Occupant and technician safety
It helps detect when high voltage may be finding an unintended path toward exposed conductive parts.
Insulation resistance testing
High voltage must stay inside the circuit — not leak into the car.
An EV insulation test checks how well the high-voltage system is electrically separated from the vehicle body. It is a safety test, a diagnostic clue and one of the most important checks after water ingress, impact damage or an isolation warning.
Checks HV to chassisThe positive and negative high-voltage paths must remain isolated from the metal body.
Finds leakage cluesMoisture, coolant, damaged cables and internal component faults can reduce resistance.
Specialist work onlyTesting follows manufacturer isolation, discharge and test-voltage instructions.
What the test checks
The test asks whether electricity has an unintended route to the chassis.
The high-voltage battery, orange cables, inverter, motor, onboard charger, electric heater and air-conditioning compressor are designed as an isolated electrical system. The metal vehicle body should not be part of that working circuit.
The vehicle continuously monitors isolation while it operates. If resistance falls too far, it may store an isolation fault, prevent charging, refuse to go ready or open the high-voltage contactors to protect occupants and technicians.
A measured value is not a parts verdict. The correct limit, test voltage, temperature conditions and connection points depend on the vehicle and the component, so the manufacturer procedure always takes priority.
Isolated HV circuitPositive and negative conductors carry high voltage without using the body as a return.
Vehicle chassisThe body is the reference point used to look for unintended leakage.
Resistance measurementA suitable tester measures resistance from the specified HV paths to chassis.
Section-by-section proofIf isolation is low, trained technicians divide the system safely to locate the affected branch.
Visual guide
See what a proper insulation diagnosis is trying to prove.
These illustrations show the test principle and common fault areas. Exact access points and limits are always vehicle-specific.
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Why it matters
The result protects people, components and repair decisions.
Insulation monitoring is designed to spot a loss of electrical separation before it becomes a greater safety or reliability problem.
Charging refusal
The car or charge point may stop a session when isolation is outside the permitted range.
No-ready condition
The vehicle can keep its contactors open rather than energise a circuit it considers unsafe.
Water and coolant clues
A fault after rain, flooding, coolant work or battery repair makes contamination history important.
Impact assessment
Underbody strikes and collision damage can compromise cables, connectors or battery enclosure insulation.
Avoids parts guessing
Separating branches can distinguish a cable or auxiliary component fault from a complete battery-pack problem.
How it is done properly
Safety first, measurement second, diagnosis third.
This is not a DIY test. High-voltage PPE, training, controlled access and the exact manufacturer procedure are essential.
Confirm the fault and preserve evidence
Record the complete vehicle scan, isolation-monitor data, freeze frames, weather, charging state and when the warning appeared before disconnecting anything.
Make the vehicle safe
Secure the work area, identify the vehicle, shut down high voltage, remove the service disconnect where specified, wait the stated discharge time and prove the absence of voltage using the OEM method.
Inspect before applying a test voltage
Look for impact marks, coolant, moisture, corrosion, damaged orange cables, disturbed connectors and previous repairs. Sensitive electronics must never be megger-tested while still connected unless the procedure explicitly permits it.
Measure at the approved points
Use a suitable calibrated insulation resistance tester, the manufacturer-specified test voltage and the correct connections from the relevant HV conductor or subcircuit to chassis.
Divide the circuit safely
If resistance is low, isolate approved branches one at a time—such as heater, compressor, charger, motor or battery—to see where the reading recovers. Never disconnect live HV connectors.
Repair, reassemble and verify
After correcting the cause, restore every interlock and connector, repeat the specified insulation checks, clear faults only when appropriate, then confirm charging and ready operation.
| What the result shows | What it can mean |
|---|---|
| Stable high resistance | The tested circuit is well separated from chassis under those test conditions. It does not prove every intermittent moisture or vibration-related fault is gone. |
| Low on the positive side | Scan data or approved testing may suggest leakage is biased toward the HV positive side, helping narrow the search. |
| Low on the negative side | A negative-side bias is another localisation clue, not an automatic component diagnosis. |
| Changes with rain or temperature | Moisture, condensation, coolant, connector sealing or temperature-sensitive internal insulation may be involved. |
| Reading recovers when isolated | The removed branch becomes a strong suspect, but its cable, connector and connected component still need separating. |
Quick answers
Questions customers often ask.
Is an insulation fault the same as a short circuit?
No. An insulation fault is an unintended leakage path between the high-voltage circuit and chassis. It may have high enough resistance that no fuse blows, while still being outside the vehicle safety limit.
Does a low reading mean the battery pack has failed?
Not automatically. The fault may be in the battery, cable, inverter, motor, onboard charger, heater, compressor, connector or contamination. The system needs to be separated methodically.
Can water cause an intermittent isolation fault?
Yes. Moisture or coolant can reduce insulation resistance, and the value can change as the vehicle dries, heats up or moves. History and environmental conditions matter.
Can I keep charging the car?
If the vehicle reports a high-voltage isolation or electrical safety warning, stop repeated charging attempts and arrange specialist assessment. Follow any stop or recovery instruction shown by the vehicle.
Isolation fault help
An isolation warning needs evidence—not a guessed battery pack.
Tell us the warning, when it occurs, whether charging still works and any recent water, impact or repair history. We can plan the correct safe diagnostic route.