Apply engineered solutions to deliver arc flash safety part 2

This is the second in a three-part series that describes planning and implementation for an effective, sustainable program. Part 1 discussed hazard awareness, safe work practices, and reducing potential for ignition of workers’ clothing from arc incidents. Part 2 will focus on applying engineered solutions, which includes electrical system analysis.

07/13/2010


In Stage 1, we were concerned about quickly implementing measures that can have an immediate impact on reducing risk of injury due to arc flash. In summary, NFPA 70E can be used immediately to set up a minimal arc hazard protection program. Then a more detailed hazard assessment can be made to refine the protection program and to identify opportunities for application of engineering solutions to eliminate or reduce risk.

STAGE 2: APPLYING ENGINEERED SOLUTIONS

In our example, Stage 2 involves performing a formal engineering analysis of the electrical system with the following goals in mind:

  • Estimating the incident energy values for all equipment where workers may be exposed such as switchgear, motor control centers (MCCs), panels, and disconnect switches
  • Determining high incident energy exposures that should be reduced through changes in design or working practices; in many cases, better coordination of existing protective devices can reduce arc flash incident energy without additional capital expenses
  • Determining the most effective flame-resistant clothing system that matches the exposures. Improvements can include better assurance that all body parts are protected, optimization of matching clothing performance to avoid under- or overuse of protective garments, and better assurance that burns from thermal transfer through protective garments will be reduced.

The work completed in Stage 2 will help the plant engineer achieve an inherently safer and more reliable electrical system. The expenses of the modeling can often be justified in the prevention of even one incident. Lost work time for the employee and downtime for the facilities often amount to large dollars very quickly. The amount of work and cost associated with conducting an arc hazard assessment depends on the availability and quality of engineering design documentation for the facility’s electrical power system.

The documents and information needed to complete the study include:

  • Power system single line drawings
  • Protective device coordination study
  • Utility short circuit data and protective device information
  • System operating conditions such as normal operating switch positions, possible tie conditions, and parallel connections
  • Information on transformers including impedance, rating, and grounding method
  • Information on cables including type, size, and approximate length
  • Manufacturer, model, trip unit, and settings of circuit breakers
  • Manufacturer, types, and sizes of fuses
  • Information on motors 100 hp and larger
  • Equipment type (MCC, panel, or switchgear)
  • Working distance for the task at each piece of equipment.

This information is entered into a commercially available power system modeling software to find the 3-phase bolted fault current at each piece of equipment being studied. Using commercially available software based on IEEE 1584™ equations, the arc flash hazard analysis can be run to determine an estimate of the incident energy for each piece of equipment. Incident energy can be measured in calories per square centimeter (cal/cm2) or joules per square centimeter (J/cm2). One cal/cm2 is equivalent to 4.18 J/cm2.

WHY A FORMAL CALCULATION?

Looking at data from a variety of bus voltages from plant sites and warehouses in mostly North America, the incident energy levels have a great deal of variability across any voltage level. Derived from studies of more than 85 industrial and warehouse facilities in North America, Figure 1 shows the percent of buses with calculated incident energy level in cal/cm2 within certain voltage classes.

For example, in the 600 V class, buses can be divided into incident energy levels, 50% at 0-1.2 cal/cm2, 20% at 1.2-4 cal/cm2, and 12% at 4-8 cal/cm2.

What is notable is that every voltage level contains buses of every incident energy level. This variability makes it difficult to predict the incident energy level of a bus without doing a formal calculation and demonstrates a limitation of selecting PPE by only using the tables in NFPA 70E section 130.7(C). This variability can lead to the use of too much or too little PPE if not calculated through system analysis.

Power system analysis programs are available to aid in the calculations of incident energy levels. The process of completing an assessment can be a great opportunity to train personnel and update documentation. Existing equipment settings and layout can be verified against existing records and discrepancies verified.

The results of the studies can be used to show why certain equipment settings have been selected and the consequences of changing settings.

Once the study has been completed, the results can be examined to determine the best course of action to deal with any points of exceptionally high incident energy levels. Exceptionally high-energy points (those greater than 40 cal/cm2) are often due to conditions in the power system such as:

  • Primary fusing on a substation transformer higher than needed: Example: 1000 kVA transformer at 35 kV primary voltage has full load amps around 16 A; typical primary fusing should be sized at 25 A. If the existing primary fuse is sized at 50 A or 65 A, the incident energy at the secondary will be high.
  • Low voltage power circuit breaker settings are all turned to maximum: The settings of switchgear circuit breakers are critical to reducing or limiting the arc flash energy. Especially important are the instantaneous settings on the breaker trip unit, if it has these settings. If these are set to the maximum, then the incident energy will be high.
  • Low fault current due to long cable lengths: Long cable lengths decrease the fault current that will flow during an arc incident. Protective devices such as fuses and circuit breakers take longer time to trip when they sense lower fault current; the longer time means higher incident energy.

