This presentation provides an overview of modern methods for minimizing the arc-flash hazard.

Transcription

1 Electricity is all around us. An uncontrolled release of electric energy is extremely dangerous, posing a threat to human life, causing equipment damage, and jeopardizing the manufacturing process that depends on it. This presentation provides an overview of modern methods for minimizing the arc-flash hazard. 1

2 An arc-flash hazard is a dangerous condition associated with the release of energy caused by an electric arc. Arc-flash events produce some of the highest temperatures on earth, up to 19,000 C (35,000 F) or more. For comparison, arc furnaces are approximately 1,600 to 3,000 C (3,000 to 5,000 F). A flash-protection boundary is an approach limit at a distance from insulated or exposed live parts, within which a person could receive a second-degree burn. The working distance is the dimension between the possible arc point and the head and body of the worker positioned in place to perform the assigned task. 2

3 Small accidents can lead to big problems. Human error accounts for many arc-hazard incidents. 3

4 Arcing faults produce the following severe worker hazards, which can have long-term effects: Infrared (IR) and ultraviolet (UV) radiation. IR (visible) and UV light levels are bright enough to cause corneal and retina eye damage. Pressure. Blast pressure waves have thrown workers across rooms and knocked them from ladders, resulting in concussions and severe brain damage. Pressure on the chest can be higher than 2,000 lb/ft 2 (100 kpa), which is sufficient to collapse a lung. Sound. Hearing loss can occur from a sound blast. The arc expands air rapidly, similar to thunder caused by lightning. Researchers have recorded magnitudes as great as 140 decibels at a distance of two feet from the arc. Shrapnel. Arcs spray molten metal in a lethal mist at high-speed pressure. The high-momentum shrapnel can easily penetrate a worker s body. Heat. Fatal burns can occur less than three feet from the arc. Serious burns are common at a distance of ten feet. Staged tests show temperatures greater than 200 C (392 F) on the neck and hands for a person standing two feet from the arc. Clothing can be ignited from several feet away and can cause more severe burns than exposed skin. Smoke. Toxic gases are formed by a chemical reaction between the molten copper dust and the atmosphere. Breathing these gases can result in thermal injury to the upper airway. Chemical injury to the lungs can cause narrowing of the airways and pneumonia. The statistic on the slide is courtesy of Electric Energy Online: Thomas E. Neal, PhD and Randell B. Hirschmann, The Myths and Realities of Arc Flash Protection, Electric Energy T&D Magazine, Vol. 8, No. 4, May-June 2004, pp Available: 4

5 The main picture on the slide shows an arc-flash incident scene. The portable light was not in place during the incident. This incident was caused by a mounting cover plate on the door of a 480-volt circuit breaker cubicle. Two substation secondaries were tied together, so the available fault energy level was higher than normal. Circumstances suggest that the cubicle door was closed before the cover plate was secured. An estimated over 60 cal/cm 2 exposure resulted in four workers being seriously injured. The inset picture shows the damaged cubicle. Notice the following*: The top cover plate stud is intact. The bottom cover plate stud has melted away. Scorch marks indicating heat are seen from the top edge of the opening down. The chain mechanism is missing. The fault started as a single-phase fault between the right-most phase and ground and then proceeded into a three-phase fault. *This information is courtesy of the case history Two Similar Incidents Two Different Outcomes, by D. Doan and H. L. Floyd, which was presented at the 2006 IEEE IAS Electrical Safety Workshop. 5

6 Energy released during an arc-flash event is directly proportional to the system voltage product, available fault current, and event duration. The first two parameters (system voltage product and available fault current) can seldom be controlled and are normally determined by the process needs. Event duration therefore becomes the most critical parameter, which must be minimized. This video illustrates the effects of two medium voltage faults with a duration set to 46 cycles and 5 cycles, respectively. A 46-cycle fault is characteristic for a time-overcurrent protected system, while the 5-cycle interruption demonstrates what can be achieved by adding a bus differential or an instantaneous light-based arc-flash detection (AFD) element. Further improvements with clearing times down to 1 cycle are possible using vacuum circuit breakers with direct drive mechanism, fast current-limiting breakers (low voltage), energy diverters (crowbar circuits), and current-limiting fuse technology. 6

7 An arc-flash problem is most pronounced in low- and medium-voltage systems. Such systems distribute power to the point of use (end loads) and are deployed in very large numbers. Low- and medium-voltage circuits are inherently associated with high-fault current levels and suffer from lack of visibility, advanced age, and lack of design focus (end equipment protection instead of operator hazard minimization and safe preservation of the manufacturing process). 7

8 Human error and power system design can contribute to initiating an arc-hazard event. Awareness of these contributing factors can lead to behavior change through safety training programs and proper design education. 8

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10 In the image on the right side of the slide, note the door imprint directly below the telephone receiver mounted at the opposing wall. The photographer vividly conveyed, I used that telephone only two days before the incident. 10

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12 Protective relays provide several ways to affect fault-clearing times. Many of the options can be implemented with existing devices or supplemented with the latest technology specifically designed for AFD and arc-flash mitigation. 12

13 In this test, an AFD relay takes 3.1 milliseconds to trip as measured from the application of current and 1.6 milliseconds as measured from the appearance of light. The arc-flash event takes 1.5 milliseconds from the application of the current to the appearance of light. This was measured from a 1,000 frames-per-second, high-speed video. 13

14 Combining AFD and high-speed overcurrent provides fast tripping and security. The combination of overcurrent and arc sensors provides independent fault detection with two separate technologies, thereby eliminating false trips from lighting. 14

15 Arc-flash protection requires a system-level approach. In addition to being fast and dependable, arc-flash systems must also be selective in order to protect the associated manufacturing process. Multiple levels of protection are advised, including the following: Dedicated arc-flash protection systems Fast breakers and improved relay settings Use of energy diverters (shorting) and fault current limiting Arc resistant switchgear Personal protective equipment (PPE) Operating procedures 15

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