High-Voltage System Disablement, Isolation, and Verification
The sequence
De-energizing is a procedure, not a single action: identify the system, secure the vehicle, move the key fob well away (10 m or more), disconnect the 12 V negative (and any auxiliary or backup battery), wait the OEM-specified time for capacitors to discharge (commonly 5–10 minutes), then remove the service disconnect or manual service disconnect plug and, where fitted, the HV fuse. Apply lockout/tagout so nobody reconnects while you are in the circuit.
Verification is the tested skill
Prove the meter on a known live source, measure DC volts at the inverter terminals and at the pack output, check for any AC component, then prove the meter again on the known source. Zero volts at one point does not mean zero everywhere: some vehicles have more than one pack, an HV auxiliary fed from the 12 V side, or an electric cabin heater with its own capacitor bank. An insulation tester then confirms the pack and cables are isolated from chassis before re-energizing.
Traps
Trusting the dash display or the silence of the vehicle; skipping the "prove meter" step in either direction; measuring only one terminal; forgetting a second battery; and restoring the service plug before the 12 V system in the wrong order. A classic: a welded contactor leaves HV present even with the disconnect pulled.
Rule of thumb
Treat any reading above 5 V DC as live and stop. Restore in reverse order and re-verify normal operation.
Worked example: 400 V pack disabled per procedure, but the meter shows 12 V at the inverter. That is not noise - a capacitor is still charged or a contactor is welded. Stop and find the source.