High-Voltage Interlock Loop (HVIL)
High-voltage interlock loop (HVIL) is a low-voltage monitoring circuit routed through selected high-voltage connectors, covers, and components.
How the loop is used
When the monitored parts are correctly connected and closed, the loop is complete. An interruption can indicate that a connector is loose or disconnected, or that a protected cover has been opened. Depending on the vehicle design and operating state, the controller may prevent the main contactors from closing, stop charging, request that they open, or store a fault.
HVIL monitors the continuity and mating state of selected parts; it does not measure the high-voltage system's insulation resistance to the chassis. Those are separate safety functions.
For contactor operation and the broader battery protection system, see High-Voltage Contactors and Pre-Charge Circuit and EV Battery Safety and Failure Management.