Their Last Line of Defense

Various calculation methods are available to aid in workers' selection of PPE.

SHORTcircuits and faults in electric power systems are nothing new--and are typically damaging and even deadly. One type of fault that has received particular attention in recent years is the arc fault, or fault current that travels through the air, which differs from the bolted fault current that flows through conductors, busbars, or other equipment optimally designed to withstand its effects.

Current flow through air releases a high amount of energy in the form of heat and pressure. When it's controlled, as in welding applications, it can be a great asset. But when it's not, as in the case of an arc flash or the uncontrolled release of energy during an arcing fault, the result usually will be equipment damage and injury or death of workers exposed to the fault. As many as five to 10 serious arc flash incidents that result in burn injuries and treatment in a burn center occur each day in the United States.

Personal Protective Equipment is the last line of defense for protecting workers from arc flash hazards and comes in a range of protective ratings. But how much PPE is enough to protect a worker . . . or too much? Several methods are available for calculating potential arc-flash "incident energy" and the "flash protection boundary" during a release. NFPA 70E-2004 Article 100-I defines incident energy as the amount of energy impressed on a surface a certain working distance from a source generated during an electrical arcing event; it defines flash protection boundary as an approach limit at a distance from exposed live parts within which a person could receive a second-degree burn if an electrical arc flash were to occur. Variables such as system voltage, arcing fault current level, fault duration, and distance of the worker from the fault source must figure in the meticulous calculations necessary for workers to choose appropriate PPE.

Five PPE Categories
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.

Figure 1

Hazard/Risk

Category

Clothing Description*

Number of Layers

Minimum PPE Arc Rating (cal/cm2)

0

Untreated natural fiber clothing

1

N/A

1

Fire-resistant shirt and fire-resistant pants

1

4

2

Cotton underwear plus Category 1

2

8

3

Fire-resistant coverall over Category 2

3

25

4

Multi-layer flash suit over Category 2

4

40

* Refer to NFPA 70E-2004 for complete clothing descriptions

Non-fire-resistant (FR) cotton has no arc rating and is allowable only 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, such as synthetic blends, is forbidden completely because this clothing can easily ignite and/or melt into the skin and aggravate a burn injury.

Many Calculation Methods
There are plenty of methods for calculating arc flash hazard potential. They range from theoretical models to code-, standard-, and equipment-specific equations and tables. Among the most important to consider are:

  • NFPA 70E-2004. Section 130.3(A) of NFPA 70E-2004 includes equations that can be used to calculate flash-protection boundary distances for systems operating at 600V or less. 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 some conditions that do fit the system, the recommended PPE may be inadequate.
  • IEEE Std 1584-2002. This standard, 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. It presents equations that cover systems at voltage levels ranging from 208V to 15 kV and for available bolted fault currents ranging from 700A to 106kA, 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 2,000A and various types of circuit breakers ranging from 100A to 6,300A.
  • Equipment-specific equations. Though IEEE 1584 is perhaps the state-of-the-art calculation method, also extremely helpful are equipment-specific equations. General equations provided in IEEE 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.

Points to Consider
Keep in mind that no single calculation method is applicable to all situations. However, there are several principles engineers can follow to ensure they arrive at the best results, so a worker can choose the appropriate amount of PPE and be safer on the job.

1. 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.

2. Use newer methodology. New 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.

3. Know which device clears the fault, and use realistic fault current values. 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. If a worker is operating in a system with a feeder breaker in the main switchboard and a main breaker and several feeder breakers in the panel, determining which breaker acts to clear the fault is paramount as an input variable in a calculation method.

4. Quantify the variables. System voltage, level of arcing fault current, and fault clearing time are very 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) rather than through the air; and system grounding, because IEEE calculations differ slightly depending on whether it's solidly grounded or ungrounded.

5. Be aware of motor contribution. 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 accounts 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.

6. Read the fine print. 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.

7. Use device-specific equations rather than general equations. As alluded to previously, equations based on testing of specific devices should be used rather than general calculation methods to provide more accurate device-specific data. If accurate information about a breaker's trip characteristics is available, it should be used along with IEEE 1584 general equations rather than the simplified circuit breaker equations.

PPE: Not a Replacement for Safety
PPE is an absolute must when working with electrical systems. 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--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. For further information on safe work practices, consult NFPA 70E-2004.

This article appeared in the May 2006 issue of Occupational Health & Safety.

This article originally appeared in the May 2006 issue of Occupational Health & Safety.

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