Pumps & Systems, April 2013
Safe, dependable power is crucial for pumping applications. Enhancing personnel and equipment safety is a top priority. At the same time, equipment needs to be up and running without interruption. Arc flash events can instantaneously generate temperatures reaching four times the temperature of the sun. Therefore, addressing the dangers of these events is critical to reducing the risks to personnel and equipment.
An arc flash is the explosive release of energy caused by an electrical arc because of either a phase-to-ground or a phase-to-phase fault. This kind of fault can result from many factors—a dropped tool, accidental contact with electrical conductors, buildup of conductive dust, corrosion or improper work conditions.
As personnel perform maintenance on electrical equipment, they must be aware of arc flash risks and how to avoid them. During the last decade, research and development has yielded a range of advances in electrical safety. Much of that research has focused on preventing arc flash events. A strategic approach to arc flash safety involves education, a robust safety program and state-of-the-art equipment engineered to prevent personal injury and property damage.
Motor control centers (MCCs) are regularly accessed during maintenance. Some are specifically designed to help protect personnel and equipment from arc flash events. These MCCs are engineered to lower the likelihood of electrical shock and reduce incidents of arc flash energy during maintenance operations.
Room for Interpretation
Arc flash dangers are not limited to switchgear, and the application of ANSI C37.20.7b to equipment such as MCCs has limitations. To meet arc-resistant standards, all MCC compartment doors need to be closed during maintenance. However, personnel routinely need to access interior components and connections.
Also, gear that meets arc-resistant standards could pose additional hazards, since the path of least resistance for the pressure wave would not be through the top, but through the open door. That is not to suggest that arc-resistant gear should be avoided. The arc-resistant classification was intended for switchgear and should be applied to switchgear.
At this time, no specific arc flash codes are designed for MCCs. Electrical codes and standards continue to evolve. Meanwhile, innovation in electrical equipment helps support arc flash prevention initiatives and practices.
An arc-flash-preventive MCC provides technology to enhance personnel safety and protect equipment.
Definitions & Specifications
The main cause of arc flash incidents in MCCs occurs when an operator plugs in or removes a unit from a live bus with the unit door open. Arc-flash-preventive designs incorporate three key strategies to maximize the safety of equipment and the operator:
- Prevention of arc flash events through the insulation and isolation of assembly elements prone to incidental occurrences during maintenance
- A design that allows units to be disconnected and reconnected from the vertical bus with the door closed
- Minimized operator exposure to dangerous events
Prevention is the foundation of arc flash safety, closed-door operation and safety interlocks. Arc-flash-preventive MCCs reduce the likelihood of arcing phase-to-phase and phase-to-ground short circuits by using insulated horizontal and labyrinth vertical bus. Additionally, active interlocks prevent the insertion or withdrawal of the MCC bucket while the stabs are engaged or extended. The MCC cabinet also incorporates two independent shutter mechanisms to automatically isolate both the vertical bus and unit power stabs when the unit is withdrawn.
For personnel, the cabinet is the main defense. MCC designs should incorporate substantial steel panels with doors that can be securely latched to suppress the effects of an arc flash, providing a barrier between a flash and the operator.
Safety Enhancing Equipment Features
Beyond the basic arc-preventive-MCC designs, a range of equipment features and accessories can help further reduce the risks of arc flash events and enhance personnel safety. Fundamentally, closed-door operation, putting increased distance between the operator and the front of the MCC, and additional visual indicators can further advance safety. MCC panel accessories include modules that provide a visual queue for personnel. These modules typically flash when dangerous voltage is present inside the unit.
Mechanisms are available that enable the unit to be disconnected from the vertical bus with the cabinet door closed. Visual indicators on these mechanisms allow for quick, positive verification of the stab position, so operators know if the stabs are retracted or extended. An additional visual indicator provides verification of the MCC bucket’s safety status by indicating whether the internal isolation shutters are open or closed.
Remote-racking systems typically extend the operator’s safety zone to about 15 feet during disconnecting or reconnecting the unit power stabs to the vertical bus during bucket replacement. Remote-racking mechanisms can also insert or remove circuit breakers. These types of mechanisms allow personnel to stand outside the arc flash boundary of the equipment. The operator can use remote controls to disconnect, test or connect the circuit breaker.