EV High-Voltage Diagnostics: Safe Working Procedure

Competence and PPE Come First

Before any procedure: technicians must hold the relevant high-voltage qualification for their jurisdiction, and the workshop must have insulated gloves rated for the system voltage, inspected and date-checked, along with insulating tools, a barrier kit and signage.

No tool substitutes for competence. Diagnostic equipment reads data; it does not make an unsafe situation safe.

Identify the System Before Touching Anything

Determine what you are working on before opening anything:

• Nominal pack voltage (commonly around 400 V, with 800 V architectures increasingly common)
• Whether the vehicle has a service disconnect / manual service plug, and where it is
• Whether the high-voltage system is already disabled — an airbag deployment or a serious collision usually opens the contactors automatically
• The location of orange HV cabling, the inverter, the DC-DC converter and the on-board charger
• Whether the vehicle has a high-voltage A/C compressor — another HV circuit that is easy to overlook

De-Energisation and Proving Dead

Step 1 — Switch the vehicle off, remove the key or smart key from the vicinity, and secure the vehicle against inadvertent activation.

Step 2 — Isolate the 12 V auxiliary battery where the manufacturer procedure requires it. Many HV systems will not open contactors without this.

Step 3 — Remove the service disconnect or manual service plug following the manufacturer procedure, and wait the specified discharge period — capacitor discharge can take several minutes.

Step 4 — Prove dead with a meter rated for the system voltage and verified against a known source immediately before and after use.

Step 5 — Re-verify at multiple test points, not just one. Confirm zero voltage between HV positive and chassis, HV negative and chassis, and across HV positive and negative.

Step 6 — Apply lockout/tagout: secure the service plug, post signage, and control access to the vehicle for the duration of the work.

What a Diagnostic Platform Does Here

An EV-capable diagnostic tool reads battery management system data: pack voltage, individual cell voltages, temperature distribution, state of charge, state of health, insulation resistance and contactor status.

Insulation resistance is the measurement that matters most for safety. A falling value between the HV system and chassis indicates degrading insulation — a fault that must be resolved before the vehicle is returned to service, not a code to be cleared.

Documentation and Handover

Record the readings before and after the work, the insulation resistance values, and the torque or verification steps performed. On handover, confirm the service disconnect is refitted correctly and that the vehicle reports no HV isolation fault before it is released.