Machine & Operator Safety
Electrical safety: Mitigating arc flash safety risk
March 13, 2012 by John Kay Rockwell Automation
In North America, about five to 10 arc flash events occur each day. Arc flashes are responsible for as many as 80 percent of all electrical-related injuries.
Arc flash safety equipment can deliver positive, business-enhancing benefits when effectively implemented, while also mitigating risk and reducing costs. But without a clear understanding of the present risks, industry standards and the role arc-resistant motor control centres (MCCs) play in helping contain arc energy, applying technology where needed on the factory floor can be a challenge.
KNOWING THE RISKS
An electric 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 vapourizes everything it comes in contact with, such as copper, insulating materials, bolts and even steel enclosures. This massive heat and energy wave can inflict serious injuries, including severe burns, damaged hearing from the powerful sound waves, impalement from projectiles and impaired eyesight from the high-intensity flash.
The causes of arc flash events are usually accidental. In industrial settings, many things could compromise the air space that acts as insulation to prevent electrical energy from igniting an electrical arc. The cause of the arc flash could be as simple as a rodent, snake, dust or water accidentally entering the electrical equipment, or it could be human error, such as an employee accidentally leaving a tool inside the enclosure or forgetting to tighten a connection.
Effectively mitigating arc flash risks begins with a risk assessment to identify the areas of arc flash risk within a plant and pinpoint the best people, processes and technologies to minimize those risks. Engineers, safety professionals and third-party safety specialists can collaborate to properly identify risks and determine the appropriate mitigation measures.
STANDARDS INTENSIFY THE FOCUS
CSA Z462 Workplace Electrical Safety is a Canadian standard intended to provide guidelines for safe working procedures to help protect workers around energized and potentially energized electrical equipment. Standards like CSA Z462 have put more focus on providing a practical, safe working area for employees by helping to reduce the hazards associated with electrical energy, including arc flash risks.
As codes and standards continue to evolve to raise the awareness of potential hazards, users of electrical control products are looking for leading-edge products capable of delivering higher levels of safety. Arc-resistant motor control centres and intelligent control systems can fulfill that need in many applications. These systems offer improved safety features along with remote operation and monitoring capabilities.
FRAMEWORK OF AN EFFECTIVE DESIGN
Arc-resistant control products are designed to contain the arc energy and direct it away from personnel. This type of equipment is designed and tested to control arc flash exposure by controlling the spread of the arc or channeling the high temperature and pressure wave away from personnel. Some systems include remote monitoring and control capabilities designed to minimize the amount of time personnel are required to be near the equipment, helping to further reduce the safety risk.
Arc-resistant control equipment, in__cluding low-voltage (LV) and medium-voltage (MV) control centres, are designed, tested and built to contain and redirect the energy away from personnel. The level of protection for those working around arc-resistant equipment is defined by an accessibility type.
The ability of arc-resistant control equipment to provide Type 2 or 2B accessibility, as defined in IEEE standard C37.20.7-2007, is another important aspect to understand. Type 2 accessibility helps shield personnel on the front, rear and sides of the enclosure from the effects of an internal arc fault. Type 2B accessibility allows control compartment doors to remain open while retaining the cabinet’s arc-resistant capabilities.
Rugged structural designs and a well-supported, isolated and insulated bus system are critical for arc-resistant equipment to withstand the effects of an arc flash event. Two-sided sheets on every section and robust bus support designs also contribute to the ability of the equipment to contain and redirect the arc flash energy.
Additional options, such as blown-fuse indication, exterior viewing windows on unit doors, arc-tested infrared viewing windows and finger-safe component barriers, are just a sample of the available options that help provide further protection and reduce the risk of personnel making contact with energized components.
One of the newest features in LV and MV MCC technology is the use of built-in networking and preconfigured software. By including a built-in industrial network, based on an open protocol, along with MCC monitoring and configuration software, users can remotely monitor, configure and troubleshoot the MCC, minimizing the need for personnel to enter into an arc flash boundary zone.
CLEARING THE CONFUSION
When considering any arc-resistant control equipment, it’s important to understand the performance criteria that must be met before the equipment can be classified as an arc-resistant design. “Arc-resistant,” as it applies to electrical equipment like MV MCCs, is a recognized industry term defined by 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.
In many cases, motor controls that are not arc resistant simply cannot withstand the effects of internal arcing faults for the tests prescribed in the IEEE standard. Instead of achieving the advanced level of protection being sought, many users are relegating their strategy (perhaps unknowingly) to one based solely on preventive measures. This limited approach may not fully address arc flash dangers and may only protect a small scope of users.
By not implementing tested arc-resistant equipment designs, users may be failing to address the possibility that an arc flash could occur in any unit, not just the unit that is being worked on. It disregards the safety of non-electrically qualified personnel who may be in the vicinity of the electrical equipment and unaware of the potential hazards of the equipment. In other words, no matter how much prevention is practised, a chance remains that an arc fault could occur in equipment during normal operation.
A fundamental approach to arc flash safety must include all personnel, electrically qualified and non-qualified. Arc faults can occur during normal operation without any specific human action and affect someone simply walking by the equipment or sweeping the floor in the equipment room.
STICKING TO THE STANDARDS
In the case of LV MCCs, 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.
According to CSA Z462, PPE levels are to be determined by either using the “default” hazard/risk category tables contained in the standard or by calculating the expected arc flash incident energy using one of the methods described in the standard. Furthermore, for the closed-door activities described in the default tables, the hazard/risk category assumes that no abnormal condition (like an arc fault) will occur, or if it does, the unit door will remain closed. This is a potentially fatal assumption if the equipment is not rated as arc resistant.
Even if reducing the risk category by keeping the door closed on a LV MCC were allowed, the real question is: Will the door stay closed in the event of a fault in the unit? The reality is that during an internal arcing fault, the doors of equipment that is not arc resistant may open, even if they were properly closed and latched. This could increase personnel exposure to the effects of the arcing fault, perhaps even exceeding the capabilities of the PPE selected based on the default tables.
DILIGENCE PAYS OFF
All users of electrical control equipment in an industrial environment are responsible for performing risk assessments to identify arc flash hazards. An assessment defines potential arc incident energy levels adjacent to particular electrical equipment and yields the required level of PPE needed when working near energized electrical equipment.
Ultimately, the best prevention against exposure to an arc flash is a safety program that complies with the requirements outlined in the CSA Z462 standard and implements arc-resistant control equipment. Beyond that, the most important advice is “shut it off.”
Because of high-production volumes and the use of multiple and differing automation systems in a single plant, identifying and significantly reducing potential hazards can be a complex task. This means manufacturers must be diligent in their training practices and highly selective in their technology choices to increase workplace safety in compliance with present safety standards. With advances in arc-resistant control and protection technology designed to help deliver improved safety, increased productivity and greater cost savings, users of electrical equipment can more easily mitigate arc flash safety risk.
John Kay is the senior product specialist, Institute of Electrical and Electronics Engineers (IEEE) Fellow, Rockwell Automation.
This column originally appeared in the March/April 2012 issue of Manufacturing AUTOMATION.