PPE may be available to protect the worker at higher incident energy, but the clothing can be restrictive and uncomfortable if worn for long periods of time. The electrical system should be reviewed to see if the hazard could be reduced or eliminated through protective device selection or other design changes.

Many parts of the design have an effect on the estimated arc flash incident energy.

A detailed assessment of an electrical system, using a commercial software program, will help in determining the value of these possible design changes. Alternatives can be modeled and the resulting arc flash energy values can be easily compared.

REDUCING HAZARDS

There are engineering solutions and technologies available that can help the plant engineer reduce arc hazards. Examples requiring no or relatively small capital investment include re-coordination of protective devices, use of current limiting fuses and circuit breakers, and application of remote racking and switching hardware.

Arc flash hazard exposure can be reduced dramatically through use of careful administrative methods and work practices, which include:

  • Job planning with careful consideration of the sequence of switching to minimize the number of exposures
  • Planning and scheduling to enable work to be done under de-energized conditions
  • Application of temporary protection schemes that change protective device settings for short periods during maintenance and switching.
  • Some devices may be able to use technological components that were installed but never fully used. If capital resources are required to obtain these technology components, the arc flash model can effectively help design an optimal system to interface with existing equipment. A long-term plan can be developed to replace system components as their life cycle comes to an end. Equipment and system design solutions include:
  • Application of zone interlocking control in low voltage switchgear
  • Arc-resistant switchgear rather than standard metal clad or metal enclosed switchgear
  • Installation and use of intelligent equipment such as motor controls that do not require the worker to open doors or take voltage and current measurements during troubleshooting

Now that the study is complete, a program of protective equipment can be developed to compliment the preventive measures noted above. Some elements of the program may be:

  • Protective equipment selection
  • Existing flame resistant arc rated clothing can be best matched with the energy levels calculated. Layering of flame resistant arc rated clothing can be used to increase the protective ability of the garments and lead to flexibility in the garments selected. Flame resistant arc rated jumpsuits worn over flame resistant arc rated shirts and pants can add protection for higher risk tasks, whereas the jumpsuit or shirt and pants alone may be appropriate for lower hazard tasks. This might help reduce the need for additional clothing purchases.
  • In addition to garments, appropriate gloves and face shields must be selected for each task.
  • High performance flame resistant arc rated switching suits are available  for exposures greater than 40 cal/cm2.
  • Standard maintenance/operating procedures
  • Once protective equipment has been selected, job plans must be updated so workers know what to wear and when
  • Labeling of equipment must be done to prevent confusion
  • A plan for protective equipment maintenance must also be implemented.
  • Training program
  • A training program with follow-up auditing must be implemented to ensure workers know what to wear and the potential serious injury consequence of non-compliance
  • Including personnel in the development of the training program and selection of protective equipment may aid in the acceptance of the protective equipment program. 

THE ONE-LINE DIAGRAM IS NOT AN OPTION

Documentation is one of the most neglected aspects of electrical safety in industry. Although the one-line diagram is the most important drawing a facility can have for understanding its electrical system, few facilities maintain accurate one-line diagrams.

A one-line diagram is a drawing in which a single line represents three phases of a 3-phase power system. If properly drawn, it shows a correct power distribution path from the incoming power source to each downstream load—including the ratings and sizes of each piece of electrical equipment, their circuit conductors, and their protective devices.

NFPA 70E states that electrical documentation, including the one-line diagram, should accurately reflect the facility electrical system. NFPA70E also requires rigorous application of an “Energized Work Permit” if the requirement to remove power to eliminate exposure to electrical hazards cannot be achieved.

Knowing how your plant is wired helps identify alternate feeds such as tie breakers. Equipment may have been rewired, modified, reconfigured, or rerouted without the changes ever being documented.

Without accurate system documentation, workers can be exposed to potential back-feeds from alternate sources—energized capacitors, undocumented switching conditions, and unknown voltages—in addition to the problem of not being able to accurately perform lockout/tagout procedures.



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