Why does the communication base station use -48V power supply?

Why does -48V DC power supply become the power supply voltage of communication base station? Communication base station power supply in the tower room power supply system is an essential and important part of the mobile communication network. The current communication power supply voltage level is divided into DC-48V (+24V), AC 220/380V. Communication industry equipment generally use -48V DC power supply, positive grounding, why? In this article, I will analyze it for you.

Why does -48V DC power supply become the power supply voltage of communication base station?

Communication base stations use -48V power supply for most historical reasons. Historically, the communications industry equipment has been using -48V DC power supply. -48V is also known as positive ground. Because the smallest communications network and communications engineering are in the telephone network, the telecom bureau power supply voltage are 48V. Later, to be compatible with earlier equipment and reduce the cost of replacement, engineering and port communications equipment use -48V power supply.

The voltage higher than 48V may do harm to the person. If it's lower than 48V, the same power of the load on its lines will carry too much current. So we have to choose a thicker power line, which cost a lot and cause line voltage drop loss. The voltage of +48V and -48V is equal, but the current flow is not the same. +48V flow to 0V, V0 flow to -48V. So -48V voltage is the communication power supply standards of many countries.

Why choose -48V as the supply voltage instead of +48V?

1. The voltage is safer. 

For example, the human body resistance is 50K ohm, the voltage is -48V. 48/50000 = 0.00096A = 0.96mA. It is life-threatening that 9mA flows through the human body.

With a negative voltage because the electron is negatively charged, it will flow in the direction of the positive voltage, which will also become current. With negative voltage will not let too many electrons gathered in the communication equipment, so that the chance of burning communication equipment at the present is reduced!    

2. The power system positive grounding can reduce the corrosion of the positive terminal of the battery.

The central battery is always grounded to the positive pole, the reason is that it can ensure that the wire potential of the components in the switch is lower than the ground potential. Because the insulation material outside the wire contains impurities, affected by moisture in the air will form negative ions such as acid root. If the central battery negative grounding, the wire potential is higher than the ground potential, it will make the wire corroded due to electrochemical effects. So until now, not only the switch, all the power supply of bureau equipment is grounded to the positive pole, and the voltage value we can contact is also -48V or -53V.

3. Early communications save wire.

Early wired telephone use "common power". The power supply of the telephone is through the telephone line by the telephone exchange. The telephone line plays a role in power supply, transmission of voice, bell stream, pick up and hang-up signal. The power supply used in the early telephone bureau is the original lead-acid battery. The material and structure are suitable for positive grounding, the voltage is a multiple of 1.2V. According to the capacity of the equipment components and lines, and to ensure that the city has a long communication distance, as well as security, 48V power supply selected positive grounding (voltage also has 60V), so it is -48V to the ground.

The current telephone and its switchboard has undergone the renewal of magnetism -- common power -- automatic exchange -- program-controlled exchange. During the renewal of these systems, except for the magnetism system, the other systems are first compatible with the power supply voltage and grounding polarity remain unchanged in the original way. These measures can ensure that the old and new equipment communication services won't interrupt and the possibility of direct information exchange. Communication technology and equipment leaps and bounds progress, but the update must be gradual and compatible which is not only a technical issue, but also a social need.

4. Why note nominal - 48V power supply but actually use - 53V?

This is because the battery pack voltage is indeed - 48V, which can be seen when using batteries in the switch power supply equipment. Because the voltage is 2V of a single bureau with high-capacity batteries. Each group consists of 24 batteries in series. So for a long time, the switch power supply voltage use 48V. In fact, the internal switch power supply is -5V, -12V, etc. While the ringing voltage to users may be as high as 100V. Daily line voltage is 60V to 80V. So choosing -48V is just right. And it will not form a large current in the circuit.  Considering that the daily power supply needs to float the battery, the supply voltage should be maintained at a relatively high level, and on the other hand, the voltage drop of the switch should also be considered, so the output of the power supply device is generally 53.52V.