Suiting up for arc flash safety

Choosing the right amount of Personal Protective Equipment can be a daunting task

by Joseph Weigel

A type of fault that occurs in electric power systems that has received particular attention in recent years is the arc fault, or fault current that travels through the air. This differs from bolted fault current that flows through conductors, busbars or other equipment that are optimally designed to withstand its effects.

Current flow through air releases a high amount of energy in the form of heat and pressure. In the case of an arc-flash, or the uncontrolled release of energy during an arcing fault, the result will usually be equipment damage and injury or death of workers exposed to the fault. Each day in the United States, between five and 10 serious arc-flash incidents result in burn injuries and treatment in burn centers.

Personal Protective Equipment (PPE) is the last line of defense for protecting workers from arc-flash hazards and comes in a range of protective ratings. Choosing how much or how little PPE to wear can be a daunting task. Several methods of calculating potential arc-flash “incident energy” and the flash protection boundary during a release are available, using variables like system voltage, arcing fault current level, fault duration and distance of the worker from the fault source. These calculations should be carefully performed so workers can choose appropriate PPE.

Five degrees of protection
Five categories of PPE are defined by NFPA 70E-2004 based on the degree of protection each provides. (See Figure 1) PPE is assigned an Arc Rating based on calories per square centimeter (cal/cm2), and defines a material’s maximum incident energy resistance.

Non-fire Resistant (FR) cotton has no Arc Rating and is only allowable at locations or working distances demonstrating extremely low available incident energy potential. Once a worker enters the flash-protection boundary, things change — as the energy level increases, the Hazard/Risk Category increases as well. Non-fire resistant clothing, like synthetic blends, are forbidden completely because they can easily ignite and/or melt into the skin and aggravate a burn injury.

There are many methods for calculating arc-flash hazard potential, ranging from theoretical models to code-, standard- and equipment-specific equations and tables. The next few paragraphs break down the most important to consider.

Section 130.3(A) of NFPA 70E-2004 includes equations that can be used to calculate flash-protection boundary distances for systems operating at 600 volts or less. The flash-protection boundary is characterized as the point where the incident energy level equals 1.2 cal/cm2, which is the threshold of energy required for a second-degree burn.

However, arc-flash hazard calculations are extremely complex and should be done by an electrical engineer familiar with calculation methods. Section 130.7(C)(9)(a) provides a method that requires little or no calculation, a table with Hazard/Risk Category values for typical work tasks for common equipment. These Hazard/Risk categories are estimates based on actual calculations, but strict attention should be paid to footnotes referenced in the table — the categories are conservative and in most cases will overstate the requirement. A worker can simply find the appropriate work task in the table, but for system conditions that fall outside the defined fault current ranges and fault clearing times, the tables shouldn’t be used to choose PPE. Additionally, for a few conditions that do fit the system, the recommended PPE may be inadequate.

IEEE Std 1584-2002, also known as the “IEEE Guide for Performing Arc-flash Hazard Calculations,” currently provides the most comprehensive set of equations for calculating incident energy levels and flash-protection boundaries, presenting equations that cover systems at voltage levels ranging from 208 volts to 15 kilovolts and for available bolted fault currents ranging from 700 amps to 106 kiloamps, which will cover a majority of low- and medium-voltage installations. Simplified equations also are provided for several common protective device types, including current-limiting Class RK1 and Class L fuses up to 2,000 amps and various types of circuit breakers ranging from 100 to 6,300 amps.

Also extremely helpful are equipment-specific equations, such as those developed for Square D Masterpact NW and NT low arc-flash (LF and L1F) circuit breakers. General equations provided in IEEE Std 1584 can’t possibly reflect the performance of every protective device in every possible situation; they may not adequately portray current-limiting action of fuses or circuit breakers and can provide overly conservative results.

Tips for success
It bears mentioning that no single calculation method is applicable to all situations. However, there are several principles an engineer can follow to ensure he or she arrives at the best results.

First, it’s best to verify that actual system conditions fall within the chosen method’s range of applicability. Many calculation methods are at least somewhat based on equations derived from test results. They’re valid over a range of system conditions where that testing was done but can’t be extended to other situations with a high degree of confidence.

The most recent test results, industry standards and calculation methods are more likely to accurately represent actual hazard levels than older methods. The latter may be based on smaller sets of test data or may be applicable over a smaller range of system conditions.

Knowing which device clears the fault and using realistic fault current values are also crucial. When determining a location’s arc-flash hazard level, two major variables to consider are the bolted fault current level at that location and the characteristics of the upstream protective device.

Quantifying variables like system voltage, level of arcing fault current and fault clearing time are significant parameters in determining a system’s arc-flash hazard potential. But also demanding consideration are the working distance, the distance from the electric arc to a worker’s face and body; the bus gap, the gap between phase conductors or from phase to ground; equipment configuration, because incident energy is amplified when it reflects off an equipment enclosure (and toward a worker) than through the air; and system grounding, as IEEE calculations differ slightly depending on whether it’s solidly grounded or ungrounded.

Being aware of motor contribution is also important. The level of arcing fault current at a location depends on the level of bolted fault current, so when motor loads are present, their contribution may add to the arcing fault current. In situations where motor contribution counts for a significant portion of total available fault current, use IEEE 1584 general equations because IEEE 1584 simplified equations and device-specific equations do not take motor contribution into account.

In the same vein, use device-specific equations rather than general equations. As mentioned previously, when equations based on testing of specific devices, such as the IEEE 1584 equations for current-limiting fuses or Square D equations for low arc-flash Masterpact circuit breakers, they should be used rather than general calculation methods to provide more accurate device-specific data. If accurate data about a breaker’s trip characteristics are available, it should be used along with IEEE 1584 general equations rather than the simplified circuit breaker equations.

Finally, when comparing results from different calculation methods, a worker should be aware that even those based on the same set of test data may have variations that make it impossible to directly compare the results.

The last line of defense
PPE is an absolute must when working with electrical systems and, when properly utilized, the various available calculation methods are a great means of determining the right amount of PPE to wear.

However, it bears repeating that PPE is a last line of defense, and not a replacement for safe work practices or engineering controls that can reduce a worker’s exposure to arc-flash hazards. For example, equipment should be placed in an electrically safe work condition whenever possible.

Joseph Weigel is a product manager for Square D Services marketing. He has been very involved in the development of Schneider Electric’s Arc Flash Safety program to educate customers on emerging arc flash safety standards. He can be reached at joseph-h.weigel@us.schneider-electric.com. For more information, visit www.squareD.com.

This article appeared in the April/May 2007 issue of MRO Today magazine.  Copyright 2007.