Meeting the letter of the revised Section 240.87 in the 2014 National Electrical Code (NEC) is relatively easy – specifying and installing one of the outlined solutions will meet its stated requirements. But meeting the spirit of the code by truly achieving a real improvement in worker safety requires careful thought to how facility distribution systems are engineered.
Design decisions made at the engineering stage can make a real difference in reducing the likelihood of any incident occurring in the first place, and address both the letter and spirit of the revised section. An engineering-based approach is more effective than personal protective equipment because it reduces the actual hazard level. Engineering controls also are more effective than administrative controls and warnings because they often don’t rely only on workers following proper procedures and safe work practices.
Back to basics
When faults do occur, the key to reducing potential harm is to reduce the duration of the fault. As the arc flash equations in the Institute of Electrical and Electronics Engineers Standard 1584-2002 illustrate, arc flash incident energy varies linearly with time; so, if you cut fault duration by half, you also cut available energy in half. Overcurrent protective devices (OCPDs) are designed to rapidly clear faults to limit available energy within a related protected zone – the challenge for engineers designing such protection is to correctly match an OCPD specification to the real-world protection needs of a client’s facility.
Meeting this challenge requires an understanding of:
- How much protection is required; and
- The extent of the zone of protection each mitigation strategy offers.
An arc flash study should be the first step in this process. It is the only way to understand the level of a prospective arcing fault current. Such a study should be conducted using the calculations in the Institute of Electrical and Electronics Engineers Standard 1584, “Guide for Performing Arc Flash Hazard Calculations.”
Then, when considering mitigation options, designers need to investigate the extent of protection each device or design strategy offers. In other words, will the selected means really provide protection where it’s needed – and is protection restricted to the equipment where the reduction means is located, or does it extend to downstream equipment, as well? Remember that future maintenance requirements might place workers at risk outside the protected zone or could expose workers to energized conductors outside the protected zone, as with the line-side terminals on a main circuit breaker.
Answering this second question may lead system designers to take a serious look at the physical location of hazard zones in relation to each other, in order to reduce potential hazards even further. For example, questions could arise regarding group-mounted UL panelboards and switchboards – in such installations, can the means be effectively located on the main device? Mounting the main circuit breaker separately, instead, could provide an inherently safer design, regardless of the selected OCPD.
Arc flash is a serious hazard, and reducing its risks can be a complicated problem. Schneider Electric can provide technical assistance with both arc-flash hazard studies and in understanding the protected zones created by specific mitigation options. And find a range of additional resources for consulting engineers at www.schneider-electric.us/consulting-engineers.