Two-thirds of arc flash incidents result from human error, according to the Occupational Safety and Health Administration (OSHA). Arch flash burns are also one of the top three most common hazards when working with energized electrical equipment. In Part 2 of our Solar Arc Flash Safety Series, we focused on worst-case scenarios and routine troubleshooting. Below we cover a hypothetical operations and maintenance (O&M) scenario as well as guidance on how arc flash labels can be made to be more effective.
Is Bob on your O&M Crew?
Bob is a 30-year card-carrying, well-trained union electrician with ten years of solar experience. Bob is doing the startup of a 12 MW solar array which utilizes a pad-mounted medium voltage recloser for the utility interconnection. When he pushes the button on the 15 kV recloser to energize the system for the first time, it immediately trips off. Rather than investigating, he tries the close button a few more times. He keeps trying until the main substation feeder finally trips and locks out, which also cuts off power to a major manufacturing facility and a hospital. When Bob was finally able to start troubleshooting a few days later, he wandered from the low voltage circuits with his handheld meter over onto medium voltage circuits, with surprisingly minor, non-fatal results as shown in the images below. The arc flash labels may or may not have already been posted on the outside of the open door in the image below, but it’s unlikely that they would have had any impact on Bob’s actions anyway, as there are plenty of the standard factory-applied warning labels visible. Personal protective equipment (PPE) is the lowest point in the Safety Pyramid to adhere to, but you can go lower. Six feet lower.
How We Can Make Arc Flash Labels Effective
The following table shows some typical arc flash energy levels for major PV Project Components based on a review of twelve different PV projects. Most of these projects were in the common community solar space of 2 MW ac and a 1.5 dc/ac ratio:
A perfect arc flash label is not necessarily an effective arc flash label. There are five PPE levels and it is generally expecting too much for everyone to be ready with the correct nuance of PPE for the job. The labels are complicated and “cal/cm2” is not a routine conversational unit of measure. Many facilities recognize this and have implemented a “two level” PPE system where the standard work uniform is a Category 2 ( 8 cal/cm2) and only requires gloves, face shield, and other PPE added on the occasion of live work needing to be performed. The other category is the full Class 4 PPE for any of the rare occasions where Class 3 or Class 4 work needs to be performed. Most people can remember the three conditions which fall into the “routine work, no problem;” “I’d rather not put on that Class 4 suit if I can avoid it;” and “we aren’t allowed to do that job” categories.
Since there is not a lot of variation in arc flash energy levels amongst similar PV projects, it could be easy to establish a common set of practical arc flash labels. Looking at the above table, the first simple rule would be to not allow any hot work on any ac equipment other than the inverter. Once you are past the main ac combiner panel(s), then normal Category 2 work clothes are acceptable, along with the lower risk. And that low risk can be further reduced by work practices and their timing. While the following two arc flash labels are not compliant with the latest NFPA-70E 2021 specifications, they are a lot easier to understand than the one of Figure 3. Maybe even to Bob.
As engineers, we can calculate short circuit currents and produce arc flash labels at any time. We often don’t get asked if all of our hard work will be used correctly in the field because the labels themselves don’t provide any guarantees. Human performance is for more critical to safety than a few little labels.1,2
It would be nice to see an increasing trend of fewer PV owners “checking the arc flash label box” and more owners doing a few minor things to help ensure that exposure to live, energized components is a rare occurrence that requires a thorough review and a written “Hot Work” permit.
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1. Human Performance in Workplace Electrical Safety: An Overview of Practical Human Performance Tools, Nehad El-Sherif; Mike Doherty; IEEE Industry Applications Magazine, 2023, Volume: 29, Issue: 4
2. Frontline Incident Prevention - The Hurdle: Innovative and Practical Insights on the Art of Safety, David McPeak, 2022