Minimizing the Cost of Arc Flash Evaluation Studies

Transcription

1 Minimizing the Cost of Arc Flash Evaluation Studies THIS ARTICLE IS AVAILABLE ONLINE AT FOR 30 DAYS FOLLOWING PUBLICATION. The electrical power system at a plant is often taken for granted. Unlimited power is assumed to be available, and the infrastructure of the power system is assumed to last forever. Even as new equipment is added or existing equipment is idled, little thought is given to the overall power system. Another overlooked area is arc flash and arc blast safety. Until recently, electrical safety was This paper examines the criteria required to perform an arc flash analysis on a steel producing facility, including what information can be gathered in-house and what must be completed by a licensed power systems engineer. A strategic program for minimizing outside costs is presented. often limited to shock hazards and lockout/tagout programs. Most safety programs focused on a worker coming in contact with energized parts. Recent changes to the National Fire Protection Act (NFPA 70E) and the National Electric Code (NEC) have compelled companies to address these areas by performing an arc flash hazard analysis. 1 2 The purpose of this analysis is to determine the potential risk of arc faults and arc blasts at every industrial panel to which a worker may be exposed. Based on this analysis, appropriate work practices and personal protective equipment (PPE) must be utilized to reduce the risks associated with arc flashes and arc blasts. A conservative method based on voltage level, type of task being performed, and standardized tables can be used to conduct this analysis. This method is simple and inexpensive in the short term, but often results in overprotection. When a worker is overly encumbered by PPE, productivity and morale are negatively impacted. This method also does not address ways of reducing PPE requirements through engineered solutions, such as using current-limiting fuses, using smaller transformers, etc. Another method of arc flash hazard analysis requires an engineered study. The available fault current is determined for each industrial panel based on voltage, overcurrent protection settings, utility contribution and equipment ratings. This type of analysis is more exact and often leads to less-restrictive PPE requirements. It also identifies problem areas and points within an electrical system that can be modified to further reduce PPE requirements by reducing the available fault current. This paper seeks to help companies minimize the cost of performing engineered studies for arc flash hazard analysis. Each step required to perform an arc flash hazard analysis will be detailed. The level of expertise required for each step will also be discussed (i.e., electrician, plant engineer, or licensed power system PE). Typical areas of concern for steel mills will also be addressed. Electrical Single Lines The basis for an engineered study is the electrical single lines. These drawings typically show the distribution of electrical power from the utility company down to the switchboard, motor control center (MCC) or panel level. Large-horsepower (hp) motors or MG sets are also included. Each item is assumed to be three-phase AC unless otherwise noted. All the breakers, fuses and protection relays that provide protection for the system are shown. Any switches, disconnects and tie-breakers are also shown (Figure 1). The vast majority of engineering studies input a plant s single lines into a software program for analysis. Since the study is only as accurate as the information put into this program, many of the single lines need to be field verified. Most plants do not have up-to-date single lines. As a result, the first step in performing an arc flash hazard analysis is walking through a plant and correcting the single line diagrams. Depending on the size of the plant, this can be a significant effort. In addition to verifying the actual connections shown on the single line, an engineered study requires the following information to be field verified: Author 134 Iron & Steel Technology Dan Laird, associate professor, Youngstown State University, Youngstown, Ohio (dlaird@ysu.edu)

2 Figure 1 Electrical single line drawing. Utility fault contribution and protective device information. Transformer nameplate information, including impedances and, if possible, X/R ratios. Nameplate fuse ratings. Nameplate breaker ratings and settings. Protective relay settings. Equipment ratings (symmetrical and asymmetrical ratings, voltage ratings, etc.). Motor/generator nameplate information (motors larger than 50 hp or large groups of motors that run simultaneously). Regenerative drive information. Conductor sizes, type and number per phase. Relative length of cable runs (±50 feet). Notation if cable is overhead (in free air) or in raceway/conduit. Bus duct ratings, lengths and loads. Gathering this information is the most critical part of the study. It is also the biggest opportunity for saving money by doing as much in-house as possible. Providing detailed information means that the power systems engineer needs to make fewer assumptions. When making engineering assumptions regarding life safety, an engineer will tend to be conservative. These conservative estimates can lead to overprotection one of the things to be avoided by doing an engineered study. The optimal condition is to have up-to-date single lines. If there is equipment missing or no longer in use, these drawings need to be updated. If there is too much information missing, an engineering firm cannot even quote the cost of performing an arc flash hazard analysis. The most cost-effective way of verifying the information is to have the plant electrician(s) red-line the single line drawings by hand. These red-lined drawings can be updated in electronic format at a later time. While the electrician is verifying these connections, nameplate data can be gathered on the transformers, motors, generators and other equipment. This can often be done by an engineering intern who has the appropriate safety training and the guidance of an engineer. A good safety practice is to always have the intern escorted by a qualified electrician at all times. (Note: Always follow the prescribed boundary distances for voltage rated equipment, even if escorted by qualified personnel.) Some information can be gathered safely only if the electrical equipment is de-energized. This includes conductor sizes, number of conductors, some relay settings, some breaker settings and fuse information. The ideal situation is to gather this information during a planned shutdown of the plant electrical system. Most of this information can be April

