Q: In the absence of an arc flash study, what PPE and boundaries should we advise our contract electricians to follow?
A: As per NFPA 70E, having an arc flash study done is the responsibility of the building owner or the equipment owner.
Often, this is not complete or communicated with the contractor. While the lack of a study could make the equipment owner liable in the event of an arc flash, the contractor must still do his or her due diligence on the job. Some contractors have begun to ask the building/equipment owner sign a limit of liability for the contractor if their arc flash PPE is not adequate. But what else can the contractor do to protect his own workers, save forcing the equipment owner to get a study?
Let’s look at two common methods to determine what arc-rated Personal Protective Equipment (PPE) is required and what boundaries to set. These boundaries apply to both low- and medium-voltage distribution systems in industrial plants.
Basics of Arc Flash
When workers undertake tasks, such as switching a breaker or disconnect, opening a door or cover of a motor control center (MCC), and performing work inside an energized industrial control panel, those workers are performing “electrical work”. When performing OSHA and NFPA 70E approved Energized Electrical Work, workers shall be provided with and utilize the appropriate PPE for protection against the shock or arc-flash hazard.
If the equipment fails, the ensuing arc flash will produce thermal and mechanical energy. The energy produced depends on enclosure dimensions, distribution system short circuit current, spacing between the buses, the protection system adopted, clearing times, maintenance, etc. Injury to the worker can occur based on the worker’s distance from the energy source, his or her orientation with regards to projectile, and the PPE being used at the time.
Two Methods Used
There are presently two popular methods used to determine the PPE and working distance. Both are based on the equipment and distribution system inputs.
1) The incident energy analysis method described in the NFPA 70E:2018 130.5(G)
2) The arc flash PPE category method described in NFPA70E:2018 130.7(c)(15)
But which method is more correct? Which method should be used?
Incident Energy Analysis
The incident energy analysis method is based on a standard that is normally embedded into a software package. These software packages offer to model the entire network and provide a relatively accurate representation for each piece of equipment if modeled correctly. A few clicks later, all arc flash incident energy values (cal/cm²), required PPE, working distances, and equipment labels are available. These benefits (strangely) introduce several of the challenges with the incident energy analysis method:
- The method is based on the IEEE 1584 standard (excluding ArcPro) and the ins & outs of the standard must be known
- The software packages are relatively expensive to procure, not used regularly, require substantial training before use, and generally require an annual licensing & maintenance fee
- Requires the expertise of an engineer or similar background who is knowledgeable on power systems and the theory of all that is arc flash related
- The NFPA 70E committee understood that many end-users may not have such resources at hand or not need such software packages. Some may have to plan a year or so advance before hiring a consultant. What can employers do in the interim?
PPE Category Method/Table Method
This leads to the NFPA 70E table method. The table method is based indirectly on the IEEE 1584:2002 method as well. It uses conservative input assumptions and directs the user to the arc-rated PPE categories. Unlike, the incident energy analysis method, the PPE category method does not produce the incident energy in cal/cm².
There are two major considerations with using the PPE category method: 1) the maximum fault clearing time (trip time), and 2) the maximum short-circuit current. Unfortunately, this method is limited and doesn’t cover the entire power system because of the short circuit and fault clearing time limitations.
This is a rather outdated method that is believed to have stuck around largely because of the cost and complexities associated with the incident energy analysis method. Also, pre-dating the IEEE1584 method and arc flash fabric testing standards, arc -rating of fabrics were categorized in terms of the number of layers.
NOTE: Fabrics have since evolved. Certain performance fabrics do not conform to the layers/rating principle. (As an aside, read this blog for the background on the “PPE Table” in NFPA 70E: How Did the ARC/HRC PPE Categories in NFPA 70E Come to Be?
Another challenge is determining the maximum clearing time. Most folks who use the PPE category method may not necessarily be knowledgeable on power system protection, a requirement for determining clearing times. Clearing time is dependent on short-circuit current. Arcing current isn’t equal to the maximum calculated short-circuit current, so that tends to complicate matters as well. Fortunately, the PPE category uses the MAXIMUM short-circuit current and fault clearing times.
Final Thoughts
Considering the above information, the incident energy analysis method should be preferred. Because it is resource-intensive, some end-users choose to use the PPE category method. It is suggested that end-users seek assistance on determining the power system parameters before using the PPE Category method.
Here are some resources to get you started:
What To Do NOW
Research has shown that having some AR PPE is better than no AR PPE. Waiting for an arc flash hazard analysis may not be the most suitable solution. It is suggested that employers roll out daily workwear rated between 8-12 cal/cm². Provide a face-shield, balaclava, and a good pair of leather gloves for arc flash protection. (Note: Shock requirements may apply). Then provide a flash suit ensemble for higher risk areas, like power/main breakers, and switchboards.
Although a 40cal/cm² suit is common, try going for a 65cal/cm² or higher. The cost difference is much less than the cost of an injury! In addition, avoid operating main service breakers (first breaker downstream of a power transformer) from the local position. Rather, use remote switching / racking devices or request the utility to switch the high voltage side.
Electrical contractors often enter facilities that have not estimated the required arc rated PPE. In this case, there will be no time for the contractor to perform an incident energy analysis and will rely on the PPE category method. It is suggested that contractors consider working with consultants to analyze several common scenarios and determine what level of arc-rated protection will work best.
Remember, in addition to arc-rated PPE, training and a Written Electrical Safety Program must form an integral part of the safety requirements.
For further resources see our Tables and Tools Class a guide to arc flash study tools for engineers
If you need an arc flash study or a fault current and coordination study, e-Hazard can help there too.
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