Arc Flash Evolution
By Control Engineering Staff -- Control Engineering, 12/11/2007
It seemed safe enough. The industrial electricians were just going to take a few measurements prior to starting the job. Everything was energized, but that should have been OK, because the electricians were going to be a safe distance away.
They were. The metal tip of their wooden yardstick wasn't. The resulting arc appeared and vanished in less than a second, sticking around long enough to catch clothes on fire. The unlucky electricians joined the estimated hundreds of arc-flash injuries a year that require hospitalization. Their co-worker standing 10 feet away became one of the thousands who annually suffer injuries that don't require a hospital stay.
Ray Clark, a consulting applications engineer with Siemens Energy and Automation, relays the tale to illustrate possible dangers. The good news, he continues, is that there are now new solutions to the problem that should help increase safety. "The awareness of arc flash has been around for several years but work in the industry is continuing to evolve and more research is being done," he says.
The most visible sign of that evolution is the requirement that equipment be marked. The mandate was broadened considerably in the 2005 National Electrical Code. The new stipulation was that switchboards, panel boards, industrial control panels, meter centers and motor control centers likely to require some kind of service while energized had to carry a warning of potential electrical arc flash hazards.
That meant a lot more equipment had to carry an arc flash warning than ever before. OSHA, the Occupational Safety and Health Administration, doesn't directly address arc-flash hazards. However, officials have stated that the industry consensus standard concerning arc flash requirements should be followed.
Warnings and calculations
These warnings have to point out the danger and also convey information. In particular, they specify a flash hazard boundary inside of which someone unprotected could receive a second-degree burn. They also have an assessment of the hazard category as per the NFPA 70E standard. The label also conveys the incident energy, the energy impressed on a body surface by an arc. That information determines the level of personal protective equipment (PPE) someone approaching a live device should wear.
Such requirements can lead to some complex arc flash calculations. Factors such as bus bar spacing, enclosures, three phase versus single phase, and how the plasma is going to react to an arc all play a role in locating the arc flash boundary and determining the incident energy.
There are some resources to make the task less challenging, notes Clark. "There are some cookbook charts that make it fairly easy to comply," he says.
Clark notes that NFPA 70E in particular has good tables for this purpose. He adds, however, that some charts and software calculators can give very conservative results, a consequence of the assumptions made. That conservative approach may actually make things less safe, since working in overprotective clothing, for example, can make accidents more likely.
The best solution is to follow the standard recommendation and only work on de-energized equipment, thereby avoiding arc flash altogether. If that's not possible and calculations are needed, Clark points to the arc flash calculator produced by the IEEE 1584 working group, of which he is a member. This calculator gives a middle-of-the-road result, so that safety isn't compromised by an arc flash or overuse of personal protective equipment. Commercial software packages for these calculations also exist.
The evolving understanding of arc flash and the dangers it presents also calls for changes in electrical protection. An arc flash isn't the typical short circuit and so requires different measures.
Bolted or arcing
Electrical short circuits are generally either bolted or arcing faults. The former occurs when current flows through bolted bus bars or other conductors. There's no direct energy release, but, as might be imagined, the amount of current flowing can be massive.
An arcing fault, in contrast, occurs when deteriorating insulation or human error creates a conductive path among phases or phase-to-ground. The resulting current flows through air, creating temperatures hotter than the surface of the sun and vaporizing all known materials. Along with intense heat, there's infrared and ultraviolet radiation, a sound blast and pressure waves.
The amount of current released in an arc flash will be less than that of a full bolted fault, perhaps substantially. But that lesser current may not be much of a safety benefit. "It's not just the magnitude that's important. It's the length of time that the arc is present," says Clark.
Modern current limiting devices clear a full bolted fault quickly, sometimes within a quarter cycle. In a 60 hertz system, that translates to just under 4.2 milliseconds. The same rapid response typically isn't present with an arc flash because of the lower current. In some cases, the arcing current can flow for several seconds. The cumulative energy discharged can thus go to extremely high levels and the damage done can be extensive.
Fast is good
The ideal solution is to implement the fastest possible tripping of protective devices. That approach has to be balanced against the need to coordinate fault protection systems, which typically require slower trip times. This is not a case, notes Clark, where one size fits all. In factories running continuous industrial processes, trip coordination may be critical. The same may not be true for something like a strip mall. There, faster trip times may reduce system damage.
Other design considerations for reducing arc flash include splitting loads into lower amp capacity units, remotely monitoring switchboards, specifying compartmentalized and insulated gear, and modifying work methods.
The use of new technology can also help, such as devices that institute a faster trip time temporarily, while work is being performed. An example of such a solution is the dynamic arc flash sentry technology found in Siemens products. Such switching can even be done automatically, with a sensor that detects worker entry into the arc flash boundary. "When someone is approaching the equipment, it will send a signal to the trip unit to switch to the low arc flash settings," says Clark.
These are just some of the ways the solutions to arc flash hazards are evolving. With a bit of luck and some hard work, the result will be fewer injuries thanks to greater yet still simplified safety.
Arc-Flash Webinar
To help manufacturers understand recent changes in Arc Flash regulations, you are invited to tickets.safetythemovie.com/ where you will find a complementary one hour recorded webinar entitled "Arc Flash Hazard: Understanding Safe Electrical Work Practices, Standards and Regulations". This Arc Flash safety webinar focuses on NFPA 70E Electrical Safety Requirements, Arc Flash Boundaries (AFB) and the specific category of personal protective equipment (PPE). Attending this webinar will help you to better understand safe electrical work practices, the NFPA 70E standard, and learn how OSHA is involved.
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