In previous articles, I’ve talked about incident energy and how it defines arc flash risk. Incident energy, measured in calories/cm2, is the amount of thermal energy contained in an arc flash event. It quantifies the risk associated with an arcing fault. The higher the incident energy, the higher the risk.
As a result, when we conduct an arc flash assessment, we want to see low incident energy numbers on the labels for the equipment. Unfortunately, we often see high numbers, which not only indicate high risk, but also warrant the use of more PPE.
However, what if we could lower the incident energy level in equipment that is high risk? In many cases we can! Let’s explore this in today’s article on arc flash mitigation.
Arc Flash Mitigation is one of the most important phases of an arc flash assessment because it identifies areas in the electrical system in which we can reduce the risk by reducing the incident energy at the equipment.
Many people think an arc flash assessment is only designed to place warning labels on electrical equipment. This is not true. Warning labels are important, but they are not the only benefit of an arc flash assessment. While arc flash labels quantify the risk by providing the worker with valuable knowledge about the electrical hazards involved with the work, mitigation actually looks for ways to lower the risk.
For this reason, mitigation is always included in our standard arc flash risk assessment at Mitchell & Lindsey. In most of our projects, we are able to identify multiple areas in which we can lower the incident energy. In many of these cases, the reduction is significant. It’s not unusual for us to be able to lower the incident energy from high risk levels such as 30 or 40 calories/cm2 to less than 1.2 calories/cm2. This not only lowers the risk, but also allows the worker to don less PPE.
I’ll use an example from one of our recent arc flash risk assessments to illustrate. Our client is a large hospital in the Midwest. The equipment is a 120/208V – 1200-amp switchboard. Current conditions have the incident energy at this switchboard as 39 calories/cm2 and an arc flash boundary of 159 inches (13 feet). If you are familiar with our PPE Charts (downloadable here), you know that working on this equipment live is a very high-risk task. By running mitigation scenarios, we were able to identify a way to lower the incident energy to 0.7 calories/cm2 with an arc flash boundary of 13 inches. To do this, we simply change the instantaneous setting on the upstream breaker from 10,000 amps to 5,000 amps. That is a huge difference, not only in risk, but in the PPE requirements.
• Incident Energy: 39 calories/cm2
• Arc Flash Boundary: 156 in. (13 ft.)
• PPE:Arc Flash Suit and Arc Flash Hood (40 calorie rating)
• Incident Energy: 0.7 calories/cm2
• Arc Flash Boundary: 13 in.
• PPE: Arc Rated Clothing not required. Shock protection still applies.
Although mitigation is not possible in all situations and results may vary, it is often possible to lower the risk drastically through simple changes in the electrical system settings and/or design. It is not enough for us to just identify the risks when doing an arc flash risk assessment. We also want to lower the risk whenever possible.
Thank you for your time.
Mitchell & Lindsey offers Arc Flash Risk Assessments and Electrical Safety Training. If we can be of service to you in these areas or if you have any questions about this article, please reach out to me at the email or phone number below.
Bobby Lindsey – CESCP
Mitchell & Lindsey – President
M: (502) 836-4217