Posts tagged ATPV
How the NFPA Defines Arc Flash Hazard Risk Categories (HRC)
11 year ago
by Todd Hoffmeister
in PandaPedia
The NFPA (National Fire Protection Association) provides five Hazard Risk Categories (HRC) to
define the level of an arc flash hazard. Each category is given as a single digit, designed to be easily
read by electrical workers from a safe distance, so that they may assess the relative danger of an arc
flash in a given situation and respond accordingly with the proper precautions and arc flash clothing.
Detailed HRC applications are described within the 2012 NFPA 70E standards, but in general, the
NFPA defines each HRC as follows. HRC 0 is generally used to describe non-contacting work
on various voltage ranges. HRC 1 describes work in an environment that may possibly produce
available incident energy of 4 cal/cm² to 7 cal/cm². HRC 2 describes work in an environment that
may possibly produce available incident energy of 8 cal/cm² to 24 cal/cm². HRC 3 describes work in
an environment that may possibly produce available incident energy of 25 cal/cm² to 39 cal/cm². And
HRC 4 describes work in an environment that may possibly produce available incident energy of 40
cal/cm² and greater.
How Much Energy is Released in an Arc Flash?
01 year ago
by Todd Hoffmeister
in PandaPedia
An arc flash can release an astonishing amount of energy. To understand just how much energy, we
first have to understand how energy is measured. In arc flash rated clothing, energy is measured in
calories. A calorie is defined as the amount of energy required to raise the temperature of one gram
of water 1°C. A candle’s flame generates 3.5 calories per second. We all know that’s more than
enough to cause a nasty burn. An arc flash that occurs in a 480V system with 20,000 amps of fault
current, and that lasts only 10 cycles at 60Hz (about 1/6 of a second), will release a mind-boggling
400,000 calories of energy! That’s enough energy to vaporize metal! So if you find yourself in a
situation where there is even the possibility that you may be exposed to an arc flash, do yourself a big
favor. Follow all precautions and wear appropriate arc flash rated clothing.
Why Worry About Arc Ratings and Arc Rated Clothing?
01 year ago
by Todd Hoffmeister
in PandaPedia
Arc Ratings are usually expressed as an ATPV (Arc Thermal Performance Value), measured in
calories per square centimeter (cal/cm²) of fabric. If a fabric has an ATPV arc rating of 8 cal/cm², that
means that this fabric can withstand 8 calories of energy (heat, flame) per square centimeter before
there is a 50% chance that the wearer will suffer a second-degree burn through that fabric. So what’s
a calorie? A calorie is the amount of energy required to raise the temperature of one gram of water
1°C. Let me put that in perspective. A candle’s flame generates 3.5 calories per second. If you put
the palm of your hand directly in a candle’s flame, hold it there, and say, “One Mississippi” slowly,
(about one second), it’s easy to imagine that you’ll get a pretty bad burn. Now, how much energy is
in an arc flash? If an arc flash occurs in a 480V system with 20,000 amps of fault current, and the
fault lasts for just 10 cycles at 60Hz, the resulting energy output is an astounding 400,000 calories!
And all of that energy can be released in about 1/6 of a second. This is enough energy to vaporize
metal! Think about what that would do to you if you are not wearing proper arc flash and flame-
resistant clothing!
Arc Ratings: The Difference Between ATPV and EBT
01 year ago
by Todd Hoffmeister
in PandaPedia
When encountering arc ratings for fabrics purporting to provide protection against arc flash, one
can see these ratings expressed as either ATPV or EBT. What is the difference? EBT stands for
Energy to Breakopen Threshold. EBT is used to describe a fabric that could actually break open or
break apart if it reaches its arc rating, thus possibly exposing flammable undergarments or skin to
arc effects at only a calorie or two above the rating, leading to the possibility of second-degree burns
or worse. However, when a fabric has an arc rating expressed as ATPV (Arc Thermal Performance
Value), that means that it will usually not break open until the fabric is exposed to energy levels at
about twice the arc rating. Obviously, this allows a much higher margin of safety and explains why
many people insist exclusively on fabrics with ATPV arc ratings.
What Does ATPV Mean in an Arc Rating?
01 year ago
by Todd Hoffmeister
in PandaPedia
ATPV stands for Arc Thermal Performance Value, and it’s measured in calories per square
centimeter (cal/cm²) of fabric. Let’s say that a certain fabric has an ATPV rating of 8 cal/cm². That
means that this fabric can protect the wearer against 8 calories of energy (flame, heat) per square
centimeter before there is a 50% chance that the fabric will burn through and the wearer will suffer
a second-degree burn. All testing of fabrics to determine their respective arc ratings is done at one
independent laboratory, Kinectrics of Toronto, Canada.
What is an Arc Rating?
01 year ago
by Todd Hoffmeister
in PandaPedia
An Arc Rating describes the amount of energy a certain fabric can defend against before there is a
50% chance that the wearer will suffer a second-degree burn through that fabric. The Arc Rating is
usually expressed as an ATPV (Arc Thermal Performance Value), measured in calories per square
centimeter (cal/cm²) of fabric. If a fabric has an ATPV arc rating of 8 cal/cm², that means that this
fabric can withstand 8 calories of energy (heat, flame) per square centimeter before there is a 50%
chance that the wearer will suffer a second-degree burn through that fabric.