Conducting Arc Flash Hazard Analysis Guidelines

1. Introduction
2. References
3. Definitions and Abbreviations
4. Standard’s Requirement
5. Method for Conducting Arc Flash Analysis

5.1 Phase-1: Collect and Enter System Information, Determine System Modes of Operation and Perform the Short Circuit Analysis
5.2 Phase-2: Perform the Coordination Study and Determine the Arc Clearing Time
5.3 Phase-3: Perform the Arc Flash Study

Conducting Arc Flash Hazard Analysis Guidelines

1. Introduction Arc Flash Hazard Analysis

1.1 Purpose

The purpose of this article is to provide guidelines on the methods for the calculation of arc flash incident energy and arc-flash boundaries in electrical systems to which workers may be exposed.

The main objective of this effort is to provide a practical safe working area for workers in respect of arc flash hazard (a dangerous condition associated with the possible release of energy caused by an electric arc) that may arise during activities such as installation, operation, maintenance and commissioning or energization of electrical equipment.

1.2 Scope

This article is intended to give general guidelines on the techniques in conducting arc flash hazard analysis and identifying the incident energy levels, arc flash protection boundaries, required PPE’s, warning labels and recommendations to limit the abnormal levels of incident energy. The guidelines will also help in determining the arc flash hazard distance and the incident energy to which employees could be exposed during their work on or near electrical equipment. It is also intended to address methods and technologies for reducing arc flash hazards.

2. References Arc Flash Hazard Analysis

Industry Codes and Standards

National Fire Protection Association

NFPA 70 (NEC) National Electrical Code
NFPA 70E-2009 Standard for Electrical Safety in the Workplace

International of Electrical and Electronics Engineers (IEEE)

IEEE 1584-2002 IEEE Guide for Performing Arc-Flash Hazard Calculations

3. Definitions and Abbreviations of Arc Flash Hazard

The following definitions are extracted from IEEE 1584 and NFPA70E with brief description for clear understanding purposes:

Arcing Fault Current (Ia): A Fault current flowing through an electrical arc plasma, also called arc fault current and arc current. Arcing fault is measured in (kA).
Arc Flash: A rapid release of electrical energy due to an arcing fault between electrical phases, neutral or ground, resulting in a plasma arc through the adjacent surrounding air. An arc flash is sometimes inappropriately called a “flashover.”
Arc Flash Hazard: A dangerous condition associated with the release of energy caused by an electric arc.
Arc Flash Suit: A complete flame resistant clothing and equipment system that covers the entire body. This includes pants/bib, jacket, and beekeeper-type head protection hood fitted with a face shield.
Bolted Fault Current (Ibf): A short circuit or electrical contact between two conductors at different potentials in which the impedance or resistance between the conductors is essentially zero. The bolted fault current is 3 phase fault measured in symmetrical RMS in KA).
Boundary, Limited Approach: An approach limit at a distance from an exposed energized electrical conductor or circuit part within which a shock hazard exists.
Boundary, Prohibited Approach: An approach limit at a distance from an exposed energized electrical conductor or circuit part within which work is considered the same as making contact with the electrical conductor or circuit part.
Boundary, Restricted Approach: An approach limit at a distance from an exposed energized electrical conductor or circuit part within which there is an increased risk of shock, due to electrical arc over combined with inadvertent movement, for personnel working in close proximity to the energized electrical conductor or circuit part.
Conductor gap (G): The distance between electrodes (buses) and between electrodes (buses) and back wall of box.
Electrical Safety: recognizing hazards associated with the use of electrical energy and taking precautions so that hazards do not cause injury or death.
Fault Clearing Time (FCT): Total time needed by protective device to completely open and clear arc fault. This time is combination of breaker opening and relay sensing times.
Flame-Resistant (FR), Flame Resistant Clothing (FRC): Materials or clothing with the inherent characteristic that combustion is prevented, terminated, or inhibited following the application of a flaming or non-flaming source of ignition, with or without subsequent removal of the ignition source.
Flash Hazard Analysis: A method to determine the risk of personal injury as a result of exposure to incident energy from an electrical arc flash.
Flash Protection Boundary (DB): The arc flash protection boundary is the distance at which the incident energy equals 1.2 calories per centimeter squared (cal/cm²).
Incident Energy (E): The amount of energy impressed on a surface, a certain distance from source, generated during an electrical arc event. Incident energy is measured in (J/cm²) or (Cal/cm²).
Incident Energy Normalized (En): This is incident energy (E) normalized for time and distance.
Working distance (mm): The dimension between the possible arc point and the head and body of the worker positioned in place to perform the assigned task.

