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Figure 3-25 Wire pairs
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Figures 3-26 Pressure transducers
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When you plug in a laptop computer, the power supply does not actually 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 basement. The batteries are quite large about two feet tall and are connected 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
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Figure 3-27 Central office battery array
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Figure 3-28 Technicians working on central office commercial power
maintains a constant charge on them. In the event that a failure of commercial power should occur, the building s uninterruptible power supply equipment, or UPS, kicks in and begins a complex series of events designed to protect service in the office. First, it recognizes that the build-
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ing is now isolated and is on battery power exclusively. The batteries have enough current to power a typical CO for several hours before things start to fail. Second, technicians begin overseeing the process of restoration and of shutting down nonessential systems to conserve battery power. Third, the UPS system initiates startup of the building s turbine, a massive jet engine that can be spun up in about a half hour. The turbine is either in a soundproof room in the basement or in an enclosure on the roof of the building (see Figure 3-29). Once the turbine has spun up to speed, the building s power load can slowly be shed onto it. It will power the building until it runs out of kerosene, but most buildings have several days supply of fuel in their underground tanks. As Al Bergman, a data center design engineer and friend told me several years ago, If we re down so long that we run out of fuel, our problems are far worse than an office that s out of service. The cables leave the cable vault via fireproof foam-insulated ducts and travel upstairs to whatever floor houses the MDF. The MDF, shown in Figure 3-30, is a large iron jungle gym sort of frame that provides physical support and technician access for the thousands of pairs of wire that interconnect customer telephones to the switch. The cables from the cable vault ascend and are hardwired to the vertical side of the MDF, shown in Figure 3-31. The ends of the cables are called the cable heads. The vertical side of the MDF has additional overvoltage protection, shown in the diagram. The arrays of plug-ins are called carbons; they are simply carbon fuses that open in the event of an overvoltage
Figure 3-29 Turbine in rooftop enclosure
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