Historically, equipment in the communications industry has been powered using -48V DC. The -48V is also known as positive ground.
If a certain degree of search on this topic, there will always be a "convention" or "there is a large number of negative charges in the air, according to the knowledge of electrochemistry, the positive grounding can adsorb negative ions in the air to protect the shell of the telecommunications equipment from rust and corrosion," this kind of explanation, it is quite ambiguous. I graduated with a degree in communications, although chemistry is a physical education teacher, but still can not help but chat about this counter-intuitive (negative power) set the reason, not necessarily rigorous, welcome to brick.
In the telephone era, the -48V power supply was used to feed power to the landline at the bureau end. Assuming that the convergence point of the telephone office to the user of this link, on the ground, water vapor, a variety of terminals and cable insulation itself is not good, which is very common, such as located in some of the wet pipe to get or in the manhole. 48V voltage, enough to carry out electrolysis of water, electrolysis process is the positive precipitation of oxygen, the negative precipitation of hydrogen, the oxidation of the positive metal materials will be quite serious. Due to the existence of grounding, assuming that the negative pole is grounded, the negative pole and the earth without potential, the positive pole appears +48V potential, the positive pole precipitation of oxygen after rapid oxidation. On the contrary, if the positive electrode is grounded and there is no potential difference between the positive electrode and the earth, the negative electrode will precipitate hydrogen after electrolyzing water. As for the leakage current return path, can go to earth or positive cable, depending on the size of the return impedance. In short, positive grounding on the cable and other metal conductors corrosion is much smaller than negative grounding. It still seems to make sense to use a negative power system for communications power.
By the way, let's talk about why use 48V? Roughly speaking, the voltage is not high enough to be safe, the cable insulation level requirements are high, and wet environments are prone to breakdown problems. Lower voltage has a high current and is also inappropriate. In addition, taking into account the use of lead-acid batteries for uninterruptible power supply systems, 48V is exactly 12 2V battery cells, for small-capacity systems, using 12V batteries, then only 4. Reasonable number of strings battery pack is easy to equalize. In short, it is considered the result of comprehensive consideration of all parties.
-48V power supply system, AC mains power by low distribution, the use of switching power supply conversion to DC 54.5-55V, the battery float charging, while the output to the equipment power supply. Plain and simple.
The voltage of 54V or so is mainly determined by the lead-acid battery float voltage, 2.2V or so per cell. Considering the charging and discharging platform of the battery, the actual allowable input voltage range of the device is generally wider, at least up to -40V~-57V for normal operation. The usual limit is -36V~-62V. Of course, in recent years, lithium batteries have also been used in the communications industry.
For the IT industry data center, generally used AC110 or 220 power supply, almost never see -48V power supply equipment. The reason for this is mainly IT equipment from the computer this kind of equipment evolved, there is no history of using DC power supply. In addition, IT equipment power density is growing, power density per U 200 watts, or 5000 watts per rack has been more common. In recent years, high-density data centers 9kw per rack, some equipment in the 15U or so chassis designed for more than 10KW power capacity. In such a density, the use of 48V power supply voltage, with hundreds of amperes of overcurrent capability of power wiring, terminals, there are also quite difficult to realize.
The use of AC power supply, as a reliability requirement, always need to deploy an uninterruptible power supply system, which brings the problem of: energy inefficiency, AC mains power by AC-DC converter, to about 300-400V DC, the battery pack for float charging (or to maintain the flywheel operation, used for the flywheel UPS), and then DC-AC inverter, to the equipment. The equipment power supply again performs AC-DC (APFC)/DC-DC conversion to the target voltage. Even if the efficiency of each conversion reaches a high level of 92%, the overall efficiency is less than 85%; in addition, the UPS construction cost is high; the AC system at the same time there are difficulties in grid connection.
In recent years, high-voltage DC power supply is a hot trend in the data center energy industry. The use of DC power supply around 240-270, low construction costs and high energy efficiency means lower operating expenses. With the popularity of IT equipment supporting high voltage DC power supply, it should be a direction of development.
