Arc Flash and Arc Blast

Basic Electricity

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• Question 1

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?

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• Question 2

What, exactly, is an electric arc? What conditions typically lead to the formation of an arc?

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• Question 3

What is the difference between arc flash and arc blast?

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• Question 4

How hot can the temperature of an arc flash become?

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• Question 5

What types of electrical systems pose a substantial threat of arc flash and arc blast?

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• Question 6

A Jacob’s Ladder is a novelty device designed to produce high-voltage arcs. It consists of a source of high voltage (usually a step-up “transformer” which takes the 120 volt AC utility power voltage and increases it to several thousand volts AC) and a pair of stiff wires or metal rods with a small air gap near the bottom and a large air gap near the top:

When powered, an arc develops at the point where the two rods are closest, then travels up the length of the rods, becoming wider and wider, until it “breaks” off the top of the rods. Once the arc is extinguished, a new arc forms at the bottom of the rods again.

Explain why the arc begins at the point where the rods are closest together. Also, explain why the arc then moves upward along the rods’ length, rather than remaining at the bottom where the gap is shorter.

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• Question 7

What type of personal protective equipment (“PPE”) is recommended for protection against arc flash and arc blast?

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• Question 8

Arc flash may occur when using a voltmeter to measure voltage in a 480 volt electrical system, if there happens to be a power line transient (from a lightning strike, perhaps) at that time. Some industrial voltmeters carry a “CAT” rating (CAT I, CAT II, CAT III, CAT IV, etc.) specifying their tolerance for overvoltage transients, and subsequent safety provisions.

Describe in detail the meaning of these “CAT” ratings, and the nature of the hazard posed by a meter that fails under transient conditions.

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• Question 9

A potential cause of arc flash, and perhaps even arc blast, is improper meter use. Describe how an ammeter could be mis-applied to a live circuit in such a way that it causes an arc flash or arc blast.

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• Question 10

What is absolutely the best way to protect yourself from the dangers of arc blast while performing maintenance work on high energy electrical circuits?

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