During this project, NRECA performed arc-flash exposure calculations that specifically focused on the types of situations encountered by electric co-ops.

This will help co-ops better understand what will need to be done to comply with the new arc-flash provisions of the National Electric Safety Code (NESC).


The 2007 edition of the NESC published a new provision, which will take effect on January 1, 2009. The provision requires utilities to perform an arc-flash exposure assessment. If the assessment determines that potential exposure is greater than a certain value, the provision requires utility workers to wear clothing that has an arc-flash protection rating of an appropriate value. This is a significant change from the current requirements, which direct the utility industry to take measures to prevent employees from wearing clothing that would cause additional injuries in the case of an arc flash.

This new requirement poses a significant problem for many electric co-ops. Few co-ops have ever done such an assessment and need guidance to conduct one. Without experience in performing these calculations, co-ops have little sense for what a reasonable result will be. Co-ops are unsure as to which segments or elements of the distribution system do or do not need not be analyzed. Without this collective information, all electric co-ops in America may incur significant costs while trying to perform assessments.


To perform the required arc-flash calculations for this study, the authors worked with CRN to identify electric cooperative volunteers who wanted to participate in the study. The requirements of the study included soliciting system load flow models in the Milsoft Windmil analysis software. The ideal models would have individual distribution transformers modeled, as well as detailed information regarding the overcurrent protection devices. In addition, the source and transformer impedances would need to be modeled to allow for accurate calculations. The authors selected two system models and, from each model, selected two feeders for a total of four distribution feeders. The selection requirements included two 12.47 kV and two 24.9 kV feeders. It was also considered optimum to include a feeder that was primarily underground. The final selection criterion involved selecting feeders that contained some transformers with secondary voltages other than 120/240 volts, including 120/208 and 277/480 volts.


The case studies provide detailed results, including a circuit diagram of the feeders, to illustrate how the distance from the source may affect the available fault and incident energy. For each case study, 15 points along the distribution system were selected. Approximately half of the points were on the low side of the distribution transformers. The cases were selected to represent typical distribution feeders found on electric cooperative systems.

  • Case Study 1 – 10 MVA transformer with an operating voltage of 7.2/12.47 kV

  • Case Study 2 – 7.5 MVA transformer with an operating voltage of 14.4/24.9 kV

  • Case Study 3 – 15/20 MVA transformer with an operating voltage of 7.2/12.47 kV

  • Case Study 4 – 12/16/20 MVA transformer with an operating voltage of 14.4/24.9 kV

Readers can use this information to better understand the differences in arc flash calculation methodologies.


Engineering and operations staff, safety and loss control professionals


Engineering, operations, Arc flash, safety, protective clothing, NESC