As part of the Model S high voltage safety philosophy, the high voltage electrical system is isolated from the vehicle. This means that there is no electrical current carrying path between the vehicle and any high voltage conductor. To ensure that this isolation is always met, a monitor is installed in the battery, which periodically measures the resistance between the high voltage conductors and the chassis. Three different conditions can introduce a conduction path to the chassis: a physical defect of the insulation material, contamination (of any size, e.g. dust, bolts, etc.), or moisture. When debugging isolation, look for signs of moisture or physical damage to HV cables, bus bars, and connectors.
