Electrical Network Models for Chassis Fault Current Analysis

The electrical power distribution system for International Space Station (ISS) payloads is designed with the single-point-ground (SPG) at the power source. The system design requires the minimum isolation resistance of one mega-ohm between the power circuits and the chassis ground at electrical loads to prevent current from flowing in ground references. Under fault conditions, solid state devices protect electrical wires by opening the circuits to interrupt power supply in the event of over-current to meet the electrical design and safety requirements. Due to the voltage potential difference between the electrical load power return and the SPG, nominal return current from power loads may flow in the reverse direction to the fault through the wire designed for nominal current load. This presents a concern that the fault current might exceed the electrical wire rating. Excessive fault current may jeopardize crew safety and equipment integrity. Therefore, the ISS payload safety review panel requires an accurate assessment of chassis fault current. The original electrical power distribution design approach adopted a simplified calculation which divides the nominal return current based on the ratio of fault resistance and the resistance of power returns wire. This approach treats each fault path independent from other loads on the same power bus and therefore, the interaction between nominal system loads and faulted circuit is ignored. The ISS payload engineering integration organization developed an improved design process network by accurately modeling power return-wire to chassis short-circuit scenarios to predict the fault current of each chassis fault path. The development of electrical network models and their applications are discussed. This methodology offers the flexibility of rapid model reconfiguration to accommodate different electrical load types and variations in fault resistance and source resistance to cover a wide range of scenarios. This analysis provid- es essential design information for the set points of the solid-state protection devices. This is an improved design process for any aerospace electrical power systems with SPG where the voltage differential between power load return and power source is significant, i.e., greater than a few hundred milli-volts