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Arc Flash Risk Assessment Done! What Next?

Presented By:
Yogi Nimbalkar
Saber Power Services
TechCon 2017

Abstract

Facilities (industrial / commercial / intuitional / utility) depend on continuous power supply and maintaining these has significantly increased the need for qualified personnel at certain times to perform work on energized equipment, thus exposing persons to arc flash risk. Working on or near electrical equipment that is energized is not considered a safe act, a risk exists at all times. Every employer is required to conduct an Arc Flash Risk Assessment (AFRA) in accordance with OSHA 29 CR 1910.132(d) (1) / NFPA 70E article 110.3 and document the results. Hence it is a requirement for any facility owner to perform an Arc Flash Risk Assessment for their facility / electrical distribution system. Benefits of performing a detailed and accurate arc flash risk assessment are: better system reliability, equipment protection, reduced downtime, employee safety, code/insurance requirements, etc. This paper will discuss the importance of Arc Flash Risk Assessment, detailed steps to perform the same, followed by important steps to be done after the assessment study is performed which are mostly ignored / unknown.

Introduction

Electrical Safety has no substitute, don’t believe anyone but yourself! The key guidelines for electrical safety are:

  1. NEC (NFPA 70) – safe installation of electrical equipment
  2. NFPA 70B – preventive maintenance of the electrical equipment assures safe operation.
  3. NFPA 70E – guide for employers to develop an Electrical Safety Program for the facility, which will help workers to ensure safe execution of work during normal operating conditions of the energized electrical equipment. Workers are exposed to energized equipment which can result in an electrical hazard (arc flash, arc blast and shock).

electrical safety explosion image

An electrical safety program shall include a risk assessment that will provide protection to qualified persons from electrical hazards. Risk Assessment is a process with several steps to ensure that the hazards are properly identified and analyzed with regards to their severity and the likelihood of their occurrence. Upon assessing these risks, appropriate control measures need to be implemented to minimize the hazards and ensure a safe work environment for qualified persons. The risk assessment must determine the voltage, incident energy, the shock boundaries, and the required Personal Protective Equipment (PPE) to protect employees from the shock hazard. The arc flash risk assessment first determines if an arc flash hazard exists. If an arc flash hazard exists, the employer then determines safe work practices, as well as the arc flash boundary and the required PPE from AFRA. Hence, to provide a practical safe working area for qualified persons it is necessary to perform an Arc Flash Risk Assessment for any facility.

Paper

NFPA 70E 2015 Article 130.5 states the detailed requirements for an Arc Flash Risk Assessment (AFRA) to be performed. To summarize the same in a few items:

  1. AFRA shall be reviewed not to exceed 5 years, or any change in the system that can invalidate the results of AFRA
  2. Results of the AFRA need to be documented
  3. To determine if a hazard exists, then determine safe work practices, the arc flash boundary, and the required PPE
  4. Arc Flash PPE shall be calculated by using one of the two methods (Incident Energy / PPE Categories or Tables), but not both for same equipment
  5. All equipment needs to be labeled followed by training of qualified persons.

Overall scope of the AFRA study includes data collection, system modeling (developing / updating software model), performing short-circuit analysis, equipment evaluation, protective device coordination, arc flash incident energy analysis and report then followed by labeling of electrical equipment.

These various steps of performing an accurate AFRA must be followed under supervision and control of a Registered Professional Engineer with a minimum of five years of experience.

1. Site kick-off meeting and scheduling data collection
This meeting could be either a conference call or held at the facility, where site/contact person information can be exchanged with the field engineer which can help in scheduling the actual data collection. This step can serve as an input to the field engineer (who would request an existing single line diagram – if any available, utility information, etc.) of the facility. During this meeting, discuss the scope of work (for e.g. grouping of < 50 HP motors, single low impedance transformers under 125kVA with secondary less than 240V, single phase ac system being ignored, etc.), methodology (software modeling preference) that will be used for AFRA, label size, etc.

2. Data Collection
Qualified persons must collect the data for electrical equipment noted below with all the necessary information. It is always a good idea to take pictures, in case it is required to verify some details later. This is a very important step and requires attention to details to accurately determine the AFRA. The old saying “Garbage-in is Garbage-out (GIGO)” is very true, especially in data collection for AFRA and/or any other power engineering studies. Data shall be complete for each equipment as a minimum, but not limited to items listed below.

