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Quality Scan: Arc Flash Safety

 

An Ounce of Prevention is Worth a Pound of Cure



By Casey B McCollum
Product Manager, MNS-MCC
ABB Inc.
Casey.mccollum@us.abb.com
 www.abb.us/lowvoltage

Casey B. McCollumAs defined by the IEEE, an arc flash is a strong electric current, and often a major explosion, that passes through air when insulation between electrical conductors is no longer sufficient to contain the voltage within them. This creates a short that allows electricity to race from conductor-to-conductor, an often fatal event for anyone working on or near energized equipment.

In recent years there has been a much needed emphasis on providing increased safety to electrical personnel, resulting in standards like the NFPA-70E. These standards are meant to assess the extent of an electrical hazard and specify the personnel protective equipment that operators conducting maintenance and upgrades to electrical systems must wear to ensure their safety.

While certainly a positive step, safety experts rightfully argue that the best method to prevent personal injury, and to protect expensive equipment, is to go to the root of the cause. That is, address the causes of arc flash incidents in addition to the protective measures in place for workers should an incident occur.

One category receiving significant attention in the design and development of safer products is Low Voltage Motor Control Centers (MCC). Primarily used for the control of electric motors, MCCs are commonly found in process industry applications such as oil drilling, steel and paper mills, chemical, mining and water/wastewater facilities, as well as in substations and critical power facilities such as hospitals and data centers.

By their nature and purpose, MCCs require more frequent maintenance and changes in configuration than other classes of low voltage power distribution equipment.
Operators working on active MCCs is a common occurrence. Many diagnostic activities must be conducted while the equipment is energized, and often a complete shutdown can result in the loss of the entire process, raising the possibility of other environmental and safety issues.

While the existing codes and standards fall short of mandating that MCCs are safe to be worked on in a live state, many electrical equipment manufacturers have clearly established a priority of developing safer MCCs for real-world applications.

Some newer MCC designs are “arc resistant” meaning that the enclosure can contain the effects of arc flash, but it does not prevent arc flash incidents from occurring. Arc resistant ratings may only be only valid when all doors and covers are closed. This level of protection is not adequate since most accidents occur when one or more MCC doors are open. The main cause of arc flashes is human error and 65% occur when an operator is working on the equipment. 

The optimal MCC design actually prevents arc flash incidents from occurring, rather than just containing them, during common operation and maintenance applications.

The recently introduced MNS-MCC from ABB Low Voltage Products is the first MCC to address the causes of arc flash incidents.  MNS-MCC’s unique withdrawable technology allows an operator to safely remove or install an MCC unit behind a closed door, without any tools, and without disturbing any field wiring. 

Traditional MCCs require the door to be opened, potentially exposing the operator to primary voltage, and for the field wiring to be disconnected prior to removing a unit.  The ease of withdrawal of an MNS-MCC bucket encourages an operator to perform adjustments and maintenance safely outside the arc flash boundary. 

Other features included in MNS-MCC that promote arc flash prevention include insulated bus bars, fingersafe IP20 connections, door-mounted voltage indication, and wireway access to control terminal blocks.

In a recent study conducted by the IEEE it was determined that more than 13% of all reported arc flash incidents occur in low voltage MCC equipment, second only to the 17% that occur in control equipment.

No small problem with arc flashes emitting heat up to 35,000° F, four times the surface temperature of the sun, and with, according to recent estimates, five to ten arc flashes occurring in the United States every day, causing one to two fatalities every day. ME

This article originally appeared in the October 2012 issue of Manufacturing Engineering magazine. Click here for a PDF of the article.


Published Date : 10/1/2012

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