4. Standard’s Requirement for Arc Flash Hazard Analysis

Arc flash hazard analysis to be conducted during the design of all new electrical equipment rated higher than 240 volts and the transformer supplying is rated more than 125 kVA, in accordance with NFPA 70E and IEEE 1584. This is to determine the arc flash protection boundary, the incident energy a worker may be subject to, and the personal protective equipment (PPE) to be worn. The analysis shall be performed in conjunction with both short-circuit and protective device time-current coordination analyses during the detailed design phase. The arc flash studies can be considered a continuation of the short circuit and coordination aspects of a power system, since the results for each are required to assess flash hazards. However, a preliminary analysis shall be conducted at early stage to identify the scope and possible mitigations strategies.

The objectives of the arc flash hazard study can be summarized as follows:
 Determine Incident Energy Levels to ensure compliance to standards
 Establish Flash Protection Boundaries
 Provide Required Personal Protective Equipment (PPE)
 Create Instructive Labels
 Implement Safety Measures
 Prevent Loss of Manpower

The most effective means to protect workers from an arc flash hazard is to de-energize the circuit in accordance with the requirements in NFPA 70E before attempting to work on or near energized equipment. However, there are some tasks where work must be performed on energized equipment such as electrical isolation, operating breakers, switches or starters, racking breakers in/out, testing electrical circuits, applying safety grounds, etc.

5. Method for Conducting Arc Flash Analysis

The IEEE 1584 equations method is recommended to be used as the most precise method for conducting an electrical flash hazard analysis. This method uses equations to accurately calculate the arc flash protection boundary and the incident energy level exposure to the worker. The IEEE 1584 arc flash calculation includes nine steps:

Step 1: Collect the system and installation data.
Step 2: Determine the system modes of operation.
Step 3: Determine the bolted fault currents.
Step 4: Determine the arc fault currents.
Step 5: Find the protective device characteristics and the duration of the arcs.
Step 6: Document system voltages and classes of equipment.
Step 7: Select the working distances.
Step 8: Determine the incident energy for all equipment.
Step 9: Determine the flash-protection boundary for all equipment.

However, to simplify the process, we have divided the analysis in to 3 major phases which are:
 Phase-1: Collect and Enter System Information, Determine System Modes of Operation and Perform the Short Circuit Analysis.
 Phase-2: Perform the Relay Coordination Study and Determine the Arc Clearing Time.
 Phase-3: Perform the Arc Flash study.

5.1 Phase-1: Collect and Enter System Information, Determine System Modes of Operation and Perform the Short Circuit Analysis

5.1.1 Collect and Enter System Information

This step is the most critical one to perform. The main difference between an arc hazard assessment and other studies is that you may need to model the system in more details, increasing the data collection time and study effort. The following items shall be considered during data collection and modeling phases for conducting the detailed arc flash hazard analysis:

o Review the single-line diagrams and electrical equipment site and layout arrangement with people who are familiar with the site. All associated diagrams have to be updated to show the current system configuration and orientation before the arc-flash study can begin. Accurate, up-to-date information on the facility electrical system is absolutely essential to the analysis.

o Model Underground cables with actual lengths. Conductor impedance’s and X/R ratios should be modeled for all equipment in order to obtain realistic short circuit values.
o Model Transformer impedances in accordance to actual nameplate impedance values.
o Verify Motor loads and breaker settings with electrical drawings.
o Obtain accurate utility data for both maximum and minimum fault contributions to determine the worst case operating scenarios.
o Consider system grounding as it can cause incident energy calculation differences when using the IEEE 1584 methodology.