3 gathered by an engineering intern under the guidance of an engineer and with the assistance of a qualified electrician. If an outage is not possible, the next best solution for saving money is to spend time in the maintenance library. Any accurate cable and conduit schedule can be used to mark conductor size and number of conductors on the single line drawings. Previous calibration reports for protective relays and breakers can be used to gather nameplate ratings for each device and give the engineering consulting company a checklist from which to work. This will save time when collecting data and reduce the overall cost of the project. If digital trip units have been added to power circuit breakers, the settings for these devices can be tabulated and provided to the outside consulting company. Any records regarding fuse information, such as infrared studies, can also be useful. Keep track of any current-limiting-type fuses that have been installed so that the engineering firm does not need to conservatively use a non-current-limiting type in the computer model. Another crucial cost-saving opportunity is in obtaining an electronic copy of the computer model from the engineering consulting firm that performs the study. These firms typically provide a hard copy of their report and analysis, but most will provide it electronically as well, if requested. Constructing the electrical system database is the most time-consuming task for the engineering firm. Once the system is built into software, it is very easy to update the study as fuses are changed, lines are added or equipment is removed. Having an electronic copy of the database ensures that a complete database reconstruction will not have to be paid for a second time, even if a new consulting firm is chosen. Since the coordination of overcurrent protection devices in a plant should be examined at least every five to 10 years, it makes sense to have a model of the power system available. Many plants purchase their own copy of this modeling software (i.e., SKM, ETAP, etc.) to examine different scenarios, such as upgrading to current-limiting fuses, opening tiebreakers or replacing existing equipment. This software can be expensive and requires training to understand how changes are implemented, so this may or may not be a good value. Short-circuit Analysis Once the power system database has been constructed, a short-circuit analysis can be performed. This analysis determines the magnitude of available current throughout the power system at various time intervals following a fault. The computer model is used to determine the bolted three-phase short-circuit current at each point in the system. Once the fault levels have been calculated, they are compared to the withstand rating of the equipment in the electrical distribution system. For example, if a breaker is rated for 80,000 amps asymmetrical/50,000 amps symmetrical, it can withstand a first 1 /2 cycle of 80,000 amps and clear a fault of 50,000 amps. If the equipment is underrated, it can fail to operate properly and cause an even greater hazard than if it did not operate at all. Most software packages will generate a list of equipment that is not adequately braced as part of an equipment evaluation. Since all panels and breakers do not list the bracing information, the software may estimate typical ratings based on voltage and amperage. Paying to have the engineering consulting firm conduct an equipment evaluation implies they will track down the true ratings for all panels that are flagged by the software as not being adequately braced. It may be a cost savings to simply ask for a list of equipment that does not pass the equipment evaluation and verify the actual ratings using inhouse resources. If the information is not found on equipment nameplates, this means the equipment manufacturer should be contacted in order to obtain the true ratings. Protective Device Coordination Analysis A coordinated system is one in which the protective devices (circuit breakers, fuses, relays, etc.) operate at the proper times to eliminate or minimize the damage to a system. In the worst case, no protective devices will operate, resulting in risk to both equipment and personnel. If set too aggressively, the protective devices may operate for normal conditions, such as transformer magnetization and motor inrush current. It takes a significant degree of skill and experience to set the overcurrent protection properly. Significant changes in loading or equipment require revisiting this analysis every few years. In general, coordination studies are conducted from the loads to the source as identified below: Individual loads. Conductors. Distribution equipment: breakers, disconnect switches, etc. Transformers. High-voltage distribution to transformers. The above process is continued for each successive voltage level until the utility-controlled devices are reached. This process needs to performed by an experienced power 136 Iron & Steel Technology

