Arc flash hazard calculations require accurate documentation

Accurate documentation of the plant electrical power system is needed for safe, reliable and efficient operation. Maintenance, troubleshooting, repair, design modifications and analysis require accurate documentation. To maintain accurate documentation of the electrical system, the most common documentation methods are: The advantage of a CAD-based model is that mechanical, electrical and proc...

02/01/2006


Accurate documentation of the plant electrical power system is needed for safe, reliable and efficient operation. Maintenance, troubleshooting, repair, design modifications and analysis require accurate documentation.

To maintain accurate documentation of the electrical system, the most common documentation methods are:

  • One-line sketch with hand-written notes

  • A simple CAD model with an equipment database

  • A power system modeling and analysis software program

  • A view-only portion of power system modeling software.

    • The advantage of a CAD-based model is that mechanical, electrical and process control systems can be documented in a single consistent program. The disadvantages are that there is no analysis capability to identify safety hazards such as undersized equipment, high arc flash energy levels or insufficient equipment protection. Operating the CAD software often requires special training, and a separate power system model must still be created when analysis is required.

      An intelligent one-line diagram and system model provides the most convenient, flexible and cost-effective method for maintaining accurate power system documentation. The advantages of using modeling and analysis software for documentation:

      • Documents the power system layout and electrical connections

      • Stores equipment details

      • Facilitates power system analysis

      • Identifies improperly sized equipment

      • Identifies and documents the arc flash hazard level for each location

      • Displays equipment damage points and the operating characteristics of protective devices

      • Schedules system maintenance and tracks maintenance history.

        • A view-only version of typical modeling and analysis software allows maintenance personnel to view the system model online to facilitate maintenance and troubleshooting without changing the model.

           



          Fig. 1. A lower arcing short-circuit current can result in higher energy. The diagram displays the melting characteristics of a fuse. The current is reported on the horizontal axis and the melt time is read from the vertical axis. Using a maximum estimated short circuit current, the fuse will melt in 0.02 seconds. At the minimum estimated short circuit current, the fuse will not melt for 1 second. At lower currents, it takes longer for the fuse to melt and clear the arcing fault. The longer the arc lasts, the more energy is released into the
          environment.


          Fig. 2. A similar example (see Fig. 1) using a circuit breaker indicates that the maximum fault current trips in the instantaneous region (0.03 seconds), whereas the minimum fault current trips in the short time region (0.3 seconds). For this case, the instantaneous or the short time delay should be reduced to minimize the incident energy.

          Reasons to maintain documentation

          In addition to the compliance issues, reasons for maintaining the one-line diagram include maintenance needs, design requirements for expansions or electrical system upgrades and safety.

          Maintenance and troubleshooting

          Maintenance requires an accurate view of the electrical system layout to identify loads affected by maintenance outages to safely de-energize and re-energize equipment, wear the proper personal protective equipment and to correctly identify equipment, wiring and supply. The diagram can also be used to store test records and maintenance history.

          An accurate one-line diagram and system model helps identify fault locations, available capacity and alternative power supply. It helps document hazards at each location and provides specifications for replacement equipment and repairs. The model can be critical in preventing human injury, damage to electrical equipment and damage to other property.

          Disaster recovery

          Accurate documentation plays a critical role in disaster recovery processes. Plants that were damaged during Hurricane Katrina and Hurricane Rita in 2005 found that having an accurate electrical model allowed them to order replacement equipment quickly and accurately. The electrical model also assists in setting the replacement protection equipment to the original design settings upon installation.

          Design and analysis

          An accurate one-line diagram must be the basis for any design or configuration change. Having an accurate one-line diagram and system model saves time and money and improves safety by eliminating the need for a site survey. It also saves time and money by providing the existing plant electrical model as an accurate starting point.

          Undertake new studies

          New system studies are required whenever changes are made to the power system. The studies are required for worker safety, protection of equipment and reliable operation. The most common studies include:

          Load analysis — verifies that equipment is sized properly for continuous loads. A new load analysis is required when existing loads are relocated, new motors or loads are added and when existing transformers are replaced or transformer taps are changed.

          Short-circuit analysis — verifies that equipment is sized properly to withstand and interrupt short circuits. A new short-circuit analysis is required when existing motors or generators are relocated, new motors or generators are added, existing transformers are replaced, transformer tap settings are changed or the utility supply is increased.

          Protective device coordination - sets protective devices to allow normal system operation while protecting equipment from damage and workers from injury. A new protective device coordination study is required when existing motors, loads or generators are relocated; new motors, loads or generators are added; the utility supply is changed; transformers are replaced; or protective device settings are changed.