  • a. Utility fault current (3 Phase and SLG) with corresponding X/R ratios at respective voltages
  • b. Protective devices shall include Manufacture, Type, Voltage/Interrupting Rating, over current and ground settings, etc.
  • c. Cables size, number per phase, length, etc.
  • d. Transformer kVA and voltage ratings, connections, % impedance, tap, etc.
  • e. Generator kVA, voltage ratings
  • f. Motor HP rating, SF, FLA, PF, efficiency, NEMA code letter, etc.
  • g. Switchgear/MCC voltage, ampacity and interrupting ratings, etc.
  • h. Modes of operation of the facility

3. System Modeling
There are several applications (ETAP, SKM, Easypower, EDSA, etc.) which contain modules to determine incident energy levels. It should be noted that the AFRA study results will only be as good as the system model. Every effort should be made to collect the actual equipment information as found in the field and select the correct protective devices from the software. Any variations can change the actual results. Facilities with simple radial systems have one mode of operation, however for larger facilities there can be multiple modes of operation, such as multiple utility sources, generators in parallel with utility, etc. Separate scenarios shall be created to consider the worst-case results for such larger facilities.

Below is a sample one line system model.

sample one line system model chart

4. Short Circuit Analysis and Equipment Evaluation
This step evaluates electrical equipment (switchgear/switchboard/MCC, etc.), protective device existing ratings and compares them to calculated short circuit ratings. Inadequacy of any equipment or protective device shall be mentioned in the report and recommendations shall be made to mitigate the same. All results need to be tabulated. The study results are also used to selectively coordinate Time vs Current Characteristics (TCC) of electrical protective devices.

Short Circuit Analysis and Equipment Evaluation chart

5. Protective Device Coordination
Log-log plots give a clear picture as to how the protective device settings coordinate within a system to comply with article 110.10 of NEC 2014. This step ensures the protective device nearest to the fault will clear the fault without extensive damage to the system electrical equipment. Proper selective coordination ensures system reliability.

Protective Device Coordination log-log plots

6. Arc Flash Incident Energy Analysis
Short circuit analysis and protective device coordination are prerequisites for arc flash incident energy analysis. Arcing current from the short circuit results and tripping time from the coordination study are used to calculate the Incident energy and arc flash boundaries referring the IEEE 1584 / NFPA 70E standards. Based on the incident energy values, appropriate PPE Level and protective equipment are selected.

The results of the study are reviewed to determine if the calculated incident energy can be mitigated to lower the incident energy by adjustments to the protective device overcurrent settings. Two main factors that can reduce the incident energy are, the short circuit and device operating time. Short circuit current cannot be easily altered, however operating time can be. The adjustments are made while maintaining selective coordination between the protective overcurrent devices. These changes should be reviewed by the facility engineers and approved before making the adjustments in the actual protective devices.

Short circuit analysis and protective device coordination are prerequisites for arc flash incident energy analysis

7. Arc Flash Labeling
Arc flash labels are generated based on the results from the arc flash incident energy analysis and applied to all electrical equipment modeled as bus in the study. Minimum information required on the labels is detailed in article 130.5. (D) of the NFPA 70E 2015 standard. The facility owner is responsible for documentation, installation and maintenance of these labels.

Arc flash labels example

8. Report
All study results shall be documented for future reference and development of a safety program. It is recommended to have two hard copies and one soft copy of all the documents used in the final report, approved by registered professional engineer.

On receipt of the hard copies of the report and all electrical equipment being labeled, are you fully complying with the AFRA? NO! WHAT TO DO NEXT!

The AFRA report is based on assumptions that the electrical equipment and protective devices are well maintained. What if a main breaker on a switchboard does not trip in case of a fault, then the AFRA report is invalid.

Below are a few important points that need to be addressed after AFRA is performed.

