Arc Flash

By Erik Kysar, PE, Principal Advisor Brown & Kysar, Inc. (BKI)

All my guys wear 8-cal clothing – that’s plenty! I don’t need to worry about doing the calculations and keeping track of the arc flash amounts, right? Unfortunately, wrong!

Some utilities have completed the calculations and use them, some have the calculations collecting dust on a shelf, and others haven’t done the calculations yet. Who really needs them? Why? And, who cares?

This article will first provide general background and recommendation, followed by some handy look up tables in the appendix for the operations folk.

What’s the purpose

Per some estimates, there are one to two deaths per day in the United States due to arc flash injuries, with approximately 20 burn injuries per day. Occupational Safety and Health Administration (OSHA) has created these rules to increase the chance you will go home to your loved ones tonight, still looking like you did when you left for work in the morning.

The rules are not perfect. As stated in the National Electrical Safety Code (NESC), section 410, the intent is to “… reduce the amount or degree of injury but may not prevent all burns.”

Who needs them

All utilities and all contractors doing work on the system. This is per OSHA 29 CFR 1926.960 (Subpart V) and 29 CFR 1910.269.

If you are a utility, have you provided the arc flash calculations to all contractors working on your system? Or at least provided them the information so they can calculate the arc flash risk? If you are a contractor, it is your responsibility to request the information from the host utility system you are working on.

Arc-flash calculations

OSHA appendix E to both 29 CFR 1926.960 (Subpart V) and 29 CFR 1910.269 provide a reasonable approach to arc flash calculations and amounts. Section 410 of the 2017 NESC also contains a nice overview with look-up tables based on actual testing and calculated values.

The problem with computer programs

Many utilities utilize various computer programs to calculate arc flash hazards such as Aspen, SKM, WindMil, etc. Most of the computer programs are based on IEEE theoretical formulas for arc flash calculations. The problem: theoretical calculations are just that – theoretical.

For example, an IEEE calculation of the arc flash hazard for a 480V three-phase transformer might demonstrate a hazard of over 30 cal/cm². However, testing has shown exposures do not exceed 4 cal/cm² (see NESC table 410-1). As a lineman, would you prefer to wear a “moon-suit” opening up a transformer, or just your typical arc flash clothes? And, being 100% honest, what if it is was 98° F while you were doing it?

Report approach

The simpler, the better. A large, complicated report is almost useless to field personnel. We recommend a simple one-line diagram for each substation, plus look-up tables for typical scenarios on the system. The report may have additional assumptions and calculations, but the aforementioned items need to be easily excerpted and handed to operational personnel in a small booklet.

Calculation approach

The use of OSHA Appendix E and NESC 410-1 is easy to use for all voltage levels. However, there are situations where these sources do not provide the needed information, such as for metal-clad switchgear (arc-in-a-box).

Where the aforementioned two sources do not provide the proper information, there are three solutions per OSHA Appendix E, table 3:

• Use the IEEE standard 1584b for 15kV arc-in-a-box equipment; this is the method most commercial software uses;
• Use ArcPro by Kinectrics for anything over 15kV; this is software based on actual testing and is the basis for the NESC tables 410-2 and 410-3. In fact, the only acceptable method to OSHA for voltages above 15kV is ArcPro;
• Use ArcPro for single-phase faults at any voltage.

Approach to work practices and equipment

Work practices and equipment have a major impact. Here are just a few examples:

• Enable “hot-line” tag on all reclosers and breakers. This can cut arc flash amounts by half or more. The key is in verifying the “hot-line” tag programming is set properly.
• Add arc flash relays in metal-clad switchgear. This can cut arc flash levels from 30 cal/cm² to less than 8 cal/cm².
• Do not work on 480V self-contained meters or panels when energized.

Summary

Take a look at the tables in the appendix. If you do not have completed arc flash calculations, this is a good place to start.

If you do have arc flash calculations, review how useful they are. Are they in a format such that you can excerpt the pertinent information and provide it to your operations folks? Are they easy to follow? Are the values reasonable?

And finally, if the calculations are collecting dust, blow it off, fix the reason it is collecting dust (complexity, overly conservative, etc.), and use it!

After all, going home to your loved one’s tonight is always a good thing!

Appendix

Who needs arc flash calculations

Anyone who has a reasonable likelihood that an electric arc will occur in an employee’s work area. In other words, if the probability of such an event is higher than it is for the normal operation of the enclosed equipment. Examples of work not needing arc flash equipment include substation or equipment readings while not holding conductive equipment and remaining outside the minimum approach distance, or maintaining and operating enclosed equipment with no evidence of impending failure. Examples of work where arc flash is a consideration include racking in a circuit breaker, substation or equipment readings while holding conductive equipment that could fall (even if outside the minimum approach distance), or if equipment shows signs of arcing, overheating or parts of the equipment are showing signs of lack of maintenance. For more detail and additional examples, please refer to OSHA 1910.269 and 1926.960 (Subpart V), Appendix E, Table 1.

Arc flash incident energy values

Following are a few examples of incident energy values taken from the Table 410-1 of the 2017 NESC, and Table 7 of Appendix E in the OSHA regulations. As long as the values are applied as prescribed by the tables, they can be used without further calculation.

• 120/240V or 208V, single-phase or three phase, (except motor control centers) are considered 4 cal/cm² or less
• 480V pad-mount transformers, CT-meters, and control wiring are all considered 4 cal/cm² or less
• 480V pedestals and pull boxes are limited to 8 cal/cm², but self-contained 480V meters can be as high as 20 cal/cm², and 480V panelboards can be greater than 60 cal/cm²
• 12.47kV and 24.9kV live-line tool exposure in open air is 4 cal/cm² or less with fault currents up to 5,000A and clearing times of up to 18 cycles

Personal Protective Equipment (PPE) Requirements

If an employee is exposed to an electric arc hazards, various personal protection equipment must be provided by the employer depending upon the area of the body. Excerpted information from OSHA 1910.269 and 1926.960 (Subpart V) Appendix E are included in the table below.