Arc-preventive MCCs promote personal safety, equipment life
Low-voltage motor control centers (MCCs) are routinely accessed during maintenance and are designed to help protect personnel and equipment from the danger of arc flash. These MCCs lower the probability of electrical shock and reduce incident arc flash energy during maintenance.
There are guidelines for arc flash prevention and equipment that are intended to help prevent injury and protect equipment. However, there is room for interpretation and the applicable standards and guidelines can be confusing.
Applicable codes and standards
The National Fire Protection Agency’s 70E standard identifies safe practices for personnel to follow while working on energized electrical equipment. The purpose of NFPA 70E is to provide guidelines to limit injury. Both the National Electrical Code (NEC) and OSHA reference the NFPA 70E standard in their arc-flash documentation.
A safe and sound electrical safety program is a key to enhancing safety. NFPA 70 Article 340.7 states that an employer must provide training and supervision by qualified personnel to:
- Explain the nature of the hazard
- Develop strategies to minimize hazards
- Provide methods to avoid and protect against hazards
- Convey the necessity of reporting any hazardous incidents.
Additionally, IEEE 1584 provides guidelines on how to calculate incident arc flash energy to develop boundaries and establish personal protective equipment requirements. Incident energy is the amount of energy impressed on a surface a certain distance from the source. Its unit of measure is in calories per square centimeter (Cal/cm2). The flash protection boundary is specified as the point where incident energies drop to 1.2 Cal/cm2, the amount of energy that begins to form second-degree burns.
Low- and medium-voltage switchgear is used to protect, control, and monitor distribution systems and protect operating and maintenance personnel from arcing faults. Arc flash hazards in switchgear are addressed through the ANSI C37.20.7 specification that lists testing guidelines for arc-resistant switchgear.
Switchgear built to meet arc-resistant standards redirects or channels the arc energy and pressure through a plenum, out the top of the switchgear, regardless of where the arc originated. The guidelines look 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.
The danger of arc flash is not limited to switchgear. However, the application of ANSI C37.20.7 to MCCs has limitations. At this time, there is no MCC arc flash guideline.
Arc-resistant gear or arc-redirection gear does provide higher levels of safety for personnel in the vicinity of the equipment, but it does not address a common cause of electrical accidents: mistakes. The overwhelming number of arc flash accidents occurs during maintenance or troubleshooting.
Arc-resistant vs. arc-preventive
A common misconception in the industry is that the use of arc-resistant switchgear in motor control centers adds significant safety margins for any electrical worker in the area. The major flaw in this assumption centers on how electrical workers perform equipment maintenance.
Most MCC compartment doors need to remain closed to meet the thrust of arc-resistant guidelines. But much electrical maintenance requires working with the doors open; eventually, personnel need to access the interior components and connections. That may be more dangerous with arc-resistant gear than with non-arc-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 at the top of the gear, but out through the open door to maintenance personnel.
That is not to say that arc-resistant gear should be avoided. The key is applying guidelines to the equipment that they were designed to address. Improving operator and electrical worker safety from arc flash incidents is necessary. It is crucial to find arc-resistant gear that lets electrical workers perform maintenance with little risk of exposure to arc flash.
At this time, the arc-resistant designation applies only to switchgear tested to meet ANSI C37.20.7. MCCs need to be accessed for maintenance, and closed-door operation requirements fall short, given normal operating procedures. MCCs are routinely accessed for a variety of reasons: connecting or disconnecting starters or feeders, adjusting trip settings, replacing fuses, adding motor loads, and general troubleshooting. To make adjustments, access to the interior of the unit buckets is necessary.
Yet this means that the MCC does not provide the highest level of personnel protection. In MCCs the predominant cause of arc flash incidents is due to the operator plugging in or removing a unit from a live bus with the unit door open.
Maintaining a dead-front barrier, like a unit closed while connecting and disconnecting MCC starter or feeder units, and providing insulated components or connections significantly reduces the possibility of an arc-flash incident.
Arc flash preventive designs are relevant to MCCs. Closed-door operation, in combination with safety interlocks, addresses this arc flash safety need.
What makes for arc-preventive MCCs?
Key strategies are used to help safeguard employees against injuries from electric shock, arc flash burns, and arc blasts:
1. Multiple insulation and isolation features enable arc flash prevention.
2. Unlike conventional MCCs, arc-preventive MCC design enables units to be disconnected and reconnected to the vertical bus with the door closed; maintaining a closed door during these operations increases operator safety.
3. A series of safety interlocks ensures that doors cannot be opened and units cannot be removed from the structure while the stabs are connected to the vertical bus.
4. Each unit contains visual indicators that report the position of the isolation shutters and stabs, providing maintenance personnel with additional assurance that dangerous voltages are not present inside the unit when service is required.
Testing by IEEE 1584P can be conducted to verify that closed-door operation provides a considerably lower risk category than the risk Category 3 assigned by NFPA 70E Table 130.7(c)(9)(a) for insertion and removal of MCC units. Beyond thermal hazards, a closed door provides better protection from shrapnel, noise, gases, and blinding light. Remote operated racking devices are available so that an operator may advance and retract the stabs from upwards of 15 ft, which places the operator outside the arc flash boundary.
Arc-preventive MCCs: Use case
When motors are geographically dispersed throughout a facility, the motor starters are aggregated in an 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 allows electrical personnel to 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 maintain power, creating arc-flash and electrocution hazards.
For example, the periodic testing and troubleshooting of motor starters requires the main power to remain on, in order to perform any meaningful tests or troubleshooting. Since the equipment is powered, electrical personnel are exposed to dangerous arc flash conditions. 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 V ac incoming voltage to 120 V ac for the control circuits. Control circuits powered by the transformer include pushbutton stations, timers, relays, and PLCs.
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 to prevent injury from electric shock, arc flash burn, and arc blast impact 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.
Mark Yerse is product manager at Eaton Corporation.