Ever wondered why telecommunications equipment widely uses a negative voltage like -48 VDC? It may seem odd, but there's smart reasoning behind this choice. In this blog post, we'll unravel the mystery behind the industry's preference for -48 VDC and explore the practical benefits that make it indispensable for telecom systems.
The Origin of -48 VDC in Telecom
Telecom and wireless networks typically operate on -48 VDC power, but why? The short story is that -48 VDC, also known as a positive-ground system, was selected because it provides enough power to support a telecom signal but is safer for the human body while doing telecom activities (such as installing telecom equipment or making repairs on any part of the infrastructure). It is currently accepted by safety regulations and electrical code that anything operating at or below 50 VDC is a safe low-voltage circuit. Another reason is that -48 VDC allows telecom operators to easily use 12 V lead-acid batteries connected in series to act as a backup power source in the event of a grid system power failure. Negative 48 VDC is still the standard in communications facilities serving up both wired and wireless services, as it is perceived to cause less (or at least inhibit) galvanic corrosion in metal than positive voltages. Figure 1 presents a simplified diagram of a typical telecommunications DC power system with an emphasis on how -48 VDC is created and distributed.
Figure 1: A Simplified Diagram of a Typical Telecommunications DC Power System
The telecom DC power system typically includes the national electricity grid system, a diesel generator, a self-acting AC automatic transfer switch (ATS), a power distribution system, solar panels or boards, controllers and chargers, rectifiers, backup batteries arranged in series, and the corresponding cables and breakers.
Battery Backup and System Reliability
The use of -48 VDC allows telecom operators to conveniently employ 12 V lead-acid batteries in series, acting as seamless backup systems. When power from the grid is lost, the diesel generator is designed to start automatically, providing AC power to the DC port system. The ATS synchronizes voltages from different sources to the equipment. Since most telecommunications equipment at the site requires a DC voltage supply, the AC power from either the electric grid or the diesel generator is converted to -48 VDC by the rectifiers. These redundant rectifiers are used to convert the AC power to -48 VDC power used to trickle charge the batteries as well as support the critical loads. The batteries, which are floating, provide the -48 VDC power to the telecom equipment or other loads if the rectifiers fail to do so. The base transceiver station (BTS) or remote radio head (RRH) doesn’t notice the difference in the actual power source, and everything keeps operating normally. When the power comes back, the rectifiers take over again. Essentially, the entire power plant works like a large uninterruptible power supply (UPS).
Corrosion Mitigation
Interestingly, another main technical reason behind the use of -48 volts DC in telecom facilities is galvanic corrosion prevention in the metallic components of telecom infrastructure. Galvanic corrosion occurs when two dissimilar metals are electrically connected in the presence of an electrolyte (like water or moisture in the air). One metal becomes the anode and corrodes, while the other becomes the cathode and is protected. The polarity of the voltage applied to metallic structures affects where corrosion occurs. So, negative voltages significantly reduce corrosion compared to positive voltages, enhancing the lifespan and reliability of telecom infrastructures.
Conclusion
The adoption of -48 VDC isn't arbitrary; it's a strategic choice that ensures safety, reliability, and operational efficiency. With growing demands on telecom networks, understanding foundational elements like this becomes increasingly crucial.
Read all the blogs in the Inside the Negative 48V Revolution series.
