Understanding short circuit currents in DC power systems when used in conjunction with standby batteries

As the telecommunications power environment is changing from one of regulated public utility companies with well-established engineering practices to one of unregulated investors supplying telecommunications infrastructure, it is apparent that a number of unknowns exist regarding DC power systems. Since traditional DC power systems were normally engineered and operated by the public utilities or a Government agency, they did not fall under the Jurisdiction of local electrical codes or the National Electrical Code® (NEC®) in the United States. In today´s marketplace things are changing. The end installation and applied engineering practices may not be controlled or reviewed by any local jurisdiction. The trend in the power industry is to have the manufacturer submit these DC power systems for review by Nationally Recognized Testing Laboratories (NRTL) but no site testing is typically performed to check compliance. Being able to define and better understand the magnitude of the possible short circuit currents that may be encountered in these systems during a fault condition is now, more than ever, a requirement for safe equipment installation and overall system design. In this paper, the authors analyze a centralized power system and smaller remote power applications. These would be representative of typical installations found in various telecommunications sites. They analyze the possible short circuit currents that may be encountered in these applications and run short circuit simulations to verify the calculated results. The importance of correctly specifying and properly coordinating overcurrent protection devices from an overall systems design standpoint is also be addressed.