering consists of acquiring the nameplate data of all electrically powered equipment, protective device settings, and load information. Source impedance data from the utility are required to accurately calculate the short circuit current. Upgrades planned within the next few years should be noted as they could affect the analyses.
Short circuit analysis — The site data are used to build a system model, which enables the short-circuit analysis to be performed. Short-circuit studies determine the magnitude of current flowing through each section of the power system at various time intervals after a fault. Next, the current magnitude data are used to determine the 3-phase bolted short-circuit current, which is used to calculate the arc fault current.
Protective device coordination analysis — Protective devices, such as fuses, circuit breakers, and relays, have curves that are plotted on a log-log graph that shows current with respect to time (Fig. 3). Protective device coordination requires setting the devices according to these curves, so that when a fault occurs, the upstream protective device closest to the fault opens as rapidly as possible to minimize risks to people and equipment, as well as to isolate the problem with minimum disruption to the rest of the plant’s electrical system.
The equipment and protective device data used to build the system model are used for protective device coordination analysis also. When making changes or upgrades to plant electrical systems, it is necessary to revisit the existing protection scheme to ensure that devices are coordinated properly. A change in load or equipment could change the timing and coordination of the protective devices.
Arc flash hazard analysis — Arc flash analysis calculates the incident energy and arc flash boundary for each location in a power system. Trip times from protective device settings and arcing fault current values from the short-circuit analyses are used in the arc flash hazard analysis. Incident energy and arc flash boundaries are calculated following the IEEE 1584 standard. Clothing or PPE requirements are specified for given tasks (see “Typical arc flash evaluation report”). Arc flash hazard warning labels showing required data can be printed on adhesive labels and placed on equipment (Fig. 4).
Equipment evaluation analysis — The equipment evaluation analysis compares equipment withstand ratings with calculated operating and short-circuit analyses. This is very important when upgrading electrical facilities — especially when increasing available power or adding/replacing transformers, motors, or generators. Systems may be operated in paralleled to increase reliability. But this has a significant impact on the available short-circuit current to a downstream device. Circuit breakers and other devices that are underrated for fault-current withstand pose a serious arc and blast hazard to anyone close to the device.
If the completed arc flash analysis identifies hazards and noncompliance, your plant’s electrical system will require engineering changes to reduce potentially high incident energy levels. Only a complete electrical system analysis can identify the level of PPE required at each location in the system.
Obviously, the best way to prevent an arc flash hazard is to deenergize the equipment. However, even if you can totally deenergize the equipment, you still must open devices upstream. It is best if this can be done remotely. But if remote operation is not possible, you must be trained and know the proper arc flash protection required for the given task.
Proper procedures for lockout/tagout call for testing for zero voltage and applying grounds. This testing also requires proper training and PPE. There is no substitution for training and following best practices in electrical work.
An arc flash hazard analysis provides you with the knowledge required to keep electrical workers out of harm’s way.
John Lane, PE is a Principal Electrical Engineer at AVO Training Institute in Dallas, TX. He performs power systems and arc flash hazard analyses, and redesigns systems that minimize the potential for arc flash incidents. The author is available to answer questions about this article. He can be reached at firstname.lastname@example.org .
Article edited by Jack Smith, Senior Editor, 630-288-8783, email@example.com .
On April 8, 2004, Plant Engineering magazine will present a webcast titled “Preventing arc flash hazards.” (Visit plantengineering.com to register for this free webcast.)
Typical arc flash evaluation report (IEEE 1584)
|Bus name||Protective device name||kV||Bus bolted fault (kA)||Protective device bolted fault current (kA)||Arcing fault (kA)||Trip/delay time (sec)||Breaker opening time (sec)||Ground||Equipment type||Gap (in mm)||Arc flash boundary (in.)||Working distance (in.)||Incident energy (cal/cm2)||Required protective fire-resistant (FR) clothing class|
|B3||C1 feeder relay||2.4||8.11||8.11||6.95||4.057||0.083||Yes||Switchgear||36||390||18||111||Dangerous! No FR class|
|B4||Sub-C main||0.48||13.71||13.71||10.56||2.76||0.083||Yes||Switchgear||18||302||18||76.1||Dangerous! No FR class|
|B5||480 feeder||0.48||13.67||13.67||10.25||0.03||0.083||Yes||Switchgear||18||28||18||2.31||Class 1, FR shirt and pants|
Electrical standards organizations focus on arc flash hazard protection
Regulations and standards that deal with arc flash were established to protect electrical workers. Some of these regulations and standards are paraphrased in the following list:
OSHA 29 CFR 1910.132 (d) requires employers to assess the workplace to determine if hazards are or could be present, provide the appropriate PPE for each affected employee, and require its use. It also requires employers to inform the affected employees regarding hazards and PPE and to verify that the required assessment was performed through a written certification identifying the workplace evaluated.
OSHA 29 CFR 1910.333 requires employees who are exposed to electrical shock hazard to be qualified for the specific task they are performing. This involves safe work practices as well as using the appropriate PPE.
OSHA 29 CFR 1910.335 (a)(1)(i) requires employees working with potential electrical hazards to be provided with, and use, electrical protective equipment appropriate for the specific parts of the body to be protected and for the work to be performed.
OSHA 29 CFR 1910.335 (a)(2)(i) requires employees to use insulated tools or handling equipment near exposed energized conductors or circuit parts.
OSHA 29 CFR 1910.335 (a)(2)(ii) requires protective shields, protective barriers, or insulating materials to be used to protect employees from shock, burns, or other injuries while working near exposed energized parts. When normally enclosed live parts are exposed for maintenance, they must be guarded to protect unqualified persons from contact.
NEC 110.16 states that equipment must be marked to warn qualified persons of potential electrical arc flash hazards.
NEC 110.9 states that the equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment.
NEC 110.10 states that the electrical characteristics of the circuit must be known to properly select and coordinate protective devices used to clear a fault. The characteristics of the system are source impedance, individual component impedances, connected loads, and short-circuit current ratings. Protective devices must be coordinated so that it protects people and equipment, and isolate the least affected part of the system.
NFPA 70 E-2000, part II, Chapter 2, paragraph 2-1.3.3 requires an arc flash hazard analysis to be performed to determine the level of hazard and appropriate PPE for given tasks.
Benefits of performing a detailed arc flash hazard analysis
The following list highlights some of the benefits of performing a detailed and accurate arc flash hazard analysis:
Provides knowledge to select the best possible PPE for electrical workers — both qualified and unqualified.
Potentially lowers insurance premiums
Brings electrical systems up to date by providing accurate one-line diagrams
Enhances system reliability
Enables easy changes and upgrades
Drastically lessens your chances of having to make a very unpleasant visit to survivors.