Why is the DC supply power used in telecom?
The equipment in the communication industry generally uses -48V DC power supply, and the anode is grounded. Why?
First, historical reasons
In the era of fixed telephone, the telephone of every household is powered by the office of telecommunication bureau.
Because the distance between the telecommunication bureau and the telephone in the home is relatively long, in order to ensure a certain service range, the power supply voltage of the local exchange cannot be too low, so the -48V power supply is adopted.
Compare some small exchange equipment, because the distance between the exchange and the telephone is relatively close, -24V power supply is generally adopted, and -48V power supply is not used, which is understood.
With the continuous progress of technology, the telephone has developed from hand-operated magnet to program-controlled telephone. In the process of development, -48V power supply has been used all the time in order to be compatible with early equipment and reduce costs.
Second, why is the positive pole grounded?
The main purpose of grounding the anode is to prevent the corrosion of the electrode.
The -48V or -24V battery in the telephone office is grounded at the positive pole, which is due to the reduction of electrical corrosion caused by poor insulation of relay or cable metal sheath, which damages the relay and cable metal sheath.
Because the metal ions move from the positive electrode to the negative electrode under the chemical reaction. If the insulation between the relay coil and the iron core is poor, a small current will flow. When the negative pole of the battery pack is grounded, the wire of the coil may be corroded.
On the contrary, if the positive pole of the battery pack is grounded, although the iron core will suffer from electric corrosion, the wire of the coil will not corrode, and the quality of the iron core will be large, which will not cause noticeable consequences. Grounding the positive pole can also protect the core wire of the external cable from corrosion when the insulation is poor.
Third, safety and cost considerations
36V is a safe voltage, which is too unsafe.
In addition, the voltage should not be too low. If it is too low, the current on the line of the load with the same power is too large, so it is necessary to choose a thicker power line, which will result in large investment and great loss of line voltage drop.
For example, due to the increase of AAU power consumption, if -48V is still used, the current is larger, the line loss is larger, and the transmission distance is shorter, which can not meet some AAU remote scenarios and can only replace cables with larger wire diameters, but this will increase the cost and the load of the tower. Therefore, the voltage should be appropriately increased (for example, -57V) to reduce the construction cost and speed up the construction pace.
IV. Other reasons
In order to unify the norms;
Early communication can use the earth as a loop to save wires;
Noise and other interference can be effectively avoided;