Nomex FR Apparel Newsletter - Volume 5

Understanding Arc Ratings for Personal Protective Equipment: ATPV & EBT

Understanding Arc Ratings for Personal Protective Equipment: ATPV & EBT

An electric arc is a potentially deadly risk that workers in utility, chemical, refinery and other hazardous industrial jobs face every day. Without proper personal protective equipment (PPE) for protection against the thermal effect of an electric arc, workers are at risk of suffering severe burn injuries or even death.

NFPA 70E, the Standard for Electrical Safety in the Workplace, is used to provide guidance for establishing safe work practices and selecting thermal PPE for workers who may encounter electric arc hazards from electrical equipment.

PPE Category - Arc Rating

This standard includes testing methods for apparel (ASTM F1959), face protective equipment (ASTM F1959) and rainwear (ASTM F1891). The arc rating of PPE estimates the amount of energy exposure that gives the wearer a 50% chance of receiving at least a second degree burn. In each of the ASTM testing methods listed above, the resulting arc rating of a material (or a multi-layer system of materials) can be expressed as either arc thermal performance value (ATPV) or energy of break-open threshold (EBT), whichever is reached first in the testing.

Important factors that can influence a protective system's arc rating—ATPV or EBT—are the weight, fabric construction and fiber components.

ATPV vs. EBT

ATPV and EBT use different criteria. ATPV estimates the amount of heat and energy that would be transferred through the PPE and result in a burn injury. In contrast, EBT records the amount of heat or energy that creates a hole large enough to result in a burn injury.

Materials reporting an EBT value as their arc rating exhibit break open before the onset of a model-predicted second-degree burn injury. When material break open occurs, there is potential for heat to pass through the material unimpeded and onto the wearer.

Is One Measure Better Than the Other?

The APTV and EBT values are expressions of different attributes of a flame-resistant (FR) material system and can result in significantly different levels of protection.

For single-layer systems, materials with arc ratings derived from the ATPV are the typical systems available in the marketplace. The materials of construction of these systems are expected to remain intact (no holes) during an arc exposure to provide thermal protection to the wearer.

On the other hand, if a material with an EBT arc rating is exposed to energies somewhat higher than the reported break-open value and a hole forms in the material, there is the potential for direct skin exposure or ignition of the non-FR under-layer of clothing, which may result in additional injuries.

For multi-layer systems, it is possible to obtain higher levels of thermal arc protection with materials that have EBT arc ratings. An FR outer-layer of material that breaks open can absorb a significant amount of thermal energy so it is not transferred to the wearer. In fact, a greater than 50% increase in arc thermal protection has been observed in some cases.

Selecting Arc Flash Clothing

Selecting Arc Flash Clothing

When evaluating a garment's arc protection, it's crucial to recognize that compliance to a standard does not always mean equal protection. The PPE system that you choose should match, or exceed, the needed protective values calculated as a result of a comprehensive hazard risk assessment required by the NFPA 70E standard. Selecting materials with arc ratings higher than the estimated exposure hazard generally leads to more protection for the wearer.

Innovative Solutions for Arc Flash Clothing

DuPont offers innovative solutions for arc flash clothing that provide proven protection for industrial workers around the world who perform specific tasks within an arc flash boundary. These solutions include:

DuPont™ Nomex® IIIA
A blend of Nomex®, DuPont™ Kevlar® and a static-dissipative fiber.

Nomex® MHP
An engineered blend of materials offering inherent multi-hazard protection against heat, flame, electric arc and small molten metal splash.

DuPont™ Protera®
A unique blend of inherent materials designed for both electric arc and flash fire protection.

Each of these solutions is available in a range of weights and all have an excellent arc rating to weight ratio. Some examples include:

  • Nomex® IIIA in a 6.0-oz/yd2 fabric has an ATPV of 6.4
  • Nomex® MHP in a 7.0-oz/yd2 fabric has an ATPV of 8.4
  • Protera® in a 6.5-oz/yd2 fabric has an ATPV of 8.5

 

Visit our website to learn more about Nomex® IIIA, Nomex® MHP and Protera®.



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