  1. Report
    After an AFRA study has been performed a report is submitted to the facility. The report should be saved in a designated location that it is easily accessible (e.g. just like the MSDS’s). Any soft copy should be stored on a public drive and file attributes should be set to read only. It is a very common practice that a report is provided in CD-ROM format which typically lands in a drawer or desk that may not be easily accessible to others, especially when it is required the most, l i k e during an electrical safety program audit, OSHA incident or event, etc.
  2. Study review and procurement of PPE.
    Facility owner or engineer should review the results of the study and determine what PPE should be purchased for their employees. All PPE should be easily accessible and checked before every use. To minimize cost, most facilities like to manage by procurement of PPE for level 2 and level 4, instead of purchasing PPE for all levels. In such cases an electrician would use level 2 PPE while working on any panels that are labeled level 1 and level 2. Similarly, one would use level 4 PPE while working on level 3 or level 4 labeled electrical equipment. Check if any recommendations (item # 4) can be implemented to reduce the PPE level, which could reduce the cost of the PPE.
  3. Single line diagrams.
    It is good engineering practice to provide a hardcopy print of the single line diagrams from the report, which will help during any troubleshooting and most importantly, for documentation. In a case of changes to the electrical distribution system, mark these changes and send the same to the study engineer to determine if there are any change to the arc flash PPE level in the overall electrical system.
  4. Implementing Recommendations
    Very often the AFRA report includes recommendations that will reduce the PPE level at the electrical equipment. Generally, the recommendations are not approved and labels are generated using the “as found” arc flash incident energy results. The most common question asked i s when and how to implement the changes. This depends on the type of recommendations. If only a setting change on the circuit breaker is recommended, typically this should be done during a slowdown or shutdown to avoid any nuisance tripping (the study engineer should consider upstream and downstream coordination to avoiding any nuisance tripping). If the recommendation includes retrofitting a circuit breaker, this will require a complete shutdown of the equipment. The equipment owner should communicate with the study engineer and classify the priority (taking the financial and production activity into account) based on report recommendations and develop a plan to implement the changes. When any recommendation is to be implemented, communicate the plan with the study engineer and most importantly create new labels for the affected equipment.
  5. Testing and Preventative Maintenance program
    An AFRA study is based on the operation of over current protective devices as per the electrical distribution system. The AFRA report provides values if the over current devices do not operate in the event of an actual fault. Testing of over current devices is a very important task of the overall electrical safety program. A proper maintenance program should be place to increase the reliability of the electricalsystem and minimize the chances of an unplanned shutdown.
  6. Training
    All operators or technicians should be properly trained to read and interpret all the information on an arc flash label. Employees should be trained on how to use the appropriate PPE and properstorage of PPE (e.g. never put gloves directly in the glove case, instead the glove opening should be at the bottom of the case). Have a PPE level description and glove class table (with pictures) in every MCC or Switchgear room to easily identify the appropriate PPE to be used.
  7. Spare labels
    Have 5-10% spare labels of each PPE level. Just in case any label is damaged or unreadable to allow a new label to be applied.
  8. Periodic review
    According to NFPA 70E, the study needs to be reviewed every five years even if there is no change to the electrical system. In case of any changes, send the marked one-line to the study engineer to verify if there is any effect on the PPE levels. It is also a good practice to document the date when the next study should be reviewed.

Conclusion

Every facility should properly protect their employees from exposure to hazardous energy levels. The best way to prevent arc flash is not to work on energized equipment. But if the equipment cannot be de-energized, qualified personnel should be trained and provided appropriate PPE for the task. Just performing an AFRA and labeling the equipment is not all that should be done to safeguard employee safety. There are other things as mentioned above that should be done after AFRA has been completed. This paper should not be considered as a comprehensive guide, only a summary of tasks after AFRA is complete.

References

[1] Occupational Safety and Health Standards, 29CFR, Part 1910, Subpart S, Electrical, Occupational Safety and Health Administration (OSHA)

[2] 2014 NFPA 70: National Electrical Code (NEC)

[3] NFPA 70B – 2016, Electrical Equipment Maintenance

[4] NFPA 70E – 2015, National Fire Protection Association (NFPA)

[5] IEEE Guide for Performing Arc-Flash Hazard Calculations, IEEE Standard 1584, Institute of Electrical and Electronics Engineers

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