Consulting-Specifying Engineer - November 2007 - (Page 49)

resistance in the conductors and through the nail is low. As a consequence, currents as high as 10,000 amps—sometimes more—can occur. This sounds serious, and it is. However, conventional circuit breakers effectively protect against this scenario. Upon sensing this high current, conventional circuits open in less than 1/10th of a second. An arc fault is similar inasmuch as conductor insulation damage is involved. However, with an arc fault, the conductors are not in direct contact with one another, nor are they connected by a metal object. Instead, because there are exposed conductors relatively close to each other, an arc forms between the two. The current in this case literally has to jump an air gap to get from one conductor to the next. Because air is a poor conductor, the resultant current is relatively small, often much too small to trip a conventional breaker. In this case, the arc is outside the cable and continues indefinitely. When protected by a conventional breaker, the resultant heat may start a fire. How AFCIs work AFCI is yet another example of an advance in solid state technology. These breakers actually monitor the waveform pattern—or signature— rather than just the magnitude of the circuit that they are feeding. When AFCIs detect a pattern similar to the blue one in Figure 2, they recognize this signature as being an arc fault. As can be seen in the figure, an arc fault is characterized by a steady, relatively low-level, high-frequency current waveform. However, one growing pain problem that this technology encountered when it first came into use involved normal, controlled, and acceptable arcing in electrical circuits. Examples of a normal arc include the arc that is seen when pulling out an electrical cord or the arc a light switch makes when it is operated. Power tools are another example, as power tools generate a sustained arc while being operated. How does the AFCI breaker know when to trip and when not to? As it turns out, all of these types of electrical arcs have subtle differences in their signatures. As AFCI technology has evolved, it has reached a point where it can effectively filter out normal and safe arcing and only trip on the occurrence of a true arc fault. Consulting-Specifying Engineer • NOVEMBER 2007 49 Input #232 at csemag.com/quickResponse http://www.yaskawa.com/E7 http://www.yaskawa.com http://www.yaskawa.com http://csemag.com/quickResponse

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - November 2007

Consulting-Specifying Engineer Contents Editor's Viewpoint Letters In The News M/E Roundtable Emergency Power for Healthcare 2007 Products of the Year Industrial-strength Lighting Hazardous HVAC Codes and Standards Management Report Jobs/Classifieds Specifier's Notebook On-Peak Performance Contents Best Battery Selector in Engine Starting Applications NEC 708: Practical Impact on Backup Power Systems Protecting Power: Specifying Outdoor Generator Enclosures Investing in Backup Power Systems Coes, Consultants, Manufacturers, and Standby Power Systems

Consulting-Specifying Engineer - November 2007

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