High-Voltage De-Energisation Procedure

1. What is High-Voltage De-Energisation?

High-voltage (HV) de-energisation refers to the standardised operating procedure carried out prior to servicing, maintaining, or handling post-collision recovery of new energy vehicles (NEVs/EVs). It involves physically isolating the complete vehicle high-voltage circuit and verifying zero voltage in the system. EV high-voltage systems typically operate between 400V and 800V, significantly exceeding the safe human threshold (36V). Hence, isolating the high-voltage circuit is the absolute first line of defence to safeguard workshop technicians and first responders.

2. Regulatory Framework & Standards

The mandatory national standard "Electric Vehicles Safety Requirements" (GB 18384-2025), effective 1 July 2026, stipulates clear compliance mandates for high-voltage de-energisation:

• Vehicles must provide a physical disconnection of the complete vehicle high-voltage circuit. For the first time, a "one-touch de-energisation" feature is officially defined as a physical isolator, entirely replacing older methods that relied solely on software-controlled shut-offs.

• With the vehicle stationary and not charging or discharging, the driver must be able to trigger the high-voltage cut-off via a single action (either a tap or a long press).

• This design guarantees reliable power isolation even under extreme conditions, such as head unit/infotainment blackouts or compromised wiring harnesses.

3. Standard Operating Procedure (SOP) for HV Isolation

HV de-energisation must strictly adhere to the following standardised steps:

Step 1: Safe Staging & Pre-Isolation Setup
Park the vehicle on a level, dry, dedicated EV service bay; shift the gear selector to Park (P) and engage the parking brake; switch the ignition/start button to the OFF position, and allow the vehicle to rest for 5 minutes.

Step 2: Disconnect the 12V Auxiliary Battery
Disconnect the 12V battery negative terminal and wait for at least 15 minutes to ensure the low-voltage electronics are completely powered down. This waiting period allows the high-voltage pre-charge capacitors inside the Motor Control Unit (MCU) to self-discharge to 0V.

Step 3: Remove the Manual Service Disconnect (MSD)
Don certified high-voltage insulating gloves, locate, and pull out the Manual Service Disconnect (MSD) plug. The MSD is commonly located in the frunk, boot, or beneath the rear seats, identifiable by a high-visibility handle or cover marked with high-voltage hazard warning labels. Note that certain models (such as the BYD Qin) omit a traditional MSD, requiring technicians to unplug the traction battery main high-voltage bus connector directly.

Step 4: Capacitor Discharge Dwell Time
Even after pulling the MSD, capacitors within the high-voltage circuitry may retain residual charge. Most manufacturers mandate waiting 5 to 15 minutes before touching or probing any high-voltage components.

Step 5: Voltage Verification (Zero-Potential Check) — The Crucial Step
Using a CAT III or CAT IV rated digital multimeter, perform the following checks:

• Measure the voltage between the high-voltage bus terminals and chassis ground; the reading must be under 5V.

• Measure the voltage across the positive (+) and negative (-) terminals of the high-voltage cables; the reading must likewise be under 5V.

• Caution: Ensure the exposed metal probe tips do not touch the high-voltage junction box grounding plate during measurement.

If residual voltage exceeds 60V, a dedicated HV discharge tool must be deployed until the voltage drops to an intrinsically safe level.

Step 6: Insulation Protection & Service Handover
Insulate and cover all exposed high-voltage connectors with approved electrical insulation tape to eliminate accidental short circuits or contact with conductive tools.

4. Personal Protective Equipment (PPE) Requirements

5. Golden Safety Rules

1. Treat Orange as Live: All orange-shielded cables must be considered live until formally proven de-energised via multimeter testing.

2. Two-Man Rule (Buddy System): High-voltage procedures require at least 2 certified personnel — one executing the procedure and one dedicated safety observer.

3. One-Hand Rule: Keep one hand away from grounded metal while testing to prevent an accidental electrical path across the chest.

4. Zero Hot-Working: Live working on un-isolated high-voltage components is strictly forbidden.

5. Lockout/Tagout (LOTO): Once the MSD is removed, secure the area with a lockout tag marked "Danger: Do Not Reconnect - Work in Progress".

6. Frameless Door Precaution: For models equipped with frameless windows, wind the door glass down slightly before cutting 12V power to prevent door seal binding or lockout issues.

6. Emergency Protocols

• Crash/Collision Response: Refer directly to the manufacturer's vehicle-specific Emergency Response Guide (ERG) to locate structural cut zones and auxiliary HV cut loops.

• Battery Thermal Runaway / Smoke / Fire: Evacuate immediately and alert emergency services (999/BOMBA); for external component fires, use dry chemical powder or CO2 extinguishers. Never apply minimal water to an uncontained high-voltage battery pack fire.

• Electrocution Incident: Immediately cut power (or use non-conductive rescue hooks) to disengage the victim from the live circuit before administering first aid.

Summary

The high-voltage de-energisation process is the ultimate safety baseline in EV service and repair. From physical disconnection and zero-voltage testing to full PPE compliance, every single stage requires strict adherence to workshop protocol. The upcoming safety regulations taking effect from July 2026 further reinforce the mandatory requirement for hardwired physical isolation mechanisms. For every EV technician, the rule "Isolate first, test to confirm, only then proceed" remains a non-negotiable lifeline on the workshop floor.