Electrical arcs form when a medium that is normally an insulator, such as air, is subjected to an electric field strong enough to cause it to become ionized. This ionization causes the medium to become a conductor which can carry current. The phenomenon of electrical arcing is as old as the world itself.
Lightning is a natural form of electrical arc. Man-made electrical arcs exist in devices such as arc furnaces. However, utilization of electrical energy invariably requires equipment where unintentional arcing between conductors becomes a possibility.
Unintentional arcing in power equipment can impose several different types of hazards:
- Heat from arc can cause severe flash burns many feet away (temperatures can reach 20,000 K, four times the temperature at the surface of the sun!).
- Byproducts from the arc, such as molten metal spatter, can cause severe injury.
- Pressure wave effects caused by the rapid expansion of air and vaporization of metal can distort enclosures and cause doors and cover panels to be ejected with severe force, injuring personnel.
- Sound levels can damage hearing.
Discussing arc flash hazards…
14 terms are of particular importance when discussing arc flash hazards:
1. Flash Hazard – A dangerous condition associated with the release of energy caused by an electric arc.
2. Incident Energy – The amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. One of the units used to measure incident is calories per square centimeter (cal/cm2).
3. Flash Hazard Analysis – A study investigating a worker’s potential exposure to arc-flash energy, conducted for the purpose of injury prevention and the determination of safe work practices and appropriate levels of PPE.
4. Live Parts – Energized conductive components.
5. Exposed (as applied to live parts) – Capable of being inadvertently touched or approached nearer than a safe distance by a person. It is applied to parts that are not suitably guarded, isolated, or insulated.
6. Shock Hazard – A dangerous condition associated with the possible release of energy caused by contact or approach to live parts.
7. Flash Protection Boundary – An approach limit at a distance from exposed live parts within which a person could receive a second degree burn if an electrical arc flash were to occur.
8. Limited Approach Boundary – An approach limit at a distance from an exposed live part within which a shock hazard exists.
9. Restricted Approach Boundary – An approach limit at a distance from an exposed live part within which there is an increased risk of shock, due to electrical arc over combined with inadvertent movement, for personnel working in close proximity to the live part.
10. Prohibited Approach Boundary – An approach limit at a distance from an exposed live part within which work is considered the same as making contact with the live part.
11. Qualified Person – One who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training on the hazards involved.
12. Working On (live parts) – Coming in contact with live parts with the hands, feet, or other body parts, with tools, probes, or with test equipment, regardless of the personal protective equipment a person is wearing.
13. Working Near (live parts) – Any activity inside the Limited Approach Boundary.
14. Electrically Safe Work Condition – A state in which the conductor or circuit part to be worked on or near has been disconnected from energized parts, locked/tagged in accordance with established standards, tested to ensure the absence of voltage, and grounded if determined necessary.
NFPA 70E – Electrical safety in the workplace
NFPA 70E chapter 1 covers personnel responsibilities (both the employer and the worker have specific responsibilities for safety), training requirements, the establishment of an electrical safety program, and the establishment of an electrically safe working condition.
In this case live work requires an Energized Electrical Work Permit, for which the requirements are given in Article 130.1 (A) (2). Some exemptions are given to the requirement for an electrical work permit, such as testing, troubleshooting, etc., performed by qualified persons.
The approach boundaries to live parts are defined above, and are illustrated in Figure 1 below. These form a series of boundaries from an exposed, energized electrical conductor(s) or circuit part(s). The requirements for crossing these become increasingly restrictive as the worker moves closer to the exposed live part(s).
The limited, restricted, and prohibited approach boundaries are shock protection boundaries and are defined in NFPA 70E table 130.2 (C).
Unqualified persons must stay outside the limited approach boundary unless they are escorted by a qualified person. Unqualified persons cannot cross the restricted approach boundary.
A flash hazard analysis must be performed in order to protect personnel from the possibility of injury due to arc flash. This analysis must set the flash protection boundary, which for voltages below 600 V is equal to 4 ft. based upon a clearing time of 0.1 second and a bolted fault current of 50 kA (5000 Ampere-seconds) or, where the clearing time x bolted fault is greater than 5000 ampere seconds or under engineering supervision, may be calculated with the equations given in the NFPA 70E text.
The means of calculating the arc flash protection boundary for voltages 600V or less is based upon the theoretical “Lee” method. The method for calculating the arc flash incident energy for a given working distance from live parts is not specified in NFPA 70E code text itself; several methods are given in Annex D of NFPA 70E. The preferred methods for performing these calculations are given in IEEE 1584.
The option is also given to use pre-prepared tables given in NFPA 70E based upon given levels of fault current and protective device clearing time to select personal protective equipment in lieu of a formal arc flash study.
The classifications for personal protective equipment (PPE)for arc flash protection are given in NFPA table 130.7 (C)(11), reproduced below as Table 1. PPE for arc flash protection is given an Arc Rating in cal/cm2, which must be compared to the arc flash incident energy for the location in question to select the proper clothing.
Face, neck, chin and eye protection must be worn wherever there is a danger of injury from electric arcs or flashes or from flying objects resulting from electrical explosion. Body protection, in the form of flame-retardant (FR) clothing as defined in Table 1, must be worn where there is possible exposure to arc flash incident energy levels above 1.2 cal/cm2; an exception allows Category 0 clothing to be worn for exposures 2 cal/cm2 or lower.
An example of a full arc flash suit is shown in Figure 2.
Table 1 – Protective clothing characteristics
|Hazard/risk category||Clothing description||Required minimum arc rating of PPE(cal/cm2)|
|0||Non-melting, flammable materials (i.e., untreated cotton, wool, rayon, or silk, or blends of these materials) with a fabric weight of at least 4.5 oz/yd2||N/A|
|1||FR shirt and FR pants or FR coverall||4|
|2||Cotton underwear – conventional short sleeve and brief/shorts, plus FR shirt and FR pants||8|
|3||Cotton underwear plus FR shirt and FR pants plus FR coverall, or cotton underwear plus two FR coveralls||25|
|4||Cotton underwear plus FR shirt and FR pants plus multilayer flash suit||40|
Full arc flash suit
Reference: Arc Flash Hazard Considerations – Bill Brown, P.E., Square D Engineering Services