5.1.2 Determine System Modes of Operation

The worst case for arc flash energy may not be the current operating configuration. All possible configurations shall be evaluated during the analysis. Some examples are on-site generation used in sole-source and parallel with the utility configurations, and tie circuit breakers to be modeled in every possible condition. The number of scenarios required increases rapidly with the system’s greater complexity. Therefore, a wide range of possible operating configurations have to be obtained and evaluated to facilitate the determination of the worst case scenario. The contribution of connected motors to the fault will increase the hazard as well. Assessment should include both the normal operating condition as well as the worst possible arc flash scenario.

For plants with simple radial service from the utility, only one mode of operation typically exists, normal. However, for larger plants, there may be multiple modes of operation. These may include:
o Multiple utility sources that may be switched in or out of service.
o Multiple generator sources that are operated in parallel or isolated depending on the system configuration.
o Emergency operating conditions.
o Maintenance conditions.
o Tie breakers which can be operated open or closed.
o Large motors not in operation.

What is important to realize is that each one of these conditions may change the level of short circuit current, which in turn changes the clearing time of the protective devices. These changes can have a significant impact on the arc-flash hazard and the PPE requirements for each piece of equipment.

In summary, Arc Flash assessment should include each operating mode for the power system to insure correct incident energies are calculated for all system conditions.

5.1.3 Perform the Short Circuit Analysis

A short circuit study shall be performed to verify all equipment duties in the system. The short circuit study shall verify the system electrical equipment is properly rated to withstand and interrupt the expected bolted and arcing faults in the system. Improperly rated and applied equipment may not protect personnel against arc flash hazards even if properly applied PPE is used.

For the arc flash study, using overly conservative short circuit data can yield non-conservative results since a very high fault current may produce a very short arc duration due to the operation of instantaneous trip elements. The highest fault current does not necessarily imply the highest possible arc flash hazard because the incident energy is a function of arcing time, which may be an inversely proportional function of the arcing current.

5.2 Phase-2: Perform the Relay Coordination Study and Determine the Arc Clearing Time

 All protective device coordination data shall be modeled on the one-line diagram and in the equipment database. Protection schemes utilized in the system such as differential and directional relays shall be included.
 Equipment data for protective device characteristics; type of device, existing settings for relays, breakers and trip units, rating amps, time-current curves, total clearing time.
 A coordination study is the examination of the electrical system and available documentation with the goal of ensuring that over-current protection devices are properly designed and coordinated. Over-current protective devices are rated, selected and adjusted so only the fault current carrying device nearest the fault opens to isolate a faulted circuit from the system. This permits the rest of the system to remain in operation, providing maximum service continuity. The study consists of time-current coordination curves that illustrate coordination among the devices shown on the one-line diagram.

 The study will show any potential problems in the protective device settings that affect either selective operation or personnel protection and will provide recommendations for changes to the settings.

Table 1 shows the required data to be entered for the protection device:

5.3 Phase-3: Perform the Arc Flash Study

The incident energy from an arc-flash event impressed upon a worker is mostly dependent upon:
 Available fault current at the arcing terminals
 Protective device clearing time for the arcing fault
 The distance of the worker to the arcing terminals.
Incident energy is affected to a lesser extent by:
 Operating voltage
 Gap length
 Type of system grounding.

5.3.1 Difference between NFPA 70E and IEEE 1584 Calculations

NFPA 70E method estimates incident energy based on a theoretical maximum value of power dissipated by arcing faults. This is believed to be generally conservative. However, the table approach must be used with caution as NFPA 70E states that the table method can be used only if the conditions, including the specific task to be accomplished, are within the assumed values for short circuit capacity and fault clearing times.

