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

1. Voltage is relatively safe, for example, human body is 50K ohmic resistance, voltage-48V, 48/50000 = 0.00096A = 0.96mA, human body flow through 9mA will be life-threatening.

2. The inheritance of history. N years ago, when electronic tubes and PNP germanium tubes were used, the positive grounding of the circuit was intuitive, simple and convenient. The anti-interference ability of negative power supply is better, but this is the reason for a long time ago. Now the digital technology has not required this. So the equipment is also useful for positive power supply. But considering the habitual universality, most of them are - 48V3. The positive grounding of power supply system can reduce the corrosion phenomenon of positive battery. 4. Reduce the system noise and reduce the interference.

5. Earlier communication can use earth as loop to save wire.

6. In order to protect the cable from corrosion due to battery reaction, the cable must be negative.

7. High voltage ratio - 24V, conducive to power transmission, reduce loss

The telephone line environment is complex. Before the invention of all-plastic cable, the line insulation is often poor. The negative power supply is less affected by the ground air noise than the positive power supply. This habit has been used up to now.

Let me explain, for your reference: Why do we need positive grounding? First of all, the power supply should be grounded, which can effectively avoid noise and other interference. Selection of positive grounding can minimize the corrosion of grounding devices.

For the reason of choosing 48V, I personally understand: the original use of communication is more than 24V and 60V, and then gradually unified to 48V. For the voltage higher than 48V, it may cause personal injury, lower than 48V, the same power load on its line current is too large, to choose a thicker power line, investment, loss of line voltage drop is large.

There are generally two modes of power supply for-48 transmission equipment, single power supply (three power lines: -48V, working place and protective area) and double power supply (five power lines: -48V, two working places and one protective ground line). Generally, single PDH equipment and 155M box SDH transmission equipment also use single power supply mode, while other rack equipment uses double power supply mode. The diameter of DC power supply line is generally 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240 kinds, in square millimeters. Line diameter S=I*L/(K*V). Among them, S-conductor cross-sectional area (mm2), I-loop current (A), L-conductor loop length (m), allowable DC voltage drop (V), conductivity of K-conductor (m/_*mm2), copper conductor K=57, aluminium conductor K=34, and general communication DC power supply system voltage drop less than 3.2V. Empirical formula: S = I*L*2/57/0.5. The result is rectified upward, and the upper line diameter is - 48V and the working ground line diameter is selected to reduce the protective ground line diameter by one grade. These are design experience values.