As electric vehicles evolve to deliver extended range and enhanced power capabilities, battery systems are increasingly adopting architectures of 800V or higher. User safety remains paramount in these high-voltage (HV) systems. Consequently, all HV components must maintain isolation from protective earth through high-resistance pathways. Insulation failures can arise from various causes; including wire harness deterioration, aging of power-handling components, or extreme electrical stress events, such as damage to Y-capacitors. The system enters an unsafe condition when any single insulation failure occurs. Electrical shock is not necessarily caused by an initial reduction in impedance directly, however, a reduction requires immediate warning as the condition presents a potential life-threatening risk if human contact occurs.

Isolated power systems, including DC fast-charging stations, must adhere to the applicable safety standards. These standards vary by region, as illustrated in Table 1-1.

In IEC 61557-8 and GB/T 18487.1, the warning (500Ω / V d.c. – 2mA) and fault (100Ω / V d.c. – 10mA) thresholds are set for the resistance of the isolation barrier. The IMD circuit monitors the insulation resistance and reports a failure in the case of insufficient insulation resistance. In the event of insufficient insulation resistance, current leakage can potentially increase above the permissible limit.

Figure 1-1 illustrates that an insulation monitoring device (IMD) is installed on both the charger side and the vehicle side. The two IMDs cooperate to supervise the isolation barrier of the DC power?supply circuit throughout the entire charging process and while the vehicle is in operation.

According to GB/T?18487.1?2023, the following sequence and requirements apply:

• During the time frame from the vehicle coupler connection to the closing of the relay, the internal insulation monitoring (including charging cables) is completed by the charger.
• After the relay closes, the insulation monitoring of the entire charging system is completed by the electric vehicle in the energy transfer stage.

Therefore, these two IMD circuits work together to verify that the insulation resistance remains within the normal range during charging and driving, in both the charging end and vehicle end.

Whether AC is charging or DC is fast charging, there is generally a residual current detection (RCD) circuit on the AC side in both cases. There is a slight difference between the functions of the RCD and IMD circuits. The RCD circuit determines the insulation failure by monitoring the total leakage current, whereas, the IMD circuit monitors the insulation resistance. Table 1-2 presents a comparison between the IMD and RCD circuits.

The existing IMD and RCD circuits cover scenarios for unidirectional OBCs. However, with the emergence of bidirectional power flow capabilities in electric vehicle (EV) onboard chargers (OBCs), these systems not only enable traditional grid-to-vehicle charging but also reverse power flow in the applications. Enhanced safety measures are critical for protecting users from potential electrical shock hazards from insulation failures, as EVs increasingly function as mobile power sources. Therefore, new requirements on insulation monitoring exists:

• In a V2X configuration, the vehicle is neither attached to a DC fast?charger nor to an AC charging station, so the insulation monitoring functions provided by the RCD or the IMD circuits in the external charger cannot be used.
• When the OBC operates in reverse?power mode (DC to AC), the power?train adopts an isolated DC–AC topology; consequently, the IMD located on the BMS side can only detect faults on the DC side and is unable to sense insulation failures that potentially occur on the AC side of the converter.
• For these reasons, an insulation monitoring device must be integrated on the AC?input side of the onboard charger to maintain coverage of the V2X operating scenarios, as highlighted by the red block in Figure 1-1.