Edited by Robert Repas
Arc flash is defined as an explosive release of energy caused by an electrical arc. Typically, the arc results from either a phase-to-ground or phase-to-phase fault created by many possible events. For example, electrical faults can arise from dropped tools, accidental contacts with exposed conductors, a build-up of conductive dust, corrosion, and improper work procedures.
The intense heat, light, and concussive force emitted by an arc flash can form severe burns, damage hearing, and lead to a loss of eyesight. It can also spew molten metal considerable distances where it can lodge in skin or ignite clothing. A 10,000-A arc on a 480-V circuit is said to have the explosive force of eight sticks of dynamite.
The National Fire Protection Agency (NFPA), Occupational Safety and Health Administration (OSHA), Institute of Electrical and Electronic Engineers (IEEE), and other organizations work together to develop regulations and standards that protect personnel and equipment against arc-flash hazards. The NFPA 70E- 2004 standard for electrical safety in the workplace was developed through these efforts and is rapidly gaining prominence. The National Electric Code (NEC) and OSHA both reference the 70E standard in their arc-flash documentation.
The 70E guide identifies safe practices and procedures for personnel to follow when working on energized electrical equipment. It also specifies how to determine the severity of potential arc-flash exposure used to develop flash-protection boundaries. The boundary identifies where energy potentials of the flash are considered hazardous to health. If personnel must work within the flash boundary, 70E also identifies the appropriate personal protective equipment (PPE) they must wear to meet compliance.
As a companion to 70E, the IEEE 1584 standard gives detailed instructions on how to calculate incident arc-flash energies to develop the corresponding flash boundaries and PPE needs. Incident energy is defined as the amount of energy impressed on a surface a certain distance from the source. It’s unit of measure is in calories per square centimeter (cal/cm″). The flash protection boundary is specified as the point where incident energies drop to 1.2 cal/cm″, the amount of energy that begins to form second-degree burns.
Arc-flash hazards in switchgear are addressed through the American National Standards Institute (ANSI) C37.20.7 specification that lists testing standards for arc-resistant switchgear. The standard looks at internal arcing faults on metal-enclosed switchgear rated up to 38 kV. Equipment tested to this standard protects against the effect of abnormal internal pressure or arc flash as long as all doors and access areas are properly secured.
However, a common misconception in industry thinking is that the use of arc-resistant switchgear in motor-control centers (MCC) adds significant safety margins for any electrical worker in the area. The major flaw in this critical assumption centers on how electrical workers do equipment maintenance.
Switchgear built to meet arcresistant standards is predicated upon redirecting arc energy and pressure through a plenum. Arc-resistant gear or, more correctly, arc-redirection gear does provide higher levels of safety for personnel in the vicinity of the equipment. But it does not address the most common cause of electrical accidents in the industry — human error. The overwhelming number of arc-flash accidents happen during maintenance or troubleshooting.
The problem lies in that most MCC compartment doors must remain closed to meet arc-flash protection specifications. The reality is much electrical maintenance requires working with the doors open. That may actually be more dangerous with arc-resistant gear than with nonarc-resistant gear. Opening the door may form conditions where the path of least resistance for the pressure wave is no longer the safe path of the plenum, but out through the open door directly into the worker.
This doesn’t mean a company should continue to use nonarcresistant gear. On the contrary, improving operator and electrical worker safety from arc-flash accidents is a necessity. The key is to search out arc-resistant gear that lets electrical workers perform maintenance with little risk of arc-flash exposure.
When motors are geographically dispersed throughout a facility, the motor starters are aggregated in a motor-control center (MCC). The motor starters are segregated into individual units or buckets within the MCC for ease of isolation and maintenance. Each bucket is connected to the MCC power bus through rear-mounted stabs.
Insertion or removal of the buckets is done manually with the MCC door open. Accepted practice lets electrical workers physically push the bucket onto the main bus by hand. While the MCC should be de-energized during this action, plant operation usually demands that the MCC maintains power creating arc-flash and electrocution hazards to the workers.
One area typically overlooked is the periodic testing and troubleshooting of motor starters. The main power stabs in conventional MCC circuits feed a control-power transformer via a short-circuit protective device such as a circuit breaker or fuse. The control-power transformer reduces the 480-Vac incoming voltage to 120 Vac for the control circuits. Control circuits powered by the transformer include pushbutton stations, timers, relays, and programmable-logic controllers (PLCs). Main power must remain on to perform any meaningful tests or troubleshooting, thus exposing electrical workers to possible electrocution, burns, noise blasts, toxic fumes, and other hazards.
A means to connect and disconnect individual unit starters with the door closed keeps the arc-flash boundary secure, while remote operating stations assure operators remain outside the arc-flash boundary. Additional safety features might include: isolation and insulation of the current carrying bus and components, finger safe covers and components, mechanical interlocks to prevent inadvertent energization and access to live components, and control circuits that use voltages below electrocution hazard levels.
One MCC that tries to address the dangers associated with arc-flash events is the FlashGard Motor Control Center by Eaton Corp. The FlashGard MCC includes those features listed above that help prevent injury from electric shock, arc-flash burn, and arc-blast impacts.
The critical component of the FlashGard is its retractable stab mechanism called a RotoTrac. It lets the electrician control power to the bucket with the door closed. The design creates a safer environment for electrical workers while maintaining power to the MCC during the installation or removal of individual starter buckets. Another safety aspect is that the control circuits operate on an intrinsically safe 24 Vdc, rather than the more common 120 Vac.
The movable assembly of the RotoTrac free wheels at the end of travel in both directions to prevent overtorquing and damaging the housing and movable components. With the stabs retracted to a “Test” position, the 480-Vac main bus is completely isolated from the bucket. Yet 24-Vdc control power is available so electrical workers can safely perform maintenance work while the compartment door is open without worry about exposure to flash or electrocution-level voltages.
However, nothing improves safety more than a safe and sound electrical safety program. The NFPA 70 Article 340.7 states that an employer is responsible for providing training and supervision by qualified personnel to: explain the nature of the hazard; develop strategies to minimize the hazard; provide methods to avoid and protect against the hazard; and, convey the necessity of reporting any hazardous incident.
Eaton Corp., (216) 523-5000,