Arc Flash Assessment

A large industrial manufacturing plant needed to have in place an evaluation of their electrical system as part of their safety program. This evaluation included an arc flash assessment. An arc flash is the unintentional release of electrical energy due to operating switchgear, equipment failure or fault currents delivered by the electric utility. Arc flashes can result in explosions, fires, equipment damage and personnel injuries.

The need for the assessment is driven by the requirement to meet safety codes such as NFPA70 (the National Electric Code), NFPA 70E (Electrical Safety in the Workplace) and 29CFR1910 (OSHA codes). Personnel protection is enhanced by reducing or mitigating potential arc flash hazards and alerting electrical equipment operators as to what Personal Protective Equipment (PPE) must be worn. Equipment reliability and production continuation would also be enhanced.

To assist the industrial plant in meeting its safety and code requirements, Varo performed the arc flash assessment of the plant’s electrical system. This involved the following tasks:

Perform field walk downs to collect plant data such as electrical system loads, equipment model numbers, fuse ratings, circuit breaker settings, cable and wire lengths and plant operating modes.
Obtain plant electrical drawings to determine the interconnection of the different components in the electrical systems.
Obtain utility data such as minimum and maximum available fault currents and X/R ratios.
Input this data into arc flash assessment software. This software is from SKM, Inc.
Create one line diagrams based on the plant drawings and collected field data.
Analyze the software output and document findings of arc flash hazard categories in a report.
Make recommendations of actions that the plant personnel could take to reduce the arc flash hazard category.
Produce arc flash hazard labels for the different busses or panels analyzed by the program. Varo affixed these labels to the analyzed equipment.

The plant receives 138kV from the electrical utility. This supply is transformed to 12.47kV where it is distributed to ten substations in the plant. From these substations, the power is transformed down to 480 VAC for use by plant machines and processes. Varo performed phase one of this assessment involving the main plant power supply and two of the ten substations. These substations had connected about 4 MCC’s, several 480 VAC distribution panels and overhead bus ways and numerous disconnect switches. In all, about 1400 data points were involved.

Varo performed the assessment on three scenarios. One scenario involved receiving full utility fault current from the utility with all plant machinery operating. Another scenario involved 85% of the utility fault current being applied with all plant machinery operating. The third scenario involved the plant in a shut down mode with minimum processes running. This assured that that the maximum fault current would be known for all situations that the plant could be in. The difference in incident energy for the different three scenarios was small and did not result in any category changes.

The final report delivered to the client consisted of:

An arc flash assessment report. This report discussed the process of performing the assessment, the findings and recommendations for reducing hazard categories.
Arc flash data including an arc flash table. This table listed the available fault currents, incident energy, arc flash hazard categories, hazard boundaries and personnel protective equipment (PPE) required for each electrical bus and panel.
A one line diagram for each of the two substations analyzed. These diagrams represented a “snapshot” of the plant at the time the information was gathered. An abbreviated example of a one line is given in Attachment 1 of this study.
Time current coordination curves for any recommendations listed in the report.
A warning label for each panel or bus analyzed. As required by NFPA 70E, each label listed the panel or bus name, the incident energy (cal/cm2), the hazard protection boundary and the PPE requirements. An example of a label is given in Attachment 2 of this study.

The report highlighted the areas with high (Category 2 or greater) hazard categories. For these areas, a determination was made if the hazard category could be reduced. If so, a recommendation was made. This recommendation involved changing the setting of a protective device, fuse types, and equipment type or other plant tasks. For any change recommendation, a time current coordination diagram was produced to assure coordination would not be impaired by implementing the recommendation. An equipment evaluation was performed.

There are several notable aspects of the assessment. The first aspect was working with 1950 vintage equipment. Varo worked with SKM to assure that we had the most up to date software and equipment libraries. Where equipment ratings were not available, similar devices were used and any changes were assessed.

The second aspect involved taking the various plant and vendor drawings, as well as field walk data and integrates them into a one line diagram that would be used for the assessment.

The third aspect was emphasizing that this assessment is only a part of an overall safety program. The importance of developing an effective lockout/tag out program and reviewing other codes was emphasized in the report.

As a quality assurance check, hand calculations were performed on some representative busses. These calculations were performed in order to gain a comparison with the output of the software. These calculations are based on examples given in the IEEE Standard 141-1993 (IEEE Red Book®).

For more information regarding this project, please feel free to contact us at (614) 459-0424.