NFPA 70E: A roadmap to compliance
Six steps can reduce arc flash hazard and protect workers and equipment
Each year electrical accidents cause over 300 deaths and 4,000 injuries in the workplace. Risks associated with shock and electrocution from inadvertent contact with energized parts have long been recognized as a threat to electrical workers.
It has only been in recent years that awareness of the dangers of arc flash events has been incorporated into electrical safety standards. Injuries and fatalities that result from these accidents are devastating to the workers and their families.
Additionally, the financial consequences of such events can be very damaging to the employer. Standards that recommend electrical safe work practices are in place to reduce these statistics. OSHA enforces these published safety standards.
There are five important steps that companies should take to reduce the occurrence of electrical accidents and better protect workers and employers from the physical, financial, and statutory consequences of electrical accidents. These steps are required as part of the National Fire Protection Association (NFPA) standard 70E-2012, which provides a detailed reference for facilities to meet the requirements of electrical workplace safety. A sixth step assists the facility owners with fine-tuning the electrical system for safety and operability.
Step 1: Establish and audit an electrical safety policy with defined responsibilities
The Electrical Safe Work Practices (ESWP) policy is a written document created by the employer that covers all areas of the company's electrical safety. It includes such things as lockout/tag-out procedures, internal safety policies, and clearly defined responsibilities for electrical safety.
While OSHA regulations may not detail what is included in an ESWP policy, through the General Duty clause, it refers to NFPA 70E for specific methods on workplace safety. NFPA 70E covers the requirements for safe work practices and administrative controls and provides the basis for the policy. A company's ESWP policy should emphasize de-energization as a priority. If that is not feasible, it should clearly outline the procedure for working on live equipment and incorporate a work permit control for working on energized electrical equipment.
NFPA 70E-2012 Edition clarifies and expands upon the requirements to audit the safety policy itself. This audit must be performed on a three-year cycle to assure continued compliance of the policies and procedures to the standard. Additionally, workers must be evaluated for compliance to the standards and the policy on an annual basis, and any deviations must be documented. The ESWP policy is a "live" document that must be audited and corrected over time.
Step 2: Perform an arc flash hazard analysis and label the electrical equipment
The methodology for performing an arc flash analysis is outlined in IEEE 1584, Guide for Performing Arc Flash Hazard Calculations. The arc flash analysis will determine, among other things, the incident energy potential of each piece of electrical distribution equipment in the facility. This incident energy potential will define the personal protective equipment (PPE) that is required.
One alternative to a detailed arc flash analysis that is permitted in NFPA 70E-2012 Edition is to use the task tables in section 130.7(C)(15)(a) to determine the required PPE Hazard Risk category. Each table has usage limitations as stated in the body of the table. They specify a range of available fault current and clearing times for the upstream overcurrent protective device. The tables may not be safely used beyond this range. Unless a short circuit and coordination study has been performed, users will usually not know these details. This commonly leads to misuse of the task tables, which can lead to either over-protection or under-protection for the worker. An arc flash analysis performed to IEEE 1584, on the other hand, provides a complete evaluation of the power system with the actual incident energies on each piece of equipment clearly defined.
No matter the methodology used to establish the proper PPE, NFPA 70E requires labeling the electrical equipment to indicate the:
1. Level of incident energy or arc rating of clothing
2. Voltage rating of equipment
3. Arc flash boundary.
Step 3: Provide adequate supplies of PPE and insulated tools
Employees shall be provided with appropriate PPE to protect against shock and arc flash. The standards also require the employer to furnish insulated voltage-rated hand tools and insulated voltage sensing devices for testing and troubleshooting energized electrical equipment.
Step 4: Train and retrain all workers
NFPA 70E defines a qualified person as "one who has skills and knowledge related to the construction and operation of the electrical equipment and systems, and has received safety training to recognize and avoid the hazards involved." This requirement means that the employee must have received safety training specific to the hazards of arc flash, arc blast, shock, and electrocution. Electrical workers are not considered to be qualified by OSHA until they have received this specific training.
The 2012 edition of NFPA 70E requires that workers be retrained at least every three years. Retraining must also occur if the supervision or annual inspection indicates that the employee is not complying with the safety-related work practices. Additional factors that require worker retraining include introduction of a new technology or a new type of equipment, or changes in procedures or job duties that necessitate different safety practices.
Step 5: Maintain equipment for personnel safety
While maintenance has long been considered to be the key to long-term reliability of equipment, NFPA 70E dedicates an entire chapter to the subject of maintenance for electrical safety. All electrical distribution systems contain active components such as fuses, circuit breakers, and protective relays that help protect the system in the event of an electrical fault. These components are also crucial when it comes to protecting workers from the hazards of arc flash and arc blast.
