Further information
All HV components should be electrically isolated from the chassis of the vehicle. However, in an electrified driveline electrical isolation is not complete isolation but instead a known high impedance through which negligible current flows: I.e. isolation resistors connect each HV bus (positive and negative) to the chassis ground while preventing harmful current flow.
Typically, insulation resistances should exceed 500 ohms for every 1 V of nominal HV battery voltage; for example, a vehicle with a 350 V battery should have a minimum insulation resistance of 175,000 ohms (175 kilohms). However, always check the vehicle manufacturer's data to find its precise insulation resistance specification.
HV systems have insulation monitoring devices that measure the insulation between the vehicle chassis ground and the HV positive and negative supply conductors. Any detected out-of-tolerance insulation resistance will cause the monitoring system to log a Diagnostic Trouble Code (DTC) and, potentially, shut down the HV system.
HV system repair
It is good practice to carry out an insulation test after removing and replacing an HV system component during repairs and before the vehicle's HV system has been powered up for the first time. This helps you to prevent faulty or incorrectly installed components causing harm or unnecessary DTCs.
HV system insulation diagnostics
As well as within DTCs, relevant insulation fault data may be found within the serial diagnostic data. Some vehicle manufacturers also supply additional diagnostic information (such as Toyota Group vehicles which provide INF codes) to provide further insight when used in conjunction with the correct reference material.
Keep yourself and all others well away from an electric vehicle when clearing DTCs from its control units' memories as by clearing them you may reactivate deactivated HV systems.
If there is evidence of, or you suspect, an HV system isolation fault, you should power down the vehicle and confirm its shut-down state with a 0 V potential test (in accordance with the vehicle manufacturer's instructions) before carrying out insulation resistance testing.
It is good practice to perform an insulation resistance test at the same location as any 0 V potential test. This will test whether there is an underlying isolation fault that can result in 0 V potential but still be an immediate cause of danger.
Once the vehicle has been safely powered down, locate and remove components from the HV circuit in order of their ease of access. Each time a component is removed, repeat the insulation test. Remove the components until the isolation resistance falls back within tolerance. The last component removed will be the likely cause of the insulation fault.
Certain insulation faults might not present themselves with the vehicle disabled: If an insulation related DTC has been set but the measured insulation resistance is within tolerance, it could be a component that only fails during operation. Typical candidates are air conditioning compressors and Positive Temperature Coefficient (PTC) heaters, which are enabled as and when the driver chooses. A good diagnostic customer interview process is important here, as it may help reveal which events and components could be causing the failure. You might also need to run the vehicle until a related DTC presents itself. However, in no circumstance, should an insulation resistance test be performed when the vehicle's HV system is live.
When testing the vehicle's electric motor phase windings, you may need to reverse the polarity of your connections: For example, apply the red test lead (positive) to the chassis side of the vehicle and the black test lead (negative) to the HV+ or HV- cabling (test both but one at a time) at the motor inverter inlet, as any current leaks through the windings then will be able to pass through the inverter diodes. Again, consult manufacturer information for further information regarding the design of your vehicle's systems and their insulation resistance test procedures and tolerances.
