SPECIAL REPORT: New solutions put us on the way to preventing arc flash incidents

Recent changes to the National Electric Code are getting us closer to the ultimate goal of preventing damage from arc flash.

06/03/2014


There’s good news about arc flash. Engineers are discovering and deploying new ways to prevent, contain, and limit its potentially catastrophic consequences.

Causing an arc flash is easy—simply dropping a wrench can power a disastrous explosion that injures employees, destroys equipment, and puts a facility out of business for weeks or months. The destructive potential of an arc flash incident is staggering. An arc flash can:

  • Severely injure or kill a person from as far away as 10 ft or more
  • Cause serious burns: at 35,000 F, arc flash temperatures are hotter than the surface of the sun
  • Produce pressure and sound waves strong enough to throw workers across a room and turn tools and equipment into life-threatening shrapnel
  • Cause vision and hearing loss from extreme light and sound outbursts
  • Cost millions of dollars per incident in health care, equipment replacement and repair, and lost production time, according to an Electric Power Research Institute study.

Plant engineers and facility managers should be very concerned about arc flash. The need to ensure worker safety is paramount in any organization. Arc flash strategies can also affect uptime, productivity, and customer safety.

Add in OSHA inspections and corporate reputation, and it’s easy to see why the electrical industry has made arc flash one of its hot buttons. Fortunately, this attention is leading to new technologies to improve arc flash safety.

 

NEC changes enable new solutions

Recent changes to the National Electric Code (NEC) are getting us closer to the ultimate goal of preventing damage from arc flash. Updated during the fourth quarter of 2013, NEC 240.87 includes several new additions to the code focusing on arc flash safety. Plant engineers and facility managers now have more protection options for managing arc flash hazard, namely:

  • Zone selective interlocking
  • Arc flash maintenance safety switch
  • Bus differential
  • Active arc flash mitigation.

Each of these protection strategies offers a unique profile of protection, selectivity, and potential injury severity reduction. Investigating each option thoroughly can help a facility determine which arc flash strategy works best for them.

Zone selective interlocking (ZSI)

ZSI connects and coordinates protection between upstream and downstream devices. A downstream circuit protective device communicates with an upstream device—via dedicated twisted pair wiring or digital communication—when it detects a current above the fault threshold. After receiving the signal, the upstream device “restrains” and shifts to a longer time delay. This allows the downstream device to clear the fault.


Should the downstream device fail to clear the fault, the upstream device jumps in to provide backup protection. Engineers can set an upstream ZSI breaker to protect with no added time delays for coordination, allowing the ZSI signal to restrain the device when needed. Faster protection can reduce arc flash incident energy and, along with it, the risk to personnel and equipment.

Arc flash maintenance safety switch

When an operator needs to work near electrical equipment, an arc flash maintenance safety switch reduces arc flash potential. An arc flash maintenance safety switch enables workers to switch to pre-set trip settings, which are usually faster. Workers and managers can quickly enable and disable switch operation, adding protection only when needed. These safety switches can be deployed for individual breakers or coordinated among a group of associated breakers. So, when a worker is in potential danger, the system is more sensitive to lower magnitude arcing faults that can cause dangerous arc flash.

Greater sensitivity also leads to a lower degree or potential lack of coordination and the associated downtime, and equipment wear that comes along with it.

 

Bus differential

Bus differential looks for differences in current flowing into and out of a zone in the power distribution system. If the bus differential relay detects a difference that exceeds the set pickup level, it trips all the protective devices feeding that zone. The result is fast, selective protection for that zone. 

A bus differential implementation connects a bus differential protective relay to dedicated current transformers (CTs) on every circuit in the protected zone. It provides fast protection and only operates when there is a fault in the zone, providing selectivity and protecting against in-zone faults.

Active arc flash mitigation

Instead of managing the tremendous energy unleashed by an arc fault, new technologies enabled by the 2013 update to NEC 240.87 manage the fault itself. Arc flash mitigation systems can contain, isolate, and dissipate a fault in as little as 8 milliseconds. These containment technologies cut off the energy at its source, making them very effective at minimizing the danger and controlling the damage of arc faults.

Active arc flash mitigation lowers arc flash force potential and helps reduce work-related injuries even when panel doors are open during routine maintenance. Some mitigation technologies can limit the incident energy released by an arc fault to between 1 cal/cm2 and 2 cal/cm2. With greatly reduced energy potential, employees can work on equipment without overly restrictive PPE.

Well on the way

The electrical industry can’t completely eliminate arc flash, but we can aspire to work together to reduce the risk of occurrence and severity of damage from arc flash incidents. It starts with engineers and manufacturers who are investing in the research and development of new technologies that help reduce risk and manage the effects of arc flash. It flows throughout our industry as companies, contractors, and everyday end users actively learn and pursue new ways to manage arc flash.

It’s important that all parties involved work together to identify the best solution for the particular circuits impacted. For instance, it’s imperative to verify that ZSI will impact the incident energy at the circuits targeted and to be cognizant if the protection settings for each circuit achieve the protection, selectivity, and arc flash results desired.

These recent code changes are evidence of the technological and philosophical progress we’re making. Working together, we can make effective arc flash safety a long-term initiative that improves operations across our industry, and not just a flash in the pan.

Marty Trivette is North America product marketing leader for power equipment and Jane Barber is intelligent systems product marketing manager, power equipment, for GE’s Industrial Solutions business. 


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