High-Voltage Safety, Personal Protective Equipment, and Service Procedures
What the candidate must know
A high-voltage (HV) system is generally defined as any circuit exceeding 60 V DC or 30 V AC; treat everything above those thresholds as live until proven otherwise. Understand the physiology: body resistance drops with moisture and broken skin, so current - not voltage - causes injury. Rough thresholds: 1 mA perception, 10–30 mA let-go, and 100 mA or more can cause ventricular fibrillation. AC at 60 Hz is more dangerous than DC at the same current because it drives the heart into fibrillation more readily, which is why HV AC and DC circuits get different isolation-resistance requirements (commonly 500 Ω/V for AC, 100 Ω/V for DC).
Equipment rules
Class 0 rubber insulating gloves are rated 1,000 V AC / 1,500 V DC, must be air-tested and inspected before every use, and are electrically retested on a six-month interval; leather protectors go over them to resist abrasion. Use a CAT III (or CAT IV where appropriate) meter, 1,000 V insulated tools, and remove jewelry and metal watchbands. Only one hand works on energized conductors; the other stays in a pocket.
Procedure sequence
Key off, remove the service disconnect or HV fuse, secure it in your possession, wait the manufacturer-specified capacitor discharge time (often 5–10 minutes), then verify zero energy at the correct test points with a proven meter, and re-verify after the test. Identify HV cable by its orange color and warning labels. Never rely on a non-contact voltage tick tester alone.
Trap and rule of thumb
Questions frequently test that the 12 V system remains live after HV shutdown, and that a disconnected pack can still hold voltage through welded contactors or capacitor charge. Rule of thumb: if your meter isn't CAT III rated to at least the pack's full voltage, you are not qualified to verify zero energy on that vehicle.