Arc Flash and Arc Blast
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.
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.