This circuit continuously checks whether traction grid and 12V grid are isolated against each other. ECE R-100 does not even require such a device for an EV vehicle, but TUEV in spite did require it. I also support this requirement because the existence of an isolation fault needs to be detected immediately to avoid dangerous “double fault” situations. Only then, one can rely that an isolation fault between traction circuit and car chassis will not have hazardous consequences.

For the isolation check, following requirements exist: * The device is not allowed to reduce the isolation resistance to a value lower than specified by R-100. Since the car has an AC motor, the harsher requirement of 500 Ohms/Volt applies - i.e. in this case 50kOhms.

* The device must be able to detect an isolation fault regardless to which point (e.g. positive or negative rail, or e.g. right in the center of the battery's cell chain) within the traction circuit.

The 22k resistor on the top left connects to a tap at the “electrical” center of the traction battery's cell chain. If the isolation is flawless, this point will be kept at 0V, referred to the 12V grid's ground. The traction battery is then “balanced”, with the positive rail at around +50V, and the negative rail at -50V referred to 12V grid ground.

The “isolation check” circuit is continuously powered (i.e. not dependent on ignition). It's function can always be verified by a test button at the rear side of the rear battery box, that simulates a slight isolation fault (see chapter “rear traction battery box” below).

*** back to top of this page ***