The term Arc Rating is crucial in the context of electrical safety, particularly when discussing protective clothing and other fabric-based protective equipment used by workers facing potential arc flash hazards. Below is a deeper look into what arc rating entails and its importance:

Understanding Arc Rating

Arc Rating is a measure used to describe the protective performance of a material against the thermal effects of an electrical arc discharge. This rating helps to determine the level of protection provided by clothing and other fabric materials under exposure to arc flash conditions. The arc rating is typically expressed in calories per square centimeter (cal/cm²).

Components of Arc Rating: ATPV and EBT

Arc rating is derived from testing and evaluation of a material’s performance under specific conditions, focusing on two key metrics:

1. Arc Thermal Performance Value (ATPV): ATPV is the amount of incident energy (expressed in cal/cm²) that a fabric can withstand before there is a 50% probability of causing a second-degree burn through the material. This measurement provides a clear threshold indicating when a fabric will likely begin to fail in protecting against heat from an arc flash.
2. Energy of Breakopen Threshold (EBT): EBT refers to the incident energy level where the material shows a breakopen, or a failure that is not necessarily related to thermal transmission but more about the material’s integrity under exposure to high temperatures and energies. Breakopen can expose skin or underlying layers of clothing, potentially leading to more severe injuries than those prevented by the fabric’s thermal resistance.

Determining Arc Rating

The arc rating is reported as the lower value between ATPV and EBT. This approach ensures that the rating reflects the material’s performance at its weakest point, whether it’s thermal resistance (ATPV) or structural integrity (EBT). This methodology prioritizes safety by focusing on the lower threshold of protective performance.

Importance in Safety Gear

The significance of knowing the arc rating of protective gear cannot be overstated:

• Safety Compliance: Compliance with safety standards such as NFPA 70E, which mandates specific levels of protective clothing based on the arc flash hazards assessed at a workplace.
• Worker Protection: Ensuring that workers are equipped with gear that can genuinely protect against the thermal effects likely to be encountered, based on realistic and worst-case scenario assessments of the energy levels involved in their specific working conditions.

Practical Application

In practice, the arc rating of a material determines how it is categorized and used in safety protocols. Protective clothing and gear are selected based on the potential arc flash energy levels identified in hazard risk assessments. This selection ensures that workers wear clothing that meets or exceeds the necessary protective requirements corresponding to the specific risks they face.

By using materials with the appropriate arc ratings, workers are better protected against the dangers of arc flash, significantly reducing the risk of burns and other serious injuries in the event of an electrical discharge.

The following definitions can be found in Article 100 of NFPA 70E:

Below is a sample of NFPA 70E. For the complete section, see the actual NFPA 70E text at NFPA.ORG. Once there, click on the free access link to NFPA 70E.

Hazard, Arc Flash. (Arc Flash Hazard). A source of possible injury or damage to health associated with the release of energy caused by an electric arc.

Arc Flash Suit. A complete arc-rated clothing and equipment system that covers the entire body, except for the hands and feet.

Arc Rating. The value attributed to materials that describes their performance to exposure to an electrical arc discharge. The arc rating is expressed in cal/cm² and is derived from the determined value of the arc thermal performance value (ATPV) or energy of breakopen threshold (E_BT) (should a material system exhibit a breakopen response below the ATPV value). Arc rating is reported as either ATPV or E_BT, whichever is the lower value.