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RECTIFIER/DC POWER SYSTEMS CHAPTER 6: POWER SOURCES • Switchmode Rectifier A switchmode rectifier offers a size and weight advantage over ferroresonant rectifiers in that its transformer is smaller and lighter than controlled ferroresonant rectifiers. This type of rectifier will provides an output of 105% of its rating for the life of the rectifier. The drawback to the switchmode rectifier is that it does not have the large transformer to absorb transient surge voltages. These transient surges can shorten the life of the switchmode rectifier. In areas prone to significant transient voltages, controlled ferroresonant rectifiers may be a better choice. The lower MTBF of a switchmode rectifier can be offset by its ease of replacement. Also, the inefficiency at low output levels of these rectifiers is not nearly as severe as that of the controlled ferroresonant rectifier; therefore, forced load sharing is not required. A switch mode rectifier that is well-filtered to prevent radiated RFI and superimposed noise on the DC output circuit should be selected. Silicon Controlled Rectifier (SCR)-based rectifier systems are not acceptable for powering Motorola systems, due to tendencies of SCRs to allow AC transients to propagate to the DC side. 6.4.1.2 REDUNDANCY An n+1 redundancy setup is recommended, at a minimum, for the rectifier system. An n+1 redundant scheme employs one rectifier more than is required to power the system. In many cases, the redundant rectifier also provides for recharging of the batteries after a power outage. 6.4.1.3 RECTIFIER SIZING In general, the power system selected should be appropriately sized based on the installation being performed. • In systems requiring 1200 A or less, 2.5 kW (50A@-48V; 100A@24V) switchmode modular rectifiers (or equivalent) are recommended. • In systems requiring more than 1200 A, modular rectifiers as described above are not typically recommended. This is based on the following: An n+1 redundancy using low capacity rectifiers may not provide sufficient reserve capacity to fully recharge discharged batteries within 24 hours. An n+2 or n+3 design may be necessary to handle recharge. However, this will affect overall system cost. Higher-output systems based on higher-current rectifiers have a theoretically higher MTBF. A -48 V, 1000 A non-redundant system using 50 A rectifiers will contain 20 modular switchmode 100 A rectifiers. The same system using 200 A controlled ferroresonant rectifiers will contain only five 200 A rectifiers. The 200 A rectifier system has 25% as many potential points of failure as the 50 A system. 6-20 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES RECTIFIER/DC POWER SYSTEMS 6.4.2 DC DISTRIBUTION The power board or DC power distribution center is the infrastructure around which the power system is built. A power board can be divided into two components: the meter/alarm and the control section/ distribution section. The distribution section of the power system can be reconfigured, expanded, and modified in many ways, however, when the meter, alarm, and control section is at capacity, any further expansion requires the replacement of the power board. Over sizing of the power board is relatively inexpensive because most of the over sizing consists of copper bus bars. 6.4.3 LOW VOLTAGE DISCONNECT DC systems, with battery back-up, shall be equipped with a Low Voltage Load Disconnect (LVLD). A Low Voltage Battery Disconnect (LVBD) shall not be substituted for the LVLD. A battery system is considered to be fully discharged when the voltage reaches 1.75 VPC (volts per cell). In a 48 volt system (24 cells) the battery plant is fully discharged when the voltage reaches 42 volts (1.75x24). Battery damage begins to occur when the voltage drops below this point. Continuing to operate the system beyond this point with the intent of providing service to the end user at the expense of damaging or destroying the batteries may not be possible. Internal power supplies that provide logic and memory voltages (typically +12V, -12V, and +5V) are designed to provide regulated power throughout a specific input voltage range. This range is typically within a few volts of the batteries operating range. When the input voltage drops below the specified range, the output voltage of the internal power supplies will no longer be within the specified limits. In many cases, the internal power system will shut down. If the internal power supplies do not shut down, damage or erratic operation may occur. Although a low voltage disconnect does protect the battery plant, it also is required to protect the load equipment. 6.4.4 OVERCURRENT PROTECTION Because fuses may not always be available, appropriate circuit breakers are recommended. Overcurrent protection should be a minimum of 50% larger than the anticipated load for the circuit. Inrush currents (the current draw when a device is first powered on) shall also be considered when sizing circuit protection. Overcurrent protection shall not exceed the ampacity rating of the conductors. 6.4.5 POWER CABLING CAPACITY WARNING UL-listed General Use or Battery cable shall be used. 68P81089E50-B 9/1/05 6-21
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© 2005 MOTOROLA, INC. ALL RIGHTS R
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TABLE OF CONTENTS Chapter 1. Introd
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STANDARDS AND GUIDELINES FOR COMMUN
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CHAPTER 1 INTRODUCTION 1 This manua
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CHAPTER 2 SITE DESIGN AND DEVELOPME
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CHAPTER 3 COMMUNICATION SITE BUILDI
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CHAPTER 4 EXTERNAL GROUNDING (EARTH
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CHAPTER 5 INTERNAL GROUNDING (EARTH
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Page 235 and 236: STANDARDS AND GUIDELINES FOR COMMUN
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CHAPTER 8 MINIMIZING SITE INTERFERE
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CHAPTER 9 EQUIPMENT INSTALLATION 9
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APPENDIX A ELECTROMAGNETIC ENERGY I
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APPENDIX B SOIL RESISTIVITY MEASURE
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APPENDIX C PROTECTING AGAINST ELECT
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APPENDIX D GROUNDING (EARTHING) ELE
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APPENDIX E GENERAL CONVERSIONS AND
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APPENDIX F R56 COMPLIANCE CHECKLIST
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I NDEX Index Index AC power cabling
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