The ATPV or Arc Thermal Protective Value is measured in calories per centimeters squared and represents the maximum arc-flash protection the fabric can protect against.

Testing a flame-resistant fabric via method ASTM 1959 can cause two things to occur:

  1. The fabric will break open
  2. The fabric will remain intact suppressing heat for a 50% chance of a 2nd degree burn occurring, all based on the Stoll curve.

Regardless of its arc-rating, once the heat threshold is passed, the fabric will no longer provide protection from injury. Usually the ATPV is measured at the heat threshold.

Simply a fire-resistant shirt may be able to protect you from the energy of a 40 calories arc while giving you a 2nd degree burn from a smaller arc. This means the shirt is rated for the smaller arc. ATPV accounts for both the energy, and heat transfer.

Many woven fabrics, such as twill, shirts, pants, or coverall materials, will not break open, and will have an assigned ATPV value.

What is an Ebt rating?

When a fabric breaks open before a second degree burn is occurred, the fabric is given an Ebt Rating.

The Energy breakopen threshold (Ebt) is an average of the top five highest energy exposure values below the Stoll curve, on fabrics that do not break open. The Ebt rating means that the fabric will break open, and you will not be burned.

If exposed to high enough energy, all flame-resistant fabrics will break open. When exposed to energies higher than the Ebt rating, there is a change that direct skin, or non-flame-resistant materials may ignite and cause additional injuries. Factors such as the fabric’s weight, construction, or fiber make-up influence the intentional ATPV or Ebt rating.

ATPV vs. Ebt for Arc Ratings 

Both ATPV and Ebt are evaluated using the ASTM F1959 test. The reported arc-rating of the fabric is the first value reached. On some fabrics, both ATPV and Ebt is reported, but only the lowest value is the arc-rating that will be assigned. According to ASTM F1506 industry standards, only the lowest value is reported on the label. To be arc rated under this standard, the fabric must pass several tests including wash testing and the standard vertical flammability test.

If Ebt is assigned to the fabric, the value will be reported as “Arc Rating (Ebt)” on the label. Many Ebt fabrics are considered more insulating than strong, while ATPV is generally stronger than insulating.

Understanding Arc Ratings and Calories

The Arc Rating measures the amount of heat the flame-resistant fabric blocks when exposed to an electric arc. This is the amount of calories the fabric can be expected to absorb if exposed to an electric arc. In simple terms, the Arc Rating of a fabric is the level of protection you can expect while wearing the fabric or clothes.

A calorie is a unit for measuring of the heat energy of an arc flash and the protective level of flame-resistant clothing. The higher the calories, the higher the heat of the flash, and the greater protection of the fabric. As long as the calories of the flash is less than the arc rating, you will be protected by the fabric.

Even thought a fabric may have either an Ebt or an ATPV value, it is important to check out the calorie level the fabric can support. A table is provided below for estimates on various levels, and arc ratings.

Level Minimum Arc Rating (cal/cm2) Maximum Arc Rating (cal/cm2)
HRC 0 0.0 0.0
HRC 1 4.0 7.9
HRC 2 8.0 24.9
HRC 3 25.0 39.9
HRC 4 40.0 +