Posts tagged arc flash
New NFPA 70E standards for 2012 now require five specific pieces of electrical equipment to
have arc flash labels. They are switchboards, panel boards, industrial control panels, meter socket
enclosures and motor control centers. Aimed at improving electrical safety in commercial facilities,
the new standards focus on electrical equipment requiring maintenance while energized. New arc
flash labels must also include additional details. Each label must now indicate nominal system
voltage, arc flash boundary, and at least one of the following: available incident energy and the
corresponding working distance, minimum arc rating of clothing, required level of PPE or highest
hazard/risk category (HRC) for a piece of equipment.
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.
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.