 cable route and very accurately determine the location of the break. As soon as the large cables have been broken down into smaller pair bundles, they leave the cable vault on their way up to the Main Distrib- ution Frame, or MDF. Before we leave the basement of the CO, we should discuss power, a major consideration in an office that provides telecommunications ser- vices to hundreds of subscribers. Figure 3-25 Wire pairs Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Telephony Figures 3-26 Pressure transducers When you plug in a laptop computer, the power supply does not actu- ally power the laptop. The power supply's job is to charge the battery (assuming the battery is installed in the laptop); the battery powers the computer. That way, if commercial power goes away, the computer is unaffected because it was running on the battery to begin with. Central offices work the same way. They are powered by massive wet cell battery arrays, shown in Figure 3-27, that are stored in the base- ment. The batteries are quite large-about two feet tall-and are con- nected together by massive copper buss bars. Meanwhile, commercial power, shown in Figure 3-28 (OK, not really), is fed to the building from several sources on the power grid to avoid dependency on a single source of power. The AC power is fed into an inverter and filter that converts it to 48 volts DC, which is then trickled to the batteries. The voltage Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) oht C 2004 The McGraw Hill Companies All rights reserved Telephony Chapter 3 Figure 3-27 Central office battery array Figure 3-28 Technicians working on central office commercial power maintains a constant charge on them. In the event that a failure of com- mercial power should occur, the building's uninterruptible power supply equipment,