          Harmonic analysis — minimizes harmonic distortion by locating its sources and causes. Harmonic analysis is also used to verify that equipment is sized properly to withstand harmonic current and voltage. A new harmonic analysis is required when existing nonlinear loads are relocated, new nonlinear loads are added or when power factor correction capacitors are added or relocated.

          Arc flash hazard analysis — calculates the incident energy released when an arcing fault occurs. A new arc flash hazard analysis is required when existing motors or generators are relocated, new motors or generators are added, the utility supply is changed, transformers are replaced or when protective device settings are changed.

          Accurate fault current values

          "Conservative" short-circuit values aren't always conservative for arc flash. In reality, these studies are not always performed before or after small changes are made to the power system because many power systems are designed for load growth and higher fault currents. However, arc flash hazard analysis requires accurate fault current values at each location and must be updated with any change to the power system.

          Lower fault currents may actually result in higher incident energy because the amount of arc flash energy is a function of both the fault current magnitude and the time it takes to clear the arcing fault. Any change that increases or decreases the available fault current or the protective device operating characteristics could substantially increase the amount of arc flash incident energy.

          The arcing-fault current and trip times depend on the 3-phase bolted fault current. Therefore, it is important to estimate the fault current as accurately as possible. It is also prudent to apply a tolerance to the fault calculations. The calculated fault current range can then be used to determine conservative arc flash hazard values.

          Maintaining accurate model

          Without accurate documentation of the electrical system, it is not possible to determine the available fault current or the arc duration. Maintaining accurate electrical system documentation in the form of an intelligent one-line diagram provides a graphical display format similar to a traditional CAD drawing, but it also integrates the electrical connection information required for analysis, the protective device operating curves required for coordination and fault clearing times and database information for maintenance.

          If you employ outside consultants for system design and analysis activities, being able to provide them an accurate as-built system model can save substantial time and money. In addition to regulatory safety requirements, benefits of maintaining an accurate system model include:

          • Saves time evaluating proposed system changes

          • Improves safety by identifying equipment where ratings are insufficient for the available short circuit current

          • Improves safety and reliability by identifying equipment that is overloaded

          • Improves safety and compliance by reporting personal protective equipment requirements and generating arc flash hazard labels and work permits

          • Reduces downtime by providing accurate system information for maintenance and troubleshooting activities.

            • If your electrical power system documentation is presently in the form of a traditional CAD-based one-line diagram, or is not current, the new arc flash hazard calculations may provide perfect justification to build and maintain an accurate power system model.

              The Bottom Line...

              • An intelligent one-line diagram and system model provides the most convenient, flexible and cost-effective method for maintaining accurate power system documentation.

              • Accurate documentation plays a critical role in disaster recovery processes.

              • Without accurate documentation of the electrical system, it is not possible to perform an accurate arc flash hazard analysis.


                • Author Information
                  Lon Lindell is an electrical industry veteran with more than 20 years with SKM Systems Analysis, Inc. In his responsibilities as General Manager, he is closely involved in engineering software design, application and training. SKM Systems Analysis, Inc. provides electrical power system modeling software.


                  NGLIA establishes its brand, launches Website

                  The Next Generation Lighting Industry Alliance has launched a new Website,

                  Solid state lighting systems are lighting applications that use light-emitting diodes, commonly called LEDs, organic light-emitting diodes or light-emitting polymers. The solid state lighting industry is attempting to integrate solid state light sources with lighting systems, as well as develop new applications in order to fully exploit the technology's potential.

                  NGLIA was formed in 2003 to foster an industry-government partnership to accelerate the technical foundation, and ultimate commercialization, of solid state lighting systems. The alliance was designated in 2005 as the "industry partner" by the U.S. Department of Energy for its Solid State Lighting program. The SSL Program is being undertaken to research, develop, and conduct demonstration activities on advanced solid state white lighting technologies based on LEDs and OLEDs.

                  The Website contains a member-only area that serves as an online repository for member-related information.

                  PLANT ENGINEERING editor to moderate Power Quality conference sessions at NMW

                  PLANT ENGINEERING senior editor Jack Smith will moderate four 50-minute sessions on Power Quality topics followed by a roundtable with panelists available to field questions.

                  Session 2P21 , "Identifying and Minimizing Harmonics in Your Plant's Electrical System" will be presented on Tuesday, March 21 from 10 a.m. to 10:50 a.m.

                  Session 2P22 , "Sags and Swells: Sources and Solutions" will be presented on Tuesday, March 21 from 11 a.m. to 11:50 a.m.

                  Session 2P23



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