Insulation Detection

Insulation detection serves as the primary line of defence for a new energy vehicle’s high-voltage (HV) safety system. Operating like a round-the-clock "insulation sentinel", it constantly monitors the insulation integrity between high-voltage components and the vehicle chassis to prevent current leakage and electrocution hazards.

⚡️ Why is Insulation Detection Necessary?

The high-voltage systems in new energy vehicles (typically 300V to 800V) operate at levels far exceeding the safe voltage threshold for the human body (36V). Under ideal conditions, the HV system remains completely insulated from the vehicle body (chassis earth).

However, prolonged driving over rough roads, high humidity, and extreme heat can lead to aging HV wiring harnesses, water ingress at connectors, or battery pack damage from road debris and collisions. Once the insulation barrier is compromised, high-voltage current can leak to the chassis, posing a severe safety risk to occupants and the vehicle itself.

🛡️ How Does It Work? (Operating Principle)

At its core, insulation detection calculates the real-time insulation resistance of both the high-voltage system's "positive-to-ground" and "negative-to-ground" circuits. If both resistance values remain sufficiently high, the insulation is intact; if both drop to critical levels simultaneously, a complete leakage circuit is formed.

Currently, mainstream on-board insulation monitoring systems rely on the "unbalanced bridge" principle. By injecting a dedicated signal into the HV circuit or toggling measuring resistors of known values, the system captures voltage variations to calculate real-time insulation resistance. This vital function is managed directly by the Battery Management System (BMS).

📊 Standards and Thresholds: When is an Alert Triggered?

Safety thresholds for insulation detection are governed by national regulatory standards and vehicle manufacturers. Once these boundaries are breached, the vehicle initiates protective measures.

Mandatory Regulatory Standards
Under GB 18384-2020 (Electric Vehicles Safety Requirements), all vehicles must feature an active insulation resistance monitoring system.

• Basic Requirement: At maximum operating voltage, the insulation resistance of DC circuits must be no less than 100Ω/V. Certain stringent standards raise this requirement to 500Ω/V.

• Example: For a 400V system, the minimum allowable insulation resistance is 400V * 100Ω/V = 40,000Ω (40kΩ).

Manufacturer Threshold Strategies
In practical applications, carmakers establish tiered alert strategies based on risk levels. For instance, CATL's specifications adopt the following graded protocol:

• Power Derating: When insulation resistance drops between 600Ω/V and 500Ω/V, vehicle power output is capped at 50%.

• HV Isolation: When insulation resistance falls between 500Ω/V and 400Ω/V, power delivery is cut to 0%.

• Immediate Power-Down: If insulation resistance drops below 400Ω/V, the system executes an immediate high-voltage shutdown, cutting drive power entirely.

🔧 How is Insulation Testing Carried Out?

Insulation testing is divided into automated on-board monitoring and manual workshop testing during maintenance.

1. On-board Monitoring (Automated via BMS)
This functions continuously during daily driving. The BMS constantly tracks resistance levels; if values drop below critical thresholds, it instantly triggers a warning on the instrument cluster, while derating power or isolating high-voltage power delivery.

2. Off-line Testing (Manual Workshop Inspection)
During scheduled servicing or diagnostic repairs, technicians use a dedicated insulation resistance tester (megohmmeter) for manual measurements.

• Standard Procedure: The 1000V test voltage range is typically selected. The black probe is connected to the chassis earth, while the red probe is attached to the HV component under test (such as the drive motor or battery pack terminals). The test button is pressed and held for 5 to 10 seconds before recording the reading.

• Pass Benchmark: Insulation resistance standards vary across different high-voltage components.

💎 Summary

Insulation detection is the cornerstone of high-voltage safety in new energy vehicles. Through continuous real-time monitoring of insulation resistance between the HV system and the vehicle chassis via the BMS, the system provides tiered alerts, derates motor power, or cuts high-voltage power entirely to prevent hazards before they occur. Understanding these principles and standards is crucial to ensuring the safety of both vehicle occupants and the high-voltage system.