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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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Although the complete theory of how this occurs is beyond the scope of the class, there are a few key features to note. Ground is our reference voltage, that which we call zero volts. All voltages are electric potential measurements from one point relative to another; typically relative to ground since that is our chosen reference point. But what happens when there is a voltage between two physically distinct areas (two buildings or two floors in a building) that we are calling ground? Well nothing would occur if no circuits where formed involving these two different grounds. However, recall we are often running long conducting copper cables around the floors of the building or between buildings to build our network. These provide ways to form complex circuits involving the different grounds and conducting human beings or conducting electronic devices. Hence the problem.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>Large buildings frequently require more than one earth ground. Separate earth grounds for each building are required in multi-building campuses. Unfortunately, the earth ground between buildings is almost never the same. Earth grounds can even vary within the same building.</p><p>When ground wires in separate locations have slightly different potential (voltage) they can present a serious problem. To understand this, assume that the ground wire for building A has a different potential than the ground wire for building B. Because of this, the outside cases of computer devices located in building A would have a voltage (potential) that is different from the voltage of the outside cases of computer equipment located in building B. If a circuit were established that linked computer devices in building A to those in building B, then electrical current would flow from the negative source to the positive source. This errant potential voltage would have the ability to severely damage delicate computer components.</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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