The OBC must meet a number of insulation monitoring requirements to maintain safety during V2L and V2H operations. The most recent regulation addressing these needs is GB/T?18487.4?2025, Electric Vehicle Conductive Charging and Discharging System – Part?4: Discharging Requirements for Electric Vehicles. Published on 25^th?April?2025, the standard is scheduled for implementation on 1^st?November?2025. The standard expands the safety?related insulation monitoring rules, previously limited to charging (GB/T?18487.1/2), and now covers vehicle?to?load (V2L), vehicle?to?home (V2H), and vehicle?to?grid (V2X) discharging operations. The following list is a concise debrief of the insulation monitoring requirements on an OBC system:

• Applicability:
  • Applies to all conductive DC to AC or DC to DC discharging functions of an EV that can feed external loads, a building, or the grid.
  • Covers both stand?alone and integrated OBC architectures. The IMD circuit must be installed between Line | Neutral and PE.
  • Requires compliance for vehicles with nominal AC output voltages ≤?250V and DC output voltages ≤?1000V.
• Accuracy:
  • Insulation resistance is the minimum value of L to PE or N to PE.
  • Fault state: R < 500Ω/V; Safe state: R > 500Ω/V.
  • Both symmetric and asymmetric insulation failures must be identified by the IMD circuit.
• Response time:
  • Real-time IMD during the discharge stage.
  • Insulation monitoring period must be less than 10s.
  • Insulation monitoring response time must be less than 100ms.
• High-potential (Hi-pot) testing:
  • 2000 VAC or 2930 VDC when the working voltage is between 300V and 690V.
  • Test 60s or test 1s with 1.1 times of the rated voltage.
  • No failures during testing and leakage current is less than 10mA.

In addition to the above-mentioned requirements, there are requirements for the DC side IMD that are not explicitly mentioned in the AC side IMD, and requirements that depend on the OEMs of different regions worldwide.

• Insulation monitoring sequence during the pre-discharge stage, post-closure stage, and shutdown stage.
• Self-diagnostics: Power-up self-test, periodic self-test, CAN reporting, and fault memory.

Interaction with other safety devices: Interaction with an RCD circuit in the AC side or interaction with the IMD circuit in a battery management system (BMS) or chargers.