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Why Do Telecom Equipment Use -48V Voltage?

Many people have a common question when using communication equipment, why do communication equipment use -48V voltage?
The answer given by experts is:
Mainly based on three considerations: 1. Historical reasons, 2. Ground wire corrosion resistance, 3. Safety factors.

This contains 2 meanings:

(1) Why is the polarity negative power supply (that is, positive ground)?

(2) Why is the voltage -48V (-36~ -72V)?

Let’s talk about the second question first. The use of -48V power supply is caused by historical reasons.
The earliest communication network used was the telephone network, and the telephones were powered by the telecommunications office. The choice of 48V was to maximize the distance between the user and the end office under the conditions at the time (36V is a safe voltage, and it is unsafe to exceed too much). Later, in order to be compatible with early equipment and reduce costs, the central office communication equipment still used -48V power supply.

Likewise, with a negative power system, the positive ground is just a convention. It turns out that there is a saying that there are a lot of negative charges in the air. According to the knowledge of electrochemistry, the positive grounding can absorb the negative ions in the air, thereby protecting the shell of telecommunications equipment from rust. In fact, this statement is not quite right. Galvanic cell reactions and electrolytic reactions will cause equipment to rust, but because they exist in microscopic forms on the equipment, they have almost no impact. For example, networks in non-communication systems are all negatively grounded (such as the computer you are using), but there is no rust. And -48V is internally isolated by DC/DC. The DC/DC output is connected to the negative pole and is grounded. There is no corrosion or rust on the single board. So no matter which pole is grounded, it’s the same.

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As for the grounding of the equipment shell (connected to PGND), this is for protective purposes, and the accumulated charge on the equipment is quickly discharged to the earth, so as not to damage the equipment and workers. Products basically use -48V power supply system, and the actual measured voltage is generally –53.5V. This is because for reliability reasons, communication equipment is equipped with a backup battery (-48V). In order to ensure reliable charging of the battery, the supply voltage needs to be slightly higher than the battery voltage.

Through the media, you may also learn that there are devices that use a -24V power supply system, which is used by some modern internal devices for design convenience. Generally, the output voltage of the power supply is measured to be 26.8V.

Generally, the equipment is required to work normally within the voltage fluctuation range of ±20%. For -48V system equipment, the required operating voltage range is -38.4V ~ 57.6V, but in fact we generally require the operating range -36V ~ -72V. The main consideration is that -48V system equipment must be compatible with –60V power supply system, which requires –48~-72V. This combination requires the operating voltage range to be about -36V ~ -72V.

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By the way, the -48V power supply system is only the communication power supply standard adopted by my country and most countries. Not all countries use this standard. For example, Russia uses a -60V power supply system, and some countries use a -24V power supply system. If products are to be sold in these regions, these different standards must be taken into account.

Mains power standards are also different around the world. For example, my country and Europe use 220V mains power systems, while the United States, Japan, etc. use 110V mains power.

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