4 Figure 2 Single line as drawn in modeling software. systems engineer. The adjustable settings on the overcurrent protection devices have a profound effect on both equipment protection and arc flash boundaries. Consider the coordination curve shown in Figure 3. Notice that all the protective relays are not set to trip before transformer T1 reaches full load current. All three relays will trip before thermal or mechanical damage occurs to the transformer. Only the relay on the secondary side of the transformer is set to trip faster than the transformer inrush. This is an example of a coordinated system. Copies of these coordination curves should come with the arc flash hazard analysis, as they provide the basis for setting available fault current. Arc Flash Hazard Analysis The next step in the study is to determine the incident energy at each point in the power system based on the previous information. These energy levels are only as accurate as the information garnered in the previous steps. There is also some latitude for engineering adjustments in performing the arc flash hazard analysis. A typical working distance of 18 inches is often used to give an average arm s length distance for the worker. The idea is to protect the face and torso, since burns over less than 25 percent of the body are less likely to be fatal (Figure 4). This is a sound estimate for working inside a panel or doing voltage checks. However, some tasks may not require the worker to be so close to the energized equipment. For example, a worker changing fuses in a fused disconnect cubicle may be more than 18 inches away from the main busbar, which carries a higher PPE rating. When switching operations are performed, it may be possible to use extended racking handles or hook sticks to increase the working distance. If areas in a plant show up on the arc flash study as having a PPE rating higher than Class 4, the study should be rerun with a working distance of 36 inches in these areas. If the result is less than a PPE rating of Class 4, a worker can perform those tasks beyond a working distance of 36 inches in PPE while the equipment is energized. This is particularly true for work done on medium- or high-voltage systems, where the only work performed while energized is switching or racking breakers in and out. Work that cannot be done from beyond 36 inches will have to be done when the system is de-energized. Another area of flexibility is in setting the maximum arcing duration in the arc flash hazard analysis. This permits the setting of a maximum (trip time + breaker time) for the incident energy and flash boundary calculations. IEEE 1584 Annex B.1.2 states, If the time is longer than 2 seconds, consider how long a person is likely to remain in the location of the arc flash. It is likely that the person exposed to arc flash will move away quickly if April

5 Figure 3 Transformer protection curve.3 it is physically possible, and 2 seconds is a reasonable maximum time for calculations. A person in a bucket truck or a person who has crawled into equipment will need more time to move away. This 2-second approximation has caused much debate among power systems engineers. Many engineering firms will run the study with a longer maximum clearing time first. If the analysis shows some restrictive PPE requirements in an area of frequent work, the study may be rerun at the 2-second clearing time. In this case, it should be stressed to the owner of the electrical system that the arc may in fact last longer than 2 seconds and that not every task is suitable for this assumption. 138 Iron & Steel Technology PPE Requirements Once the incident energy is calculated at the working distance for each point in the system, suitable PPE can be prescribed for that level of energy. Table 1, from NFPA 70E, shows the PPE hazard category and minimum arc thermal performance exposure values. Many companies provide their workers with a uniform that meets Class 1 requirements and have Class 2 and Class 4 PPE kits available for all other tasks. In this case, a worker will wear Class 4 PPE for hazard categories 3 and 4. A common mistake is to purchase PPE before the study is completed, only to find out that it is inadequate or overkill for most tasks performed in the plant. There is also the possibility that PPE requirements can be relaxed

