How motor control centres help reduce arc flash hazards

Companies continually look for ways to improve plant safety and a growing area of focus is reducing the potentially serious hazards caused by arc flash events. This article examines the causes of arc flash and discusses the standards guiding arc flash safety. It also explains the role that arc-resistant motor control centres (MCCs) play in helping to contain arc energy.

An arc flash is the result of an arc fault that superheats the air around it, expanding and creating a pressure wave within an electrical enclosure. This arc plasma vapourises everything it contacts, such as copper, insulating materials, bolts and the steel enclosure. This massive heat and energy can inflict serious injuries, including: severe burns from the vaporised materials; damaged hearing from the sound and pressure waves; and impaired eyesight from the high-intensity flash.

Over the past 15 years, arc flash safety compliance has been introduced at many industrial sites in Australia and New Zealand. In fact, arc flashes are responsible for about 80% of all electrical-related injuries. One of the characteristics that make an arc flash such a dangerous event is the extreme temperatures involved. In some cases, temperatures can reach 19,000°C - almost four times greater than the temperature of the sun’s surface. Also, the pressure wave from the blast is equivalent to that of a hand grenade.

The causes of an arc flash are usually accidental. They can be as simple as a rodent, snake or water accidentally entering the electrical equipment, or the cause could be human error, such as an employee accidentally leaving a tool inside the enclosure or forgetting to tighten a connection.

Standards that address arc flash safety

Historically, electric codes and safety standards didn’t directly address arc flash hazards; they only addressed protection from fire, electrocution and shock. Standards such as IEC61641, AS3439-ZD and NFPA 70E - Electrical Standard for Safety in the Workplace are putting more focus on arc flash risks and helping to reduce the associated hazards.

In 2004, numerous additions were made to NFPA 70E, including definitions and formulas to calculate arc flash, shock hazard boundaries and default tables for arc flash levels. The IEC61641 and AS3439 still do not address this issue.

The revisions also outlined personal protective equipment (PPE) requirements for specific tasks and provided mandates for electrical safety programs, energised work permits, safe work practices and training.

Features of an arc-resistant MCC design

Arc-resistant MCCs and intelligent control systems can offer improved safety features along with remote operation and monitoring capabilities. Arc-resistant equipment controls arc flash exposure by extinguishing the arc, controlling the spread of the arc and directing the arc pressure wave away from personnel.

An arc-resistant MCC also can provide Type 2 accessibility as defined within IEC 61641, IEC 60947 and IEEE standard C37.20.7-2007. Core features should include: structural integrity through a solid, robust design; two side sheets on every section; automatic vertical bus shutters and unit isolation. The MCC should employ a solid grounding system along with a well isolated and insulated horizontal bus and vertical bus. For added safety, spaces that accommodate plug-in units also should include automatic shutters that immediately isolate stab openings when units are removed.

Selecting an MCC that uses smaller bus and main disconnect sizes also helps reduce the available energy in an application to reduce the intensity of an arc flash event if it does occur. Intelligent MCC designs also include remote monitoring and control capabilities designed to minimise the amount of time that employees are near the equipment.

One of the newest features in MCC technology is built-in networking and preconfigured software. By including a built-in industrial network, based on an open protocol, such as EtherNet/IP or DeviceNet, along with MCC monitoring and configuration software, users can remotely monitor, configure and troubleshoot the MCC. This minimises the need for personnel to enter into an arc flash boundary zone.

How do you know it’s truly arc resistant?

When choosing an arc-resistant MCC, it’s important to understand the performance criteria that must be met before the MCC can be classified as an arc-resistant design. ‘Arc resistant’, as it applies to electrical equipment such as low-voltage MCCs, is a recognised industry term defined by IEC61641 or IEEE C37.20.7-2007. The standard defines the test requirements that must be met and the expected performance the equipment must deliver in the event of an arc flash.

Some vendors might use terms like ‘arc flash resistant’ to describe their product, with the implication that it offers substantiated arc-resistant capabilities. The truth is that arc flash resistant is not a standard industry term and has no relevant meaning behind it. This type of inference provides a false sense of security for users expecting an arc-resistant design.

In many cases, ‘commercial grade’ MCCs simply can’t withstand the effects of internal arcing faults for the tests prescribed in the IEC or IEEE standard. Instead of achieving the advanced level of protection they want, many users instead are relegating their strategy (perhaps unknowingly) to one based solely on preventative measures. This limited approach doesn’t fully address arc flash dangers and only protects a small range of users.

Does a closed door help?

Another area of confusion centres on the claim that keeping the doors of an MCC closed during insertion and removal of power stabs provides a lower risk and therefore allows users to adhere to a reduced level of required PPE. The reality is that no industry standard allows users to reduce the risk category of an MCC application just because the door is closed.

Will the door stay closed in the event of a fault in the unit? The reality is that during an internal arcing fault, doors of non-arc-resistant equipment will likely blast open due to the pressure wave, even if they were properly closed and latched per the manufacturer’s specifications. This would increase worker exposure to the effects of the arcing fault, perhaps even exceeding the capabilities of the PPE selected, based on the default tables.

Any person intending to open the door to work on the unit needs a level of PPE based on NFPA 70E guidelines for working on an energised unit. Only when using an MCC designed and tested in accordance with AS3439-AZD, IEC 61641 or IEEE C37.20.7-2007 should a user have any expectation of maintaining closed doors during an internal arcing fault. Otherwise, the worker is exposed to increased risk if an arc fault occurs in the MCC.

Diligence pays off

Employers are responsible for performing arc flash hazard analysis that defines potential arc energy levels adjacent to particular electrical equipment and providing the required level of PPE for working near energised electrical equipment. The best prevention against exposure to an arc flash is an in-house safety program that complies with the NFPA 70E standard. Beyond that, the most important advice is ‘shut it off’.

Take the initiative to include in-depth safety programs and invest in current equipment designs to provide an improved level of safety for employees that also helps reduce substantial financial costs associated with electrical incidents. The good news is that advances in control technology make it easier with an expanded array of solutions designed to deliver improved safety, increased productivity and greater cost savings.

Reducing arc flash risks

The following guidelines are effective in reducing the risk of arc flash: perform an arc flash hazard analysis on all electrical equipment; label all electrical equipment for arc flash hazards; install arc-resistant equipment; employ remote monitoring/operation; conduct ongoing safety training; ensure workers are appropriately protected with suitable personal protective equipment (PPE); implement an equipment maintenance program; perform lockout/tagout (LOTO) to work on equipment; and test the voltage on each conductor for verification before working on equipment.

By Michael Terry, Product Manager - LV MCC, Rockwell Automation