How to Perform an Arc Flash Calculation Study

This article by Jim Phillips provides an overview of how to perform an arc flash study.  It was originally presented at the 2010 NETA Conference.  InterNational Electrical Testing Association.

Arc Flash Calculation Study

Many separate codes, standards and related documents are available regarding electrical safety and arc flash. However, a standardized recommended practice or guide that integrates all of the components into an Arc Flash Calculation Study does not presently exist.

The 2009 Edition of National Fire Protection Association’s NFPA 70E Standard for Electrical Safety in the Work Place, Article 130.3 requires that an arc flash hazard analysis shall be performed to determine the arc flash protection boundary (AFPB) and personal protective equipment (PPE) that people within the AFPB shall use. One method for determining the PPE is to perform an incident energy analysis that is at the heart of an arc flash calculation study.

According to NFPA 70E Article 130.3(B)(1) the Incident Energy Analysis:
“shall determine, and the employer shall document, the incident energy exposure of the worker (in calories per square centimeter). The incident energy exposure level shall be based on the working distance of the employee’s face and chest areas from a prospective arc source for the specific task to be performed.”

Once the incident energy has been determined, typically from performing detailed calculations, it can then be used to select the appropriate level of flame resistant (FR) clothing and associated PPE. The basic concept of PPE selection from the results of an arc flash calculation study is simple. PPE is selected that has an arc thermal performance value (ATPV) greater than the calculated incident energy.

The arc flash protection boundary is the distance from a potential arc source where the incident energy falls to a value of 1.2 cal/cm2. This value is considered to be the point at which the onset of a second-degree burn can occur. When an arc flash hazard exists, persons standing beyond the arc flash protection boundary are not required to wear PPE, although the risk of some injury may still exist.

The most commonly used method for calculating incident energy and the arc flash protection boundary is based on the Institute of Electrical and Electronics Engineers IEEE 1584 IEEE Guide for Performing Arc Flash Hazard Calculations. The IEEE equations were empirically derived from tests and are valid for systems operating from 208 volts up through 15 kV with short circuit currents ranging from 700A up through 106kA. These equations are based on the incident energy from a three phase arc flash. Even though the majority of arc flash events may begin with contact from one phase to ground, it is assumed the conducting plasma that is produced will quickly engulf the other phases and escalate it into a larger three phase arc flash.

Data Collection

The first step of the study process is to obtain data that defines the electrical system characteristics. Depending on the system’s size, age, and complexity, as well as what data is available from previous studies, this step could require a significant amount of manpower.
Data requirements can be broken down into the following categories:

• source/utility company
• impedance data
• overcurrent device data
• equipment parameters

Single Line Diagram and System Modeling

An arc flash study requires an up-to-date single line diagram to document and organize the data. If a single line diagram already exists, it must be verified and updated with any changes that may have occurred. Where a single line diagram does not exist, one will need to be created.
Many power systems will have various operating modes that can lead to different levels of incident energy. The single line diagram can be used to assist in defining the different operating scenarios. In addition to the normal base case condition, “what if” scenarios can be created to determine whether special conditions exist that might produce results worse than the base case.

Bolted and Arcing Short Circuit Current

IEEE 1584 provides equations for calculating the arcing short circuit current based on a known bolted short circuit current obtained from a traditional short circuit study. This arcing current is used to evaluate the upstream overcurrent protective device’s time current characteristic and obtain the clearing time that is used in the incident energy calculations. In addition the system voltage, arc gap length, and whether the arc is in a box or in air must be known.

Coordination Study / Protective Device Clearing Time

The arc duration used in the calculations is based on the clearing time of an upstream protective device and is determined by evaluating time current characteristics. IEEE 1584a recommends evaluating two different levels of arcing current to determine the clearing time. The first uses 100% of the estimated arcing current, and the second uses a magnitude based on 85% of the estimated arc current. Since the actual arcing current can vary from the estimated value, using 85% of the current might reveal that a device operates more slowly depending on its time current curve and setting adjustment. You can then compare the incident energy based on both 100% and 85% of the arcing current to see which yields the worst-case incident energy.

