By Michael Terry, product manager-LV MCC, Rockwell Automation
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 staff. One of the characteristics that makes an arc flash such a dangerous event is the extreme temperature involved. Temperatures can reach 19,000 degrees Celsius – almost four times greater than the temperature of the sun’s surface. The pressure wave from the blast is equivalent to that of a hand grenade.
Core features in an arc-resistant motor control centre (MCC) should include structural integrity through a solid, robust design, two side sheets on every section, automatic vertical bus shutters and unit isolation.
MCC also provide Type 2 accessibility as defined within industry standards IEC 61641, IEC 60947 and IEEE standard C37.20.7-2007.
An efficient 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 cuts back the need for personnel to enter into an arc flash boundary zone.
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 use terms like “arc flash-resistant” to describe their products 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 arcresistant design.
In many cases, “commercial grade” MCC 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 preventive measures.
This limited approach doesn’t fully address arc flash dangers, and only protects a small range of users.
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 personal protective equipment (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? 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.
Employers are responsible for performing arc-flash hazard analyses that define potential arc energy levels adjacent to particular electrical equipment, and yield the required level of PPE needed when 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.
Advances in control technology make it easier with an expanded array of solutions designed to deliver improved safety, increased productivity and greater cost savings.
For more information:
Rockwell Automation New Zealand is a subsidiary of Rockwell Automation Inc, the world’s largest company dedicated to industrial automation
Tel: 09 276 3070
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