Designing for transients on the -48V distribution

This paper addresses several issues related to system design for robustness to transients on the -48 V bus and its verification. After a brief review of the methods available to the system designer for ensuring survivability of the system during and following the occurrence of transients, the paper uses a model of the complete telecommunications system (power plant, power distribution, and powered telecommunications equipment) to analyze the effects of short circuits occurring at the load sides of both primary and secondary distribution. The results obtained by simulation are then compared to the results obtained by testing to the three segments of the transient specified in ANSI T1.315: impulse, overvoltage, and undervoltage. The insights provided by both simulation and testing are then applied to the various aspects of a typical system design. Some inherent limitations of the simulation are identified, together with additional considerations for less common powering arrangements. The paper provides several conclusions on the applicability and usefulness of both simulation and testing as means for demonstrating system robustness in accordance with the requirements of the applicable standard. In addition, the paper highlights the importance of overall system configuration and front-end power distribution topology for meeting the specified reliability, survivability, and robustness objectives of the complete telecommunications system.