electrical_safety_electrical__arco_flash

safety

1. Arc Flash An arc fault happens when electric current flows through air gaps between conductors. The fault current magnetic fields make conductors separate, producing an arc. The expansive vaporization of conductive metal can escalate into a three phase arcing fault in less than a 1/1000 of a second. The heat energy and intense light at the point of the arc is called arc flash.

2. Arc Flash The product of the arc fault current and voltage results in enormous energy released in several forms. Large amounts of heat can severely burn human skin and set clothing on fire. Temperatures can reach four times the temperature of the sun's surface. High arc temperatures vaporize the conductors in an explosive change in state from solid to vapor.

3. Arc Flash Copper vapor expands to 67,000 times the volume of solid copper. The duration of the arc is determined by the time it takes for protective devices to open the circuit. Explosive expansion of the surrounding air can create pressure waves able to blow personnel off their feet. In addition to the blast, boiling metal sends out a shower of molten droplets in the vicinity of the arc.

4. Arc Flash Arc Flash Hazards Severe burns, damaged eyesight, ruptured eardrums, collapsed lungs and in some cases death. Total force on a person standing in front of an open enclosure can exceed 1,000 lbs. Enough to crush a person's chest breaking bones, puncturing lungs and other organs. Even if the blast is not powerful enough to propel a worker, severe head tossing often occurs, causing whiplash type injuries with the possibility of brain damage.

5. Arc Flash Intense UV light can damage the retina, cause blurred vision and severe headaches. Hot vapors injure lungs. Hearing can be lost or damaged. Sudden pressure changes exceeding 720 lbs/ft 2 for 400 milliseconds can rupture eardrums.

6. Arc Flash Arc Flash Incident

7. Arc Flash Incident Energy The incident energy is a measure of thermal energy at a working distance from an arc fault. The unit of incident energy is cal/cm 2 . Incident energy is inversely proportional to the working distance squared and directly proportional to the time duration of the arc and to available fault current.

8. Arc Flash The threshold value of incident energy for 2nd degree burn of human skin is about 1.2 cal /cm 2 . One cal/cm 2 is equivalent to the amount of energy produced by a cigarette lighter in one second. PPE PPE includes clothing, gloves, tools, face protection, and glasses. The head and chest areas are the most critical and must be protected.

9. Arc Flash Flash Protection Boundaries The safe approach distance from energized equipment or parts. This boundary has been established as a default at 4 feet for low voltage (<600 V) systems where the total fault exposure is less than 5,000 amperes-seconds. Persons crossing into the flash protection boundary are required to wear the appropriate PPE.

10. Arc Flash To protect qualified and unqualified workers, arc flash protection boundaries should be established around an area where energized work is being performed by observing the limits of approach set forth by NFPA 70E. NFPA 70E establishes four different approach boundaries for worker safety.

11. Arc Flash Flash Boundary – If an energized conductor is exposed, no one may approach closer than this boundary without wearing arc protection. Limited Approach – No unqualified person may approach any exposed energized conductor closer than this boundary. Restricted Boundary – Qualified persons are not allowed to approach closer than this boundary unless wearing PPE and have a written, approved work plan. Prohibited Boundary – Crossing this boundary is considered as actually contacting the exposed energized part. In addition to the Restricted Boundary requirements, personnel must perform a risk assessment before crossing this boundary.

12. Arc Flash

13. Arc Flash Arc Flash Warning Labels Located so they are visible to the personnel before examination, adjustment, servicing, or maintenance of the equipment.

14. Arc Flash While the arc flash hazards associated with AC systems have been identified, no such standardization exists for DC systems such as the battery banks. In the absence of guidance from adopted standards, it is recommended that workers use Category 1 or better PPE and face shield when working with systems over 50 VDC.