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<html> <head> <meta content="en-us" http-equiv="Content-Language"> <meta content="text/html; charset=UTF-8" http-equiv="Content-Type"> <link type="text/css" rel="stylesheet" href="../../../../../css/ccna.css"> <script src="../../../../../js/framesetHelper.js" language="JavaScript"></script><script language="JavaScript"> window.scrollTo(0,0); function newPage(pageID) { alert("COMMING SOON!!\nThis feature is currently in development."); } function launchMoreInfo(oid) { dash = oid.indexOf('-'); sub1 = oid.substring(0,dash); sub2 = oid.substring(dash+1); // this is an URL base on the // system that generated the static // download of the curriculum. httpref = "http://nomar.sdf.cisco.com/servlet/org.cli.delivery.rendering.servlet.MIServlet/Static=true,LMSID=DTUI,Engine=dynamic,Theme=cnamstheme,Style=ccna,Language=en,Version=2,RootID=knet-v214aCV47051/CHAPID=null/RLOID=null/RIOID=null/"; newref = httpref + sub1 +"/"+ sub2 +"/moreinfoframeset.html"; //alert("new newref :\n" + newref); window.open(newref,'MoreInfo','toolbar=no,status=yes,menubar=no,location=no,scrollbars=yes,height=442,width=756,left=20,top=20,resizable=yes') } function launchContentResource(contentID) { top.currentSupport = 0; 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The concept of ground is difficult for the beginning electronics student, and worthy of some review. Of particular importance are signal reference ground (the zero Volts line, or reference line, for determining the polarity of our networking signals) and power line earth ground. Differences between signal reference ground and earth ground, or between the power line grounding of two different networking devices, can cause noise problems as well as dangerous shock conditions.</p> <p>Grounding connections and practices vary around the world; feel free to teach whatever standards apply to you locally.</p> <p>After covering this target indicator, a quiz game - such as "Jeopardy" -- might be in order. The Categories could be Voltage versus time graphs; electronic materials; electric circuits; the water analogy; and grounding. Students can select a category and a level of difficulty and attempt to answer your review questions, earning "points" for themselves and their team.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>For AC and DC electrical systems, the flow of electrons is always from a negatively charged source to a positively charged source. However, for the controlled flow of electrons to occur, a complete circuit is required. Generally speaking, electrical current follows the path of least resistance. Because metals such as copper provide little resistance, they are frequently used as conductors for electrical current. Conversely, materials such as glass, rubber, and plastic provide more resistance. Therefore, they do not make good electrical conductors. Instead, these materials are frequently used as insulators. They are used on conductors to prevent shock, fires, and short circuits.</p><p>In the United States, electrical power is usually delivered to a pole-mounted transformer. The transformer reduces the high voltages used in the transmission to the 120 or 240 volts used by typical consumer electrical appliances.</p><p>Figure <img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47542/RIOID=knet-v214aRIO121155/knet/v214adataimage1/1.gif" width="12" height="12"> shows a familiar object, electricity as supplied through wall outlets in the US (other nations have different wall outlet configurations). The top two connectors supply power. The round connector, called the safety ground connection, protects people and equipment from shocks and short circuits. In electrical equipment where this is used, the safety ground wire is connected to any exposed metal part of the equipment. The motherboards and computing circuits in computing equipment are electrically connected to the chassis. This also connects them to the safety grounding wire, which is used to dissipate static electricity.</p><p>The purpose of connecting the safety ground to exposed metal parts of the computing equipment is to prevent such metal parts from becoming energized with a hazardous voltage, resulting from a wiring fault inside the device.</p><p>An accidental connection between the hot wire and the chassis is an example of a wiring fault that could occur in a network device. If such a fault were to occur, the safety ground wire connected to the device would serve as a low resistance path to the earth ground. The safety ground connection provides a lower resistance path than your body.</p><p>When properly installed, the low resistance path, provided by the safety ground wire, offers sufficiently low resistance and current carrying capacity to prevent the build up of hazardously high voltages. The circuit links directly to the hot connection to the earth.<img border="0" src="../../../../../CHAPID=knet-v214aCH47504/RLOID=knet-v214aRLO47542/RIOID=knet-v214aRIO121155/knet/v214adataimage2/2.gif" width="12" height="12"></p></td> </tr> </table> </td> </tr></table> <p> </p> <table width="100%" cellspacing="0" cellpadding="2" border="0" valign="bottom"> <tr> <td></td> </tr> </table> </body></html>⌨️ 快捷键说明
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