6 Figure 4 if fuses or breaker settings are changed, more information becomes available at the next planned outage, or equipment is replaced. If a plant has only a few Class 4 locations, it may not make sense to purchase Class 4 PPE for every maintenance worker. The application of arc flash labels, employee training and issuing of PPE should ideally happen at the same time, but all these tasks should be based on the results of the study. Otherwise, it may be necessary to do these tasks twice and incur additional costs. Items Particular to the Steel Industry Electrical power systems for steel producers have some unique features. Below are some common concerns for arc flash hazard analysis: DC Systems There is currently no arc flash standard for DC systems. The IEEE 1584 method currently used to perform most arc flash evaluations is based on experiments conducted in high-power laboratories on AC systems. Until enough experimental data exists for DC systems, arc flash analysis must stop at the rectifiers converting AC to DC. When a standard is developed, it will pay to have the electronic copy of the database on hand to quickly rerun the arc flash hazard analysis for the plant. Probability of survival after a burn injury. Regenerative Systems Bridle and winder motors are often running in regeneration mode, i.e., they are sending energy back to the incoming line during normal operation. On a fault condition, this added energy from regenerating motors must be applied to the short-circuit study. The engineering consulting firm should be assisted in identifying which motors operate in regeneration, especially if the firm is not familiar with the steel industry. Open Commutators Many commutators on large motors and generators in a steel mill are open to ambient air. This creates a situation where entire motor rooms are in the flash zone of the motor or generator at all times. Since these rooms often house variable speed drives, MCCs or switchgear, restrictive PPE must be worn even for routine tasks. It may be necessary to partition motor rooms to avoid cumbersome PPE for everyday tasks. Exposed Busbar Many steel mills have exposed busbar on slate panels, crane rails or processes like electrolytic cleaners. Again, these conductors are considered exposed at Table 1 NFPA 70 E PPE Hazard Category and Minimum Arc Thermal Performance Exposure Values Minimum arc thermal performance exposure value (ATPV)* or Hazard/risk Clothing description Total weight breakopen threshold energy (E BT )* Category (number of clothing layers in parentheses) (oz./yd. 2 ) rating of PPE cal/cm 2 0 Untreated cotton (1) N/A 1 FR shirt and FR pants (1) Cotton underwear plus FR shirt and FR pants (2) Cotton underwear plus FR shirt and FR pants plus FR coverall (3) 4 Cotton underwear plus FR shirt and FR pants plus double-layer switching coat and pants (4) *ATPV is defined in the ASTM P S58 standard arc test method for flame-resistant (FR) fabrics as the incident energy that would just cause the onset of a second-degree burn (1.2 cal/cm 2 ). E BT is reported according to ASTM P S58 and is defined as the highest incident energy which did not cause FR fabric breakopen and did not exceed the second-degree burn criteria. E BT is reported when ATPV cannot be measured due to FR fabric breakopen. April

7 all times, and workers would be within their flash zones under normal circumstances. The solution may not be PPE, but finding creative ways to either contain the energized parts or keep workers at a safe distance. Arc Furnaces The process of making steel can require very high-powered electrical systems and large transformers. Working in these areas may affect not only electrical maintenance workers, but production workers as well. The approach boundaries for unqualified individuals can be prohibitive. Engineering creativity is again needed to shield workers from the exposed energized equipment at greater distances than currently maintained in most steelmaking facilities. Summary Companies are required to perform an arc flash hazard analysis by NFPA 70 E. Choosing to do an engineered study will provide a safe and accurate assessment of arc flash hazards and provide options for making a plant a safer place to work. These options can include PPE, design changes or equipment changes. These studies should be performed by an experienced power systems engineer. Much of the information required by the power systems engineer can be obtained using in-house resources as outlined above. The cost of future studies can be minimized by obtaining an electronic copy of the modeling database. The cost of equipment evaluation can be minimized by acquiring equipment bracing information using in-house resources. Calculating the working distance based on the task performed can reduce PPE requirements. Creativity can be applied to increase the safe working distance and ultimately reduce PPE requirements. Care should be taken to avoid ordering the incorrect PPE. It is recommended that a PPE plan be developed based on the results of the engineered study. The steel industry has its own unique concerns relative to arc flash. They can be considered and acted upon prior to performing an engineered study in many cases. References 1. National Fire Protection Association Inc., NFPA 70E: Electrical Safety Requirements for Employee Workplaces, 2004 Edition, National Fire Protection Association Inc., National Electric Code 2005, Article , Barr, Edward, Coordination of the Kraft Facility Philadelphia, Pa., Reuter Hanney Inc., May American Burn Association, National Burn Repository 2002 Report: Percent Survival Versus Age Range, March IEEE, IEEE Standard Guide for Performing Arc-flash Hazard Calculations, This paper was presented at AISTech 2006 The Iron & Steel Technology Conference and Exposition, Cleveland, Ohio, and published in the AISTech 2006 Proceedings. DID YOU KNOW? December 2006 Steel Shipments Down 11.8 Percent From Previous Year The American Iron and Steel Institute reported that, for the month of December 2006, U.S. steel mills shipped 7,609,000 net tons, an 11.8 percent decrease from the 8,513,000 net tons shipped in December 2005, and a 5.0 percent decrease from the 7,991,000 net tons shipped in the previous month, November A year-to-year comparison of year-to-date shipments shows the following changes within major market classifications: service centers and distributors, up 2.1 percent; automotive, up 7.5 percent; construction and contractors products, up 10.6 percent; oil and gas, up 19.6 percent; machinery, industrial equipment and tools, up 4.6 percent; appliances, utensils and cutlery, down 6.0 percent; containers, packaging and shipping materials, up 1.2 percent; and electrical equipment, up 12.8 percent. 140 Iron & Steel Technology