The incident energy calculation is used to determine how much energy can reach a person standing at a specific distance from the source of the arc. Depending on the type of equipment, this “working distance” as it is called, is typically defined as either 18, 24 or 36 inches however other distances may be used. The magnitude of incident energy available during an arc flash is directly dependent on the short circuit current flowing through the air gap and the time it takes an upstream protective device to clear the fault. In general, the greater the short circuit current the greater the incident energy, however this is not always the case.

It is a commonly held belief that the greater the available short circuit current is at a given location, the more damage can occur. When it comes to evaluating a protective device’s interrupting and withstand capability, this is a true statement. However, in the case of arc flash, it is quite possible that a lower short circuit current can cause the upstream protective device to take longer to operate and actually increase the overall incident energy exposure.

The incident energy is also dependent on whether the arc flash occurs in open air or in a box type of environment such as an enclosure. When an arc occurs in open air, energy can radiate spherically in all directions, and less incident energy is concentrated towards the worker. However, when an arc occurs in a box, the energy is focused out of the box towards the worker, resulting in much higher incident energy.

Arc Flash Protection Boundary

The Arc Flash Protection Boundary is considered to be the minimum distance from a potential source of an arc flash where the incident energy falls to 1.2 calories per square centimeter. Since this energy level is the threshold of a second degree burn, it is the minimum distance that people not wearing appropriate PPE should be located when an arc flash hazard exists. Several methods have been defined to calculate the AFPB the IEEE 1584 method is based on taking a known incident energy at a known working distance and calculating the required distance where the incident energy drops to 1.2 calories / cm2.

The results of an arc flash calculation study will normally contain many different AFPBs based on each piece of equipment and its location’s unique characteristics. With such a multitude of boundaries, implementation of the study results can become confusing.
A simpler approach is to adopt a standardized AFPB. This requires reviewing the various AFPB results and adopting the largest boundary within reason. The term “within reason” is used because it is possible to have an unusually large AFPB that may not be realistic. The existing IEEE 1584 formulas use a protective device’s clearing time as one of the many input variables. If the arcing short circuit current is low, a protective device’s time current characteristic may indicate an unusually long clearing time, perhaps tens of seconds.

PPE Selection

Arc flash FR clothing and PPE is designed to protect the worker against the thermal energy exposure. The capability of this protection is defined by the Arc Thermal Performance Value (ATPV) and rated in calories per square centimeter. To properly select the protective equipment, the prospective incident energy that is calculated at each location is compared to the ATPV rating of the PPE. The PPE must have a total ATPV rating greater than or equal to the calculated incident energy.

Arc Flash Warning Labels

Presently there are only minimal requirements regarding the content and format of arc flash warning labels. The NEC and NFPA 70E both require labels warning of the potential arc flash hazard. NFPA 70E further requires that either the PPE or the calculated incident energy is listed on the label. A typical arc flash label generated from the results of an arc flash calculation study includes the calculated incident energy, arc flash protection boundary and PPE which are part of the arc flash hazard analysis.

NFPA 70E also requires that prior to performing live work, a shock hazard analysis must be performed. Although technically not part of an arc flash study, it has become common practice to also list the shock protection information such as voltage, electric shock PPE and approach limits on the label. With all of this information listed and posted visibly on the equipment, the required arc flash and shock hazard analysis data is in plain view for use by the qualified person.

Report and Recommendations to Reduce Incident Energy

Once the various arc flash calculations have been completed, a formal report should be developed that contains, at a minimum, a listing of the input data, study assumptions, calculation results, PPE recommendations, single-line drawing, a description of the study procedure, as well as recommendations on how to further reduce incident energy exposure.

Recommended solutions for reducing incident energy can be divided into two categories depending on cost and ease of implementation. The first category would contain low-cost or no-cost solutions, such as changing overcurrent device settings. The second category could contain changes requiring some level of expenditure in order of priority based on their costs and benefits.

Electrically Safe – The Best Method

OSHA and NFPA 70E both advocate the best solution is to only work on equipment that is placed into an electrically safe condition. This means it has been de-energized, locked out, tested for the absence of voltage and safety grounds installed if necessary. Only then is the system truly safe to work on.

(C) 2010 Jim Phillips/Brainfiller, Inc.