Criteria for selecting arc flash protection techniques, Part 2

Conducting an arc flash study to identify hazards and proper PPE and tools is critical in reducing the risk of arc flash incidents.

04/10/2014


Learning Objectives:

  1. Understand the causes and consequences of arc flash incidents.
  2. Properly categorize different types of arc flash mitigation techniques.
  3. Learn the pros and cons of different types of arc flash mitigation techniques.

Part 1 of this three-part series focused on arc flash incident characteristics and the causes of said incidents, as well as standards for arc flash safety. This installment will explore work safety practices, the importance of arc flash testing, and the different types of arc flash protection with a focus on passive arc resistant switchgear.

Arc flash study

When equipment must be worked on energized, it is best to limit arc flash hazard exposure through engineered solutions or processes. Important solutions and processes that can be implemented include:

  • Identifying hazards by means of an arc flash study, warning labels, and live work permits
  • Establishing, documenting, and training workers on proper maintenance procedures
  • Redesigning a system to remove the need for live work if possible
  • Using proper personal protective equipment (PPE), tools, and training.

An arc flash study determines the available incident energy and the arc flash boundary for equipment under study based on the available fault current and arcing fault durations in the system. This helps to determine the proper PPE needed in each of the protection zones. You may discover that the incident energy level exceeds currently available PPE, so you would not be able to work on the equipment energized without other means of reducing the potential arc flash incident energy level.

Figure 1: This diagram shows NFPA 70E image C.1.2.4 depicting arc flash boundaries. Courtesy: Schneider ElectricNFPA 70E – 2012 table H.3(b) explains the necessary PPE for different levels of incident energy exposure. At 1.2 cal/cm2 and below, maintenance personnel must wear long sleeve/pant leg clothing made of protective nonmelting (in accordance with ASTM F 1506-08) material or untreated natural fiber. Above this energy level, personnel must wear arc-rated clothing and equipment with an arc rating equal to or greater than the incident energy determined in the arc flash hazard analysis. The specific articles of clothing and equipment are further specified in NFPA 70E, and that standard should be consulted in regard to determining proper PPE to be worn in arc flash hazard situations.

NFPA 70E and the NEC drive labeling of equipment with arc flash hazards. Energized work permits must be used to set proper procedures in place before work begins on live equipment. Again, we can’t stress enough that proper maintenance significantly reduces the possibility of an arc flash event. Clean equipment reduces the possibility of arc flash incidents due to dust, debris, and contaminants in switchgear. By properly maintaining equipment, the probability is increased that circuit interrupting equipment will operate as intended and as quickly as possible during a fault incident.

Providing proper PPE is only one part of overall protection. Tools that protect against accidental contact with live parts are also important. Remote operating mechanisms or racking mechanisms are always good options because they increase the distance between maintenance personnel and the potential arc flash hazard.

It is important to train maintenance personnel on the use of the PPE and tools provided. Safety must become a habit, and repetitive training develops that habit. PPE is the final barrier against arc flash hazard for the individual. It does not prevent the arc flash, but gives the wearer a better chance of surviving the event. Workers must wear this equipment when in the arc flash boundary. The available incident energy level dictates the level of PPE required for working in that area. This PPE includes, but is not limited to, a face shield, hearing protection, cotton or flame-resistant clothing, gloves, and insulated blankets. Arc-rated clothing is normally defined by the calories per square centimeter of energy (i.e., 40 cal/m2). This is why an arc flash hazard study should be conducted. It must be stressed that the protective clothing mentioned here does not protect against the pressure wave, intense light flash, or shrapnel that may be generated during the arcing event.

Arc flash protection methods

Arc flash protection techniques can be categorized into three basic groups: passive arc resistant, arc flash mitigating, and active arc resistant. Passive arc resistant protection concerns the build and physical structure of switchgear that helps to channel arc flash energy in a more controlled manner. No arc detection beyond standard circuit protection is implemented. Arc flash mitigation involves advanced detection techniques for arc flash. Arc flash mitigation methods attempt to detect arc flash events and signal breaker tripping faster than normal circuit protection methods. Active arc flash resistance techniques take arc mitigation one step further. In addition to quick detection of the arc flash, a method of energy redirection is implemented to move the dangerous energy of an arc flash to a more controlled state.

Table 1: The following is a listing of arc-resistant classifications according to IEEE C37.20.7. Courtesy: Schneider ElectricPassive arc resistant switchgear is designed to redirect the dangerous energy (pressure wave, heated air and metal, etc.) created during an arc flash event. Arc resistant switchgear is tested to and classified according to the IEEE C37.20.7 test guide (see Table 1).

There are several benefits to passive arc resistant switchgear:

  • The arc resistant features are built into the physical structure so no additional detection or activation is required.
  • Arc resistant switchgear tested to IEEE C37.20.7 is “intended to provide an additional degree of protection to the personnel performing normal operating duties in close proximity to the equipment while the equipment is operating under normal conditions.”
  • Arc flash incident energy is better controlled as long as there is no physical compromise of the enclosure, such as having a medium-voltage compartment doors open while the equipment is energized. Proper PPE must be worn in this instance according to the established arc flash incident energy.

There are disadvantages to this method compared to the other arc resistance and mitigating methods:

  • Passive arc resistant switchgear does not protect the equipment. The interior of the switchgear is still subject to the destructive forces of the arc blast and heat rise events. In most cases, the arc resistant enclosures will still need to be completely replaced, which are among the most expensive methods of arc flash protection.
  • Incident energy is not lowered with this method, as this method still relies on the standard circuit protection methods to trip upstream breakers and extinguish the arc.
  • In many cases the equipment requires the addition of plenums to provide a channel for the expulsion of arc flash materials (hot gases, molten metal, vaporized metal and chemicals) from the electrical room. In cases where the plenums are not required, this material is forced out of the switchgear and into the room with the switchgear and personnel.

The final installment of this three-part series will discuss the remaining methods for arc flash protection, arc flash mitigation systems and active arc flash resistant switchgear, as well as the benefits and disadvantages of each.


Ken Joye is staff marketing specialist at Schneider Electric. He has worked for Schneider Electric for 39 years, specializing in medium-voltage equipment applications for more than 20 years. Joe Richard is senior marketing specialist at Schneider Electric. He graduated with a BSEE from the Georgia Institute of Technology in 2007. He has worked for Schneider Electric for 6 years, specializing in medium-voltage switchgear and applications.



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