System Design and Arc Flash Labeling - The Importance of Protecting Against Worker Injuries
The incident energy from an arc flash event can cause severe worker injuries, shut down a facility, and cripple a business. Since the electrical industry is focused on safety and reliability, it is no surprise that everyone, from engineers and system designers to facility managers and electricians, is focused on minimizing the risk of arc flash incidents.
One common practice for keeping workers safe is arc flash labeling. But with the advances in arc flash prevention/containment/mitigationsolutions, there may be some confusion regarding how to appropriately label locations. Developments such as arc flash mitigation equipment and strategies like instantaneous zone selective interlocking and reduced energy let-through technology (RELT) affect incident energy and therefore, labeling details.
Arc flash labeling requires properly identifying the electrical system incident energy available during an arc flash event and labeling equipment accordingly. This first step enables operators to develop proper workplace safety procedures, like the use of personal protective equipment (PPE). The labeling requirements are in accordance to the National Fire Protection Agency under NFPA 70E 130.5 arc flash Hazard Analysis (2012 Edition) standards:
(C) Equipment Labeling. Electrical equipment such as switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control centers that are in other than dwelling units and are likely to require examination, adjustment, servicing, or maintenance while energized, shall be field marked with a label containing all the following information:
(1) At least one of the following:
a. Available incident energy and the corresponding working distance
b. Minimum arc rating of clothing
c. Required level of PPE
d. Highest Hazard/Risk Category for the equipment
(2) Nominal system voltage
(3) Arc flash boundary
Exception: Labels applied prior to September 30, 2011, are acceptable if they contain the available incident energy or required level of PPE. The method of calculating and data to support the information for the label shall be documented.
Some industry experts believe a good safety label should go a bit further and contain more details. Below is a sample GE label that includes additional information companies should consider for their labels, such as study date, case number, equipment description, and name.
Equipment construction and system-wide strategies affect labeling
Electrical manufacturers are continuously developing products and strategies to limit arc flash incident energy. Companies providing arc flash studies need to understand the widely varying incident-energy reduction potential of these products and strategies so they can be properly applied and labeled correctly.
Barriers offer a basic level of difference
Available for most low and medium-voltage equipment today, barriers work by providing electrical insulation and even isolation of an arc, impeding its propagation. Although products with isolation barriers may not have reduced incident energy levels that alter label requirements, they offer a great initial step at impactful protection of personnel and equipment.
Arc resistant equipment absorbs energy, but does not reduce it (doors closed)
Arc resistant equipment is specially designed to handle arc flash blast energy while channeling expanding hot gases away from personnel. This equipment usually requires less PPE when performing certain maintenance or diagnostics activities.
Arc resistant equipment ratings require closed and latched primary compartment doors or panels. If the breaker panels are opened for inspection or maintenance, the equipment is no longer protecting personnel. Incident energy calculations and therefore, labeling, is the same regardless of arc-resistant or non-arc-resistant construction because the calculation is a factor of the system’s available fault energy.
Arc flash mitigation equipment delivers greater protection (doors open)
Arc flash mitigation solutions reduce the incident energy potential of energized equipment by significantly decreasing the duration of an arc flash event. Mitigation technologies today include faster, more-coordinated circuit breakers and solutions. One technology can sense an arc flash event and transfer the incident energy into a containment dome until an upstream device interrupts the circuit. A few milliseconds reduction in event duration can lead to a very significant reduction in incident energy, altering the arc flash rating label.
System-wide protection and control strategies maximize protection
The most important engineering process in an arc flash scenario is the thought process. Rethinking overall system designs and strategies can limit arc flash duration and incident energy. Approaches can range from zone selective interlocking (ZSI) and bus differential to technologies like waveform recognition (WFR) for faster detection of over current and arc flash events. These changes can positively affect system labeling.
RELT technology is a common practice for safer equipment maintenance. Also known as a maintenance switch or arc flash switch, it requires the user to manually trade off selectivity for arc flash protection while near or within the arc flash boundary. The arc flash label in a facility employing this strategy will indicate the 24x7 level of incident energy, but incident-energy potential will be lower when the maintenance switch is enabled.
Zone Selective Interlocking systems (I-ZSI) can deliver safety and selectivity
I-ZSI is a new and unique strategy for lowering incident energy and improving safety across a facility. This offers a significant reduction in incident energy as compared with traditional ZSI. It delivers a powerful combination of selective coordination and arc flash mitigation, no longer requiring a tradeoff of one technology for the other. I-ZSI and WFR technologies enable multiple layers of circuit breakers to operate as one unified system. When responding to a fault, equipment has the intelligence to direct circuit breakers to respond either immediately or as a backup, depending on the situation.
Fast circuit breakers and over-current protection devices
Fast protection is crucial to reducing incident energy. Modern circuit breakers can operate in three cycles or less on a fully coordinated 24x7 system. Engineers must specify and install fast and reliable protection to minimize incident energy.
Retrofitting to reduce incident energy and safety requirements
Older equipment often will have slower protective devices, like circuit breakers or large fuses coordinated by relatively slow short-time or long-time trip bands. Most installed equipment can be readily retrofitted or retro filled with coordinated instantaneous protection. Quite frequently, aftermarket equipment that upgrades the slowest device in an electrical system can dramatically decrease the incident energy and allow relabeling.
Designing in remote operation
The more workers can remain outside the arc flash boundary the better. Today it is possible to design an electrical system with remote routine diagnostics and maintenance. Examples range from remote racking of circuit breakers to fully functional remote human machine interface operation that enables real-time system-settings changes, health monitoring, alarms, lock-out, and overall status updates. Some systems can even be configured to notify operators of alarms via email or text messaging.
It’s about the big picture and the little details
There’s a world of variables when it comes to arc flash prevention, mitigation, and minimization. That means there is no one arc flash solution and no single answer for arc flash labeling. Engineers, system designers, and facility managers should reach out to companies with demonstrated arc flash expertise for help with labeling and for other important services, like arc flash studies. Relying on the experience of resources that are focused on arc flash solutions is a sound strategy for safe, reliable system improvements.
Marty Trivette is the liaison between global product management and sales for GE’s Industrial Solutions business. He was the senior product manager for medium and low-voltage switchgear in North America. His broad experience in electrical distribution equipment includes switchgear, motor controls, reclosers, and transformers. GE is an IEC National Platinum Partner.
Andrew Sohn is responsible for overseeing strategic development for installation, start-up, commissioning, testing, maintenance, repair, retrofit, and upgrade services of electrical distribution systems at industrial facilities worldwide. As chair of GE’s Industrial Arc Flash Council, he oversees the development and integration of new arc flash standards into services offerings.