WHY AN ARC FLASH STUDY?
Commercial and industrial facilities depend on reliable electric power without interruption. This is especially the case for facilities with mission critical systems. Localized generators, UPS systems and other equipment is typically installed prevent a momentary loss of power to critical loads. Because the distribution equipment remains energized during routine maintenance, the workers can be exposed to both electric shock and Arc Flash Hazard. It is therefore essential to provide a work plan that identifies the arc flash hazard category level and Personal Protective Clothing and Equipment to reduce the risk of injury or death.
As part of the electrical safety program, per NFPA 70E ® -Article 110.7 The employer shall implement and document an overall safety program that directs the work activity appropriate for the voltages, energy level, and circuit conditions. If the work involves electrical hazards Article 130.3 states that a an Arc Flash Hazard Analysis shall determine the Arc Flash Protection Boundary and the personal protective equipment that people within the Arc Flash Protection Boundary shall use. As a result of the employer is responsible to promote electrical safety in the work place. As facility managers work to implement arc flash safety it may help to consider the answers to a typical questions:
Why Not Use the Arc Flash Tables Only?
PSCi recommends that the incident energy and Hazard Category Results (HRC) be determined by analysis, and that before the analysis is performed and the arc flash labels are installed, and that the Tables in NFPA-70E be used to determine the PPE requirements whenever working on an energized system during this interim period. Consider some important reasons why the incident energy analysis is recommended.
Section 130.7 (15) “Selection of PPE When Required for Various Task”, provides the information that helps to answer? This section explains that, where PPE is selected in lieu of the incident energy analysis of 130. 5(B) (1), Table 130.7(C)15(a) and Table 130.7(C)15(b) shall be used to determine the HRC and PPE for Shock Hazard for a specific task.
Note: When using these tables, the maximum short circuit current and fault clearing times for the protective devices are assumed for each task category. Section 130.7 (15) also states that, “for power systems with greater than the assumed maximum short circuit current capacity or with longer than the assumed maximum fault clearing times, an incident energy analysis shall be required in accordance with 130.5.”
It is highly recommended to determine the clearing times for the protective devices by analysis for large systems. The clearing time for each protective device is determined in the coordination study that is a major component of the arc flash study. The clearing time is based on the recommended settings to achieve optimum coordination and service continuity.
Also consider the fact that the maximum fault current used to determine the incident energy is assumed. The calculated fault values based on the utility source impedance are often lower than the assumed maximum values in the table or input utility values based infinite bus. The lower fault value, determined by analysis, may require a longer clearing time and produce a much higher incident energy. Using the calculated arcing fault current, the analysis software compares the clear times for a high and low arcing fault tolerance and reports the highest incident energy.
This meets te condition stated in Section 130.5 Information Notes 1 and 2, and 130.5(B) as follows:
130.5 Arc Flash Hazard Analysis (Notes)
Informational Note No.1: Improper or inadequate maintenance can result in increased opening time of the overcurrent protective device, thus increasing the incident energy.
Informational Note No.2: Both larger and smaller available short-circuit currents could result in higher available arc flash energies. If the available short-circuit current increases without a decrease in the opening time of the over-current protective device, the arc flash energy will increase. If the available short-circuit current decreases, resulting in a longer opening time for the over current protective device, arc flash energies could also increase. Informational Note
Protective Clothing and Other Personal Protective Equipment (PPE) for Application with an Arc Flash Hazard Analysis.
Where it has been determined that work will be performed within the arc flash boundary, one of the following methods shall be used for the selection of protective clothing and other personal protective equipment (PPE):
Incident Energy Analysis. The incident energy analysis shall determine, and the employer shall document, the incident energy exposure of the worker (in calories per square centimeter).
To determine the incident energy by analysis also provides documentation and field references, training, and troubleshooting, and tracking system changes. The analysis also provides a means to approach arc flash mitigation when necessary to lower the hazard risk category during maintenance operations, by means of temporarily adjusting the protective device settings. For these reasons an Arc Flash Study with incident energy analysis is the best approach to obtain the Arc Flash HRC Results, the required documentation promote electrical safety in the work place, that can also be used to implement system improvements and increased reliability.