A Simple Method to Insure Bus-to-Bus Safety in Dual-Voltage Automotive Systems 2014-01-0244

In some automotive electrical systems, it is advantageous to use power supplies and loads at two or more voltages. Often it is desirable to retain the single wire power architecture, with the car body providing the return circuit. A major difficulty in achieving this end is the matter of dealing with the possibility of a short circuit between feed wires at different voltages. It can be shown that source-side fuses cannot be relied upon to return the system to a safe state in all cases.Substantial effort was applied to this problem in the early years of the 21st century, but the results were less than completely satisfactory. Using entirely separate cable harnesses for each voltage, with physically separated routing, minimizes the risk of such a short occurring in the harness. However, the use of more than one harness imposes a substantial cost penalty, and this solution does not provide an answer for the case where the short occurs at or within a load being serviced by more than one voltage.Complicated sensing circuits with floating solid state switches (requiring expensive drivers) are also unattractive.The authors offer a simple solution which can be implemented exclusively with ground-reference switches concentrated in the fuse block. This simple modification, coupled with the application of a very reasonable and easy-to-implement design rule for selecting wire gauges for the cable harness, can result in the system returning to a safe state following every fuse operation.