High Voltage Interlock Loop or HVIL is a critical safety design method used in electric and hybrid vehicles. HVIL monitors all components connected to the high-voltage cables in an electric vehicle via low-voltage signals. It detects the integrity of the electrical connections in the high-voltage system circuit and activates safety measures when the circuit is interrupted or its integrity is compromised.
Why is HVIL Important?
In high-voltage systems, potential leakage currents or connection failures pose risks to both the vehicle’s electronics and users. HVIL instantly detects such conditions and initiates a power cutoff. This ensures that during an accident, the vehicle’s high-voltage system is rapidly deactivated, protecting both occupants and first responders. It also safeguards technicians during maintenance and repair operations. Standards such as IEC 61851 and ISO 26262 mandate the use of HVIL.
How Does HVIL Work?
The HVIL system consists of a low-voltage signal loop running parallel to the high-voltage circuit. Any interruption in this loop (e.g., disconnecting a connector, cable damage) is detected by the system, triggering intervention by the VCU (Vehicle Control Unit) or BMS (Battery Management System).
EV power connectors include auxiliary pins to monitor connector status. When a connector is plugged in, these two pins are short-circuited, enabling status monitoring. During connection, the power pins make contact first, followed by the HVIL pins. This sequence ensures the battery contactors only engage power after the connector is fully seated, preventing arcing.
The HVIL loop passes through all high-voltage components in the vehicle. In the automotive industry, voltages above 60V are considered high-voltage.
What Happens When an HVIL Fault Occurs?
When an HVIL fault is detected, the BMS or VCU issues appropriate commands to safely disconnect the high-voltage system and prevents high-voltage operation until the fault is resolved. During a fault condition, the following responses may occur:
- Fault Warning
- Whether the vehicle is in motion or stationary, when an HVIL fault is detected, the driver is notified via dashboard warning lights or audible alerts about the vehicle status.
- Power Shutdown
- If an HVIL fault is detected while the vehicle is stationary, the vehicle control system may immediately cut off high-voltage power.
- Important Note: If the vehicle is in motion, the high-voltage connection cannot be immediately interrupted as this could cause sudden power loss and compromise passenger safety.
- Limited Operation Mode (Limp Home Mode)
- When a potential hazard is detected during driving:
- a) The system first alerts the driver via warnings/alarms
- b) Gradually reduces motor output power
- c) Systematically decreases vehicle speed
- This allows the high-voltage system to operate at reduced load, giving the driver adequate time to safely pull over.
- When a potential hazard is detected during driving:
Different manufacturers employ various solutions for fault detection, but most commonly utilize either a fixed voltage/current source or a PWM signal.
In Tesla’s system, the BMS applies a constant current to the HVIL loop and performs voltage measurements at specific points to monitor voltage drops. The image below illustrates this system’s working principle for better understanding.
In the future, HVIL systems will become increasingly intelligent as battery voltages rise and autonomous driving/V2X technologies advance. These systems may even expand beyond the automotive sector, potentially becoming ubiquitous across various industries.
References
- HVIL Connectors: Safety in High-voltage, High-current Environments, RENHOTEC
- What is a high-voltage interlock?, CMVTE
- Tech Note: HVIL Diagnostic Guide, NHTSA
- Automotive High-Voltage Interlock Reference Design, TI Designs: TIDA-01445, Texas Instruments
- The air conditioner of a new energy pure electric vehicle cannot cool, and the computer reports an HVIL fault, NEV School
- Detailed implementation guide for DFMEA (Design Failure Mode and Effects Analysis) of high voltage wiring harnesses for new energy vehicles, Automotive Electrical
See also: HVIL, High Voltage System