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Reduce Arc Flash Risks With Motor Control Centers

March 8, 2021
The right technologies and proper installation can reduce exposure to arc-flash events and other electrical hazards.

Industrial companies have put a greater focus in recent years on addressing a top electrical safety risk: arc flash events. Arc flashes can be created by a wide range of actions, from a dropped tool, to the build-up of conductive dust, to improper work procedures. And the resulting massive heat and energy waves created by them can reach temperatures of up to 35,000 degrees Fahrenheit.

Arc flash events create serious risks in nearly all industrial operations. The proper use of appropriately sized and applied low- and medium-voltage motor control centers (MCCs) can reduce the likelihood of arc flash events, but the risk still exists. Fortunately, MCCs can be enhanced to further reduce risks to both people and equipment.

There are three primary safety-enhancing technologies worth knowing about for an MCC, and it’s important to understand not only the technologies but also the importance of proper installation. This can help ensure critical safety capabilities are retained during and after equipment implementation.

Remote Access and Monitoring

The safest way to interact with any MCC is from a distance through the use of remote connectivity. By not requiring workers to approach the MCC in the first place, companies can reduce the risk of people being exposed to voltage and power.

Low- and medium-voltage MCCs are available with built-in Ethernet networking and software that make remote monitoring and access possible. The MCCs allow operators to monitor energy usage and operating conditions, receive notifications and diagnose issues or faults—and even make programming changes—all from the comfort and safety of any remote location.

These capabilities are built into the MCCs and can be preconfigured to reduce integration and setup time. 

Arc-resistant Equipment

When workers do need to physically approach an MCC, an enhanced, arc-resistant version of the MCC can further mitigate risk and provide increased protection from arc-flash events.

Arc-resistant MCCs incorporate multiple design features to contain and control the pressurized arc event byproducts and then safely direct them away from personnel.

Some equipment offers different arc-resistant design options to better meet the needs of specific applications. For example, a device-limited design option can contain an arcing fault for the time it takes a pre-tested main protective device to clear a fault. This can be ideal for applications that need a wider variety of structural or unit options. A duration-limited design option can contain an arcing fault for a predetermined current level and tested amount of time. This option is recommended for applications that require high bus short circuit currents levels or specific types of mains.

When choosing arc-resistant equipment, it’s important to make sure a vendor’s proposed solution has been tested for compliance with the rigorous arc-resistance requirements in standards such as  IEC/TR 61641 or IEEE C37.20.7. Equipment is only arc resistant if the following criteria are met during testing:

  • Doors and covers do not open
  • Parts are not ejected from the equipment
  • The arc does not burn holes in the tested structure’s exterior
  • Untreated cotton test indicators are not ignited or perforated
  • Grounding connections remain effective.

Of course, choosing properly designed and compliant arc-resistant equipment is just one consideration. Making sure it’s installed in accordance with the manufacturer’s requirements is just as critical.

The Importance of Proper Installation

A lack of familiarity with arc-resistant equipment creates the risk that installers may not follow the equipment manufacturer’s specific installation requirements. For example, they may leave screws out of panels, replace or not use supplied cover plates where wires are to enter, use oversized or misaligned conduits, or not properly install or seal the arc-gas ducting system.

These practices—often used to save time or money during an installation—can result in openings in what effectively disable the sealed system of the enclosures and arc-gas control devices. That can create a very dangerous situation for personnel.

If an arc event does occur, pressurized arc gases will seek the point of least resistance. And if extremely hot gases and vaporized metals escape through unintended openings, they could harm people, damage nearby equipment, or ignite dust or other flammable components in the area.

To help make sure arc-resistant equipment functions as designed—and to reduce risk to workers—it’s essential that the equipment be installed according to the manufacturer’s requirements. Fortunately, there are some measurable steps that users can take to help make sure electrical contractors install equipment correctly.

Reinforcing Correct Equipment Installation

To date, electrical standards don’t provide specific guidance for installing arc-resistant equipment. However, they do require that the equipment be installed according to the supplier’s requirements.

In particular, the latest version of NFPA 70E requires that users verify the equipment is properly installed and maintained. The phrase “properly installed” means the equipment is installed in accordance with applicable industry codes and standards as well as the manufacturer’s requirements.

How can users make sure that arc-resistant equipment is installed correctly?

First, they should include the equipment vendor’s installation documentation in their request for quotes (RFQ). Doing so will give electrical contractors an understanding of the scope of work before they bid on it. The contractors can then incorporate into their budget and project plan any additional time required to install the specialized equipment.

Users of arc-resistant equipment should also reach out to the equipment manufacturer. Especially for first-time buyers, the manufacturer can serve as a helpful resource, such as by answering questions about the installation or use of equipment. The manufacturer can also supply useful equipment documentation and images that can be used as part of the RFQ process.

Another way users can reinforce proper installation practices is by incorporating it into their electrical safety program. For example, they should have a process for verifying that equipment has been properly installed and that ongoing maintenance and inspection requirements are performed according to the vendor’s requirements.

Closed-door Power Disconnect

A low voltage MCC that allows operators to disconnect and reconnect power from an individual MCC plug-in unit without opening the unit door can reduce exposure to electrical shock and hazards.

Here’s how it works: When an operator turns the unit to the “disconnect” position, the power stabs that connect the unit to the MCC vertical bus to establish power are disconnected and retract inside the stab housing. Once the power stabs are securely inside, the stab housing shutters close and the disconnected state can be verified. When operators turn the unit back to the “connect” position, the power stabs reconnect with the vertical bus.

Lock-out mechanisms can also be used to help prevent the power stabs from being connected and the unit from being placed back into service.

Another option to increase safety is with remote operation. This allows operators to use a wired or wireless controller to disconnect and connect unit stabs from up to 300 feet away, which can remove the operator from the arc flash boundary and further minimize exposure to hazards.

Benefits Beyond Safety

Selecting a complete MCC solution that combines these three technologies can help reduce personnel exposure to electrical hazards. What’s more, the technologies can also deliver valuable business benefits beyond safety.

For example, the built-in technology that enables remote monitoring and reduces the need for operators to approach the MCC can also give plant teams access to valuable information, such as real-time diagnostics and advanced warnings. These insights can help speed up troubleshooting, reduce downtime and ultimately benefit the bottom line. 

John Kay is a principal application engineer with Rockwell Automation, a provider of industrial automation and information products. He is a Fellow Member of the IEEE and is actively involved with several technical committees, including various standards development and review subcommittees for the IEEE and UL. 

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