Arc Flash Hazard – Safe practices
The Institute of Electrical and Electronic Engineers (IEEE) published the IEEE 1584 “Guide for Performing Arc Flash Hazard Calculations”. It contains detailed methods and data that can be used to calculate arc flash hazards for the simplest to the most complex systems.
IEEE spent many years developing these methods. They are based on empirical testing of class RK1 and class L fuses, low voltage molded case circuit breakers (MCCB), insulated case circuit breakers and low voltage power circuit breakers as well as theoretical modeling.
Included in IEEE 1584 are spreadsheet programs that simplify the calculation of incident energy and flash-protection boundaries.
IEEE 1584 does not address the Safety related Work Pratices in the same manner as NFPA 70E. It concerns itself primarily with performing the calculations that may be necessary to determine safe practices. The calculation methods in Annex D of NFPA 70E are based on IEEE 1584, but do not contain all the data or descriptions of how these methods were developed.
IEEE 1584 outlines 9 steps necessary to properly perform an Arc Flash hazard calculation //
Collect the system and installation data.
Depending on whether you are doing a complete site analysis or looking at one individual portion, this step can take a few minutes or several weeks to perform. Begin by reviewing the latest up-to-date single line diagram(s) of the equipment or system you are analyzing. If single line diagrams are not available, you must create them! The utility can provide you with the available fault MVA and X/R ratio at the entrance to your facility.
If you generate your own electricity, or if you have emergency or standby generators and large motors, a more detailed analysis must be performed.
In order to calculate the bolted fault current available at the point of your application, you must record on your single line diagram all transformers and their ratings, circuit breakers or fusible distribution circuits and their ratings, MCC’s, and all other equipment between the power source and the area you are concerned with.
All transformer data must be recorded including MVA ratings and impedance, and all overcurrent protective devices must be identified with their specific characteristics or trip ratings recorded.
Determine the system modes of operation.
Most installations have only one mode of operation with one utility connection. However, larger industrial or commercial buildings or manufacturing plants may have two or more utility feeders with tie switching of two or more transformers, or co-generators running in parallel (example shown below).
Each mode can be very complex and require a detailed hazard analysis.
Determine the bolted fault currents.
You can perform hand calculations or use commercially available software programs to calculate the bolted fault currents at all points between the utility and the distribution or control equipment you are analyzing.
It will be necessary to plug in all of the data you have recorded about the transformers, cable sizes and lengths, and type of conduit, etc. used in each installation to determine the bolted fault currents.
Determine the arc fault currents.
After determining the bolted fault currents, IEEE 1584 provides a formula to calculate the predicted arc fault current due to typical arc impedance and other factors.
The predicted arc fault current for system voltages under 1kV depends on the bolted fault current, system voltage, arc gap, and whether the arc would most likely occur in the open air or in an enclosed box configuration.
Find the protective device characteristics and the duration of the arcs.
From the data collected in Step 1 and the predicted arc fault current determined in Step 4, the next step is to establish the total clearing time of the overcurrent protective device immediately on the LINE side of the equipment you are analyzing.
NOTE // This step can be omitted if the overcurrent protective devices are those already tested and listed in the IEEE 1584 document.
Document the system voltages and classes of equipment.
Make sure you document the system voltages and class of equipment such as 15kV switchgear, 5kV switchgear, low voltage switchgear, low voltage motor control centers (MCC) and panelboards, or cable runs.
Select the working distances.
IEEE 1584 has established three typical working distances for different classes of equipment. As previously discussed, incident energy calculations and Hazard Risk Categories will depend on the working distances selected.
Determine the incident energy for all equipment
You can use formulas included in the IEEE 1584 document or commercially available software to calculate the incident energy possible in cal/cm2 at the working distance selected.
Determine the flash protection boundary for all equipment
The formulas given within IEEE 1584 can be used to determine the distance from the arc at which the onset of a second degree burn will occur to unprotected skin. This distance must be established and will vary based on system parameters.
If the overcurrent protective devices (OCPD) are something other than those covered by IEEE 1584, or if the voltage levels and short circuit currents exceed the IEEE 1584 limitations, then the opening times of the overcurrent protective devices must be analyzed and the corresponding Flash Protection Boundary and incident energy must be calculated by another method.
Reference // Electrical Safety Hazards Handbook – Littelfuse