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Arc Flash and Arc Blast
10 questions By Tony R. Kuphaldt
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Question 1 of 10
Electromechanical relays used to start and stop high-power electric motors (called “contactors” or “starters”) must be considered a possible source of arc flash. Explain why this is. What is it about the construction or operation of such a relay that invites this dangerous phenomenon?
Reveal answerElectromechanical relays interrupt circuit current by drawing pairs of metal contacts apart, separating them with an air gap. Because this contact motion is not instantaneous, it is possible to generate an arc across the air gaps of such magnitude that it becomes an arc flash.
Notes:Arc flash is just as hazardous to electrical technicians as electric shock, yet I have seen (and worked with) people who pay no attention to the dangers! It must be understood that motor starters are by their very nature arc-generating devices, and that under certain unusual conditions may generate lethal arc flashes. You might want to ask your students what sorts of unusual conditions could lead to a contactor producing an actual arc flash (rather than merely a few small sparks).
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Question 2 of 10
What, exactly, is an electric arc? What conditions typically lead to the formation of an arc?
Reveal answerAn electric “arc” is a discharge of current through the air. Arcs may be caused by excessive voltage ionizing an air path between two conductors, or from accidental contact (and subsequent re-separation) between two conductors.
Notes:Discuss the subject of “ionization” of gases. Air is normally an insulator of electricity, but it becomes a conductor when ionized. Ask your students what happens to a gas when it becomes ionized. Ask them to tell you what the word “ionized” means.
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Question 3 of 10
What is the difference between arc flash and arc blast?
Reveal answer“Arc flash” is the extremely high-temperature discharge produced by an electrical fault in air. “Arc blast” is a high-pressure sound wave caused by a sudden arc fault.
Notes:Ask your students where they obtained this information. I have found much on this subject, including some very readable primary research, on the internet.
