The radiant energy released by an electric arc is capable of permanently inuring or killing a human being at distances of up to ten or even twenty feet. Photo: TestGuy.
Numerous workers are injured and/or killed each year while working on energized electrical equipment.
Many of these workplace injuries are a result of arc flash, a type of electrical explosion that results from a low-impedance connection to ground or another voltage phase in an electrical power system.
Arc flash temperatures can reach or exceed 35,000 °F (19,400 °C) at the source of the arc. The massive energy released in the fault rapidly vaporizes the metal conductors involved, blasting molten metal and expanding plasma outward with extraordinary force.
Arc Flash Incident Energy
Incident energy of an arc flash can best be explained with a simple campfire analogy. If you sit right next to a campfire, you would receive more radiant (incident) energy than if you sit farther away.
The radiant energy released by an electric arc is capable of permanently inuring or killing a human being at distances of up to ten or even twenty feet. In most cases, arc flash incident energy depends on three elements:
- Intensity - How much power the arc has. This is calculated using the system voltage and the maximum available fault current.
- Distance - How far away a from the arc worker will be. Also known as the "working distance", this measurement is estimated to be 18 inches.
- Duration - How long the arc will last. If there is enough power available, an arc flash could last an extended period of long time. This is why its important that protective devices be properly set and coordinated in order to identify the beginnings of an arc and quickly interrupt the fault.
Incident energy is measured in calories/cm2. If unprotected skin is exposed to an incident energy of 1.2 cal/cm2, a second-degree (curable) burn is expected. Higher incident energy can cause third-degree (incurable) burns. A large arc flash can easily generate incident energy in the range of 40 cal/cm2
This video demonstrates the devistating effects of a large arc flash. Arc flash temperatures can reach or exceed 35,000 °F (19,400 °C) at the source of the arc. The massive energy released in the fault rapidly vaporizes the metal conductors involved, blasting molten metal and expanding plasma outward with extraordinary force.
An Arc Flash Study or Analysis is a calculation performed by Professional Engineers to determine the incident energy found at each location which determines the various arc flash boundaries and what Personal Protective Equipment (PPE) must be used in approaching each boundary.
NFPA 70E Arc Flash and Shock Hazard Boundaries
As a result of accidents in the workplace related to arc flash, the National Fire Protection Association (NFPA) has developed specific approach boundaries designed to protect employees while working on or near energized equipment.
NFPA 70E stipulates two approach boundaries in addition to the arc flash protection boundary that must be known and observed. The shock hazard boundaries are dependent on the system voltage and can be found in Table 130.4(D).
An update to the 2012 version of Table 130.4 includes voltages up to 800kV. The Prohibited Approach Boundary was removed from NFPA-70E in 2015.
1. Flash protection boundary (outer boundary)
The flash protection boundary is the farthest established boundary from the energy source. If an arc flash occurred, this boundary is where an employee would be exposed to a curable second degree burn (1.2 calories/cm2). When an energized conductor is exposed, you may not approach closer than the flash boundary without wearing appropriate personal protective clothing and personal protective equipment.
2. Limited approach boundary
The limited approach boundary is the minimum distance from an exposed live component where unqualified personnel may safely stand. No untrained personnel may approach energized parts any closer in this boundary unless under the supervision of a qualified worker and using proper PPE. A qualified person must use the appropriate PPE and be trained to perform the required work to cross the limited approach boundary and enter the limited space.
3. Restricted approach boundary
The restricted approach boundary is the distance from an exposed part which is considered the same as making contact with the live part. Only qualified personnel wearing appropriate personal protective equipment (PPE), having specified training to work on energized conductors or components, and a documented plan justifying the need to perform this work may cross the boundary and enter the Restricted Space. Insulated gloves, tools, and equipment are required within this boundary.
Who is a qualified person?
The NFPA 70E 2012 definition of a qualified person is: "one who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training to recognize and avoid the hazards involved."
It is important to note that a person can be considered qualified with respect to certain equipment and methods, but still be unqualified in other situations.
A qualified person shall know the hazards involved, the applicable workplace safety practices, and have received specific safety training. Only a qualified person can be an escort into an area with exposed energized parts. Employees who respond to medical emergencies must also participate in refresher training.
What skills are required to work within the limited approach boundary?
Qualified Personnel permitted to work inside the Limited Approach Boundary of exposed electrical conductors and circuit parts operating at 50 volts or more shall have the following skills:
- The skills and techniques necessary to distinguish exposed energized electrical conductors and circuit parts from other parts of electrical equipment.
- The skills and techniques necessary to determine the nominal voltage of exposed energized electrical conductors and circuit parts.
- An understanding of the approach distances specified in Table 130.4(D) and the corresponding voltages to which the qualified person will be exposed.
- The decision making process necessary to determine the degree and extent of the hazard and the personal protective equipment and job planning necessary to perform the task safely.
The 2015 version of NFPA 70E adds the requirement that such a person shall demonstrate the ability to use — and not just be familiar with the proper use of — the following:
- Special precautionary techniques
- PPE including arc flash suits
- Insulating and shielding materials
- Insulated tools and test equipment
Who is considered to be an unqualified person?
A person with limited electrical knowledge and little or no electrical training on avoiding the electrical hazards associated with work on or near exposed energized parts is considered to be unqualified personnel.
Where there is a need for an unqualified person to cross the Limited Approach Boundary, a qualified person shall advise him or her of the possible hazards and continuously escort the unqualified person while inside the Limited Approach Boundary.
Under no circumstance shall the escorted unqualified person be permitted to cross the Restricted Approach Boundary.
Why is 50 volts and above considered to be "live?"
NFPA 70E Article 130 states that "Energized electrical conductors and circuit parts that operate at less than 50 volts shall not be required to be deenergized where the capacity of the source and any overcurrent protection between the energy source and the worker are considered and it is determined that there will be no increased exposure to electrical burns or to explosion due to electric arcs."
50 volts is considered to be a lethal voltage based on a simple calculation using ohms law (V=IR).
- 1/10 of an amp (0.10A) through the body is considered to be a fatal level of current (I)
- The average body resistance of a human being is considered to be 5000 ohms, however body resistance while working can be reduced to 500 ohms (R).
When you plug these numbers into ohms law you get the following result: 50V = 500ohms x 0.10A