At PG&E’s outdoor lab at the Applied Technology Services building in San Ramon, an arc flash is simulated in a 21-kilovolt pad-mounted switch, which is a common electrical box.
PG&E stages explosions—specifically, arc flash incidents.
Creating an Arc Flash event helps to determine what type of protective clothing an electrical employee must wear and how far they must stand from the equipment they’re working on and the extent of damage received based on distance and protective clothing.
PG&E has tailored its safety rules accordingly, and research has led to revisions this year in the National Electric Safety Code, allowing other utilities to follow PG&E in protecting workers against arc flashes.
What exactly is an arc flash? It’s essentially a short circuit that causes a sudden release of electricity through the air. The arc and resulting blast are intensely hot and bright, and can cause serious burns to anyone in the blast area. Although an Arc Flash can happen spontaneously, the more common cause is human error.
In 2005 PG&E engineer Marcia Eblen began her arc flash research, as PG&E began outfitting its electrical workers with flame-resistant (FR) clothing—because she wasn’t satisfied with industry-accepted formulas for predicting arc flash heat. Her thermal calculations for flashes in large customers’ meters were three times higher than protective limits of the best FR clothing and were not supported by actual incidents across the industry.
So, she convinced PG&E’s meter department to donate 100 480-volt meters and began simulating arc flashes in the Applied Technology Services outdoor lab in San Ramon. The maximum energy measured during these test blasts was significantly lower, much more consistent with the types of injuries workers had seen over the years. Turns out, the formula assumed the flash would last longer, but arcs in these types of meters quickly self-extinguish.
Those tests got the ball rolling for PG&E’s arc flash research. In 2009, the utility teamed with the Palo Alto-based Electric Power Research Institute to have other equipment tested.
More surprising things were then discovered about how electrical equipment affects the intensity of arc flashes. For example, some flashes were worse than expected, because of the arrangement, spacing and mass of the “bus bars,” which carry current to supply several circuits. When bus bars are close together, the arc flash sustains itself longer because it has more copper to ignite. When bus bars are aligned horizontally in a tight space, the copper vapor is thrust toward the worker.
The tests were extended to medium-voltage equipment after an arc flash incident involving a PG&E employee working on a 21-kilovolt pad-mounted switch, a common electrical box. The worker’s injuries were more severe than calculations would have predicted. Once again, the test explosions confirmed that bus bar configuration was the main culprit.
With the knowledge of how arc flashes behave in specific types of equipment, PG&E has adjusted its clothing and work-distance requirements. For employees working on medium-voltage pad-mounted switches, for example, PG&E increased the minimum safe distance to 48 inches from 28 inches, and the required clothing from single to double layer, plus a face mask.