Modern, properly adjusted overcurrent protective devices that have been well maintained are able to detect an arcing condition and clear the fault quickly. The action of overcurrent protective devices results in significantly reducing the amount of incident energy that is released.
In the past, attention to maintenance and the condition of electrical equipment was not a primary concern for most facility owners. In many cases it was not clearly understood that poor condition or inadequate maintenance of the devices presented an elevated safety hazard for workers. With the current focus on workplace hazards and electrical safety, companies will be more vigilant when it comes to properly maintaining their electrical systems.
Step 6: Develop solutions to the arc flash problem
There have been significant efforts on the part of electrical equipment manufacturers to develop strategies for arc flash reduction. Arc flash encompasses many physical phenomena, and the continuing research will yield innovative technologies in the years to come. Yet, the methodologies to reduce arc flash energy or to mitigate its effects fall under two basic categories. The first category is a collection of engineering controls that are intended to reduce the arcing time. The second category encompasses equipment and techniques to remove the worker from the danger zone.
Reduce arcing time
The calculated arc flash energy may be high enough to hinder some normal tasks from being performed within the arc flash boundary of the equipment. Consider this example: The calculated arc flash energy at a 480V-rated motor control center is 35 calories per square cm. Workers may view the PPE requirements, that is, heavy clothing and flash hood, as a hindrance to mobility and vision when performing basic troubleshooting tasks. Since motor starters in the motor control center require routine maintenance and testing, this could become a significant operational problem. It necessitates the development of strategies to reduce the arc flash incident energy level on this equipment.
Arc flash incident energy level is a function of the arcing time, which is a function of the speed of operation of the upstream overcurrent protective devices. System design, protective settings, and equipment selection can all affect this parameter. The main strategies to reduce arcing time include the following:
1. Overcurrent device trip settings can be adjusted to reduce the incident energy with minimal impact to device coordination. This is done by performing a coordination study. If the upstream circuit breaker is located in a separate compartment, the arc flash energy on the downstream equipment may be reduced.
2. Various relaying technologies exist that may help reduce the arcing time. Light sensing technologies, zone selective interlocking, and differential protection all act to provide a tripping signal that can be used to trip a circuit breaker upstream. This may reduce the energy released during an arc flash event. These technologies are effective where the upstream circuit breaker is installed in a separate compartment.
3. A virtual main protection system is the most effective methodology to reduce arc flash energy on the low- voltage side of a unit substation. A set of current transformers, located on the secondary side, sense the fault current and send a trip signal to a circuit breaker on the high-voltage side of the transformer.
4. Fast fault-making devices introduce a "controlled" bolted 3-phase fault on the switchgear bus to divert the energy and extinguish an arcing fault. A relaying scheme such as the light sensing technology discussed above provides the trip signal for the system. The overall operation is fast enough to significantly reduce the arc flash energy that is released.
Remove workers from the danger zone
Keeping workers out of the arc flash protection boundary when equipment is energized is another way to improve workplace safety. Infrared windows, remote racking systems, and wireless temperature monitoring are three methodologies to consider.
1. Infrared windows allow thermographic inspections to be performed at any time without removing equipment covers or panels. Thermography is a valuable tool for preventive maintenance, allowing workers to determine load, connection, component fatigue, overheating, or phase problems. Infrared windows can be installed in both low-voltage and medium-voltage distribution equipment.
2. Remote racking systems can avert arc flash injuries by moving the worker out of the flash boundary during circuit breaker racking operations-an activity during which a significant percentage of arcing faults occur. The system allows racking operations to be remotely controlled and monitored without direct human contact.
A motorized racking mechanism is retrofitted into each medium-voltage circuit breaker cell. The racking operation is initiated and monitored via a remotely located operator interface. Remote racking systems maintain the equipment's original interlocks and can be retrofitted into existing cubicles.
3. A wireless temperature monitoring system embeds temperature sensors on key components of electrical switchgear, such as bus joints and circuit breaker stabs. The sensors communicate temperature readings through a wireless communication system. Installing a wireless temperature monitoring system can reduce the need for annual thermographic inspections. As a result, equipment covers do not need to be removed and workers are not exposed to energized components.
Electrical hazards are a significant safety and financial risk for electrical workers and their employers. While the threat of shock and electrocution from inadvertent contact with energized parts has long been recognized, the arc flash and arc blast hazards have only fairly recently been incorporated into the electrical safety standards.
The continued focus on arc flash and arc blast hazards will play a critical role in reducing the frequency and severity of electrical accidents over time. The process of complying with the safe work practices dictated by NFPA 70E presents an opportunity to re-examine your electrical system and procedures to gain a better understanding of potential issues.
Implementing new engineering philosophies and designs will enhance workplace safety for employees and reduce the financial risk for your company.
Reza Tajali is power system engineering manager at Schneider Electric.