In contrast, IEEE 1584 estimates incident energy with empirical equations developed from statistical analysis of measurements taken from numerous laboratory tests. The IEEE method was intended to be more realistic.

Table 2 – Conditions for which the IEEE 1584 Equations are Applicable

At the conclusion of the Arc Flash Hazard Analysis, the study shall determine the following important parameters:

o Arcing current
o Arcing time
o Incident energy
o Flash protection boundary
o Hazard/risk category
o Appropriate personal protective equipment (PPE)
o Arc flash warning labels
o Recommendations for arc flash energy reduction

5.3.2 Arcing Current

The predicted three-phase arcing current must be found so the operating time for protective devices can be determined.
Following equations are utilized to determine the predicted three-phase arcing current:

The high-voltage case makes no distinction between open and box configurations.
Calculation is recommended to be conducted at arcing current of 100% and 85% of the IEEE 1584 estimate for low voltage equipment.

Table 3 – Typical Gap between Conductors and X Factors

5.3.3 Arcing Time

Estimate arcing time from the protective device characteristics and the contributing arc current passing through this device for every branch that significantly contributes to the arc fault. The two seconds are the maximum clearing time for arc flash calculations. If the time is longer than two seconds, consider how long a person is likely to remain in the location of the arc flash.

5.3.4 Incident Energy

First find the log10 of the incident energy normalized. This equation is based on data normalized for an arc time of 0.2 seconds and a distance from the possible arc point to the person of 610 mm.

Table 4 – Classes of Equipment and Typical Working Distances

5.3.6 Hazard/Risk Category

Hazard/risk category (HRC) is specified as a number representing the level of danger, which depends upon the incident energy. Table 5
provides the classification guide for the risk category number.

Table 5 – Hazard/Risk Category Levels

5.3.7 Appropriate Personal Protective Equipment (PPE)

Based upon the hazard assessment the appropriate PPE must be selected and provided to the workers. Workers must wear the PPE properly, provide care and maintenance of the PPE, inspect it before every use and dispose of it after its useful life has expired.
Properly selected PPE is designed to minimize the worker’s risk of sustaining more than a second- degree burn during an electrical arc flash incident (defined as occurring at 1.2 cal/cm²). Choosing inadequate PPE can lead to even more severe burns. Injuries occur because of flash burns from the heat generated by the electric arc and by flame burns from the ignition of clothing or other combustible materials. Clothing that is not flame retardant (FR) can increase the severity of burns. Even FR fabric can ignite, but it will self-extinguish after the external ignition source (flame or arc) has finished burning. Non-FR fabrics that ignite (start to burn) will continue to burn even after the ignition source has been removed.
Employees must wear and be trained in the use of appropriate protective equipment for the possible electrical hazards with which they may face.

5.3.8 Arc Flash Warning Labels

The label shall be machine printed, with no field markings and include the following information:
o Location designation
o Nominal voltage
o Flash protection boundary
o Hazard risk category

o Incident energy
o Limited approach boundary
o Restricted approach boundary
o Prohibited approach boundary
o Source protective device

5.3.9 Recommendations for Arc Flash Energy Reduction

According to SAES-P-100, the maximum allowable incident energy shall not exceed 8 Cal./cm² to qualify for the Hazard Risk Category (HRC) up to 2. In case, this incident energy level is exceeded, then other mitigation methods shall be evaluated and applied. Exposure to arc flash can be limited in three ways:
1. Avoiding arc flash accidents.
2. Reducing the level of incident energy.
3. Using the proper personal protective equipment (PPE).
There are several ways and strategies to reduce the incident energy levels through reduction of the exposure time as accidents may occur despite precautions taken to avoid them. These include but not limited to:
o Bus differential protection
o Reducing safety margin for relay and breaker operation
o Zone selection interlock
o Instantaneous tripping during maintenance using maintenance switches
o Arc detection system through optical sensors that trip the breaker in the event of arc flash.

“Your safety depends upon your own actions”