XEV High-Voltage EV Safety — practice test

High-yield XEV topics

The material that shows up year after year. If you're short on time, start here.

• Verify zero energy, the whole ritual. Key off, fob out of range, PPE on, pull/open the MSD and keep it under your personal control, apply LOTO, wait the OEM capacitor-discharge time (commonly 5-10 minutes), then measure. Measure HV+ to HV-, AND each leg to chassis - both legs, or you miss a leg-to-chassis fault. Prove the meter live-dead-live on a known source before and after. Below roughly 30 V (or the OEM limit) is de-energized; 60 V DC is the hazard threshold, not the pass mark.
• Meter and glove ratings as numbers, not vibes. CAT III 1000 V minimum for a 400-800 V bus, and the leads and probes must carry the same rating as the meter - CAT III 600 V does not cover an 800 V pack. ASTM D120 / OSHA 1910.137 Class 0 = red label, 1000 V AC / 1500 V DC max use, which still covers 800 V DC. Visual plus air (roll-up) test before EVERY use, electrical retest at least every 6 months in service, 12 months if tested but never issued. Leather protectors go OVER the rubber.
• Isolation resistance math. The 500 ohms-per-volt DC criterion means a 400 V pack needs at least 200 kilohms bus-to-chassis; UN ECE R100 / GTR 20 give 500 ohms/V DC and 100 ohms/V AC for the motor phase side. Test with a megohmmeter at the specified voltage (commonly 500 V DC), not a DMM ohms range, and disconnect the modules first or you will kill semiconductors. Healthy systems read megohms. Also know ISO 6469-3: equipotential bonding under 0.1 ohm - a missing bond strap is a defect, not cosmetic.
• The floating bus and the two-fault problem. A single HV+ to chassis fault does not complete a circuit and will not pop the pack fuse, so the car drives fine while the IMD screams. The second fault on the other leg is the short and the shock path. Reading roughly 0 V from HV+ to chassis and full pack voltage from HV- to chassis means HV+ is grounded to the body. Common real causes: coolant intrusion into the pack, chafed cable at the shield, moisture after a rainstorm (reading recovers when it dries), and a just-replaced HV component like the electric A/C compressor.
• Interlocks and startup logic. HVIL is a low-voltage loop routed through HV covers and connectors; break it and the BMS drops the main contactors. The MSD interlock pins are shorter than the power pins so the loop opens before the main contacts part. Contactors are normally open and fail-safe on loss of 12 V; precharge limits inrush into the DC-link caps and the precharge contactor drops out once the bus is up. Know the failure modes: welded contactor = full pack voltage on the bus with the car off, open precharge resistor = bus never reaches voltage and no READY.
• Damaged and burned vehicles. Stranded energy means the pack is energized even with the 12 V disconnected and the cables cut - never cut orange cables to de-energize; use the ERG cut loop and disable points (SAE J2990). LiPF6 electrolyte decomposition releases hydrogen fluoride: SCBA and turnout gear, not a dust mask. Quarantine a fire- or crash-damaged EV at least 50 ft (15 m) from vehicles, structures and combustibles and monitor 24-48 hours - re-ignition hours later is normal behavior, not a freak event. Flatbed only, drive wheels off the ground, because rolling wheels spin the motor and generate HV.

Where techs lose points on XEV

Good technicians miss these — not from lack of skill, but because the question is built to catch them.

• "I pulled the service disconnect, so it's dead." Wrong, and it shows up in three different question skins. The MSD splits the pack's internal HV, but the DC-link capacitors in the inverter still hold a charge, so the wait time and the measurement are both mandatory. Same trap with the 12 V battery: dropping 12 V opens the contactor coils, it does not discharge anything. Any answer that ends the sequence before a personally verified measurement is the wrong answer.
• PPE picked from voltage alone. Shock boundaries scale with voltage (301 V - 1 kV DC row: restricted approach 1 ft 0 in., limited approach 3 ft 6 in. for a fixed part), but arc flash PPE comes from an incident-energy assessment - available fault current and clearing time. The arc flash boundary is where incident energy hits 1.2 cal/cm2, and it can land inside or outside the limited approach boundary. Category minimum arc ratings are 4/8/25/40 cal/cm2, so Cat 2 = 8 cal/cm2. Do not confuse the 1.2 boundary number with a PPE rating.
• Tagout treated as lockout, and the disconnect left lying around. Under 1910.147 the lock comes off by the person who put it on, a tag is not a lock, and "I'm standing right here" is not a control. The removed MSD goes in your pocket or your locked box, not on the bench where the next guy reinstalls it. Test writers love the plausible-sounding Technician B who is casually skipping a control.
• Look-alike answers on gear and testing. A CAT II 600 V meter is a real meter and a wrong answer. A DMM that reads 50 megohms is a real meter and still cannot do an isolation test. Gloves that passed their 6-month proof test still get an air test today. Gloves stored loose in a drawer with brake cleaner and rags go tacky and swollen from chemical attack - not "normal wear" - and fine cracks at flex points are ozone damage. And a chafed HV cable with the shield exposed does not get soldered and heat-shrunk; most OEMs require replacing the assembly.

A study plan that works for XEV

Roughly two weeks of real preparation, in the order that actually builds on itself.

1. Days 1-3: Architecture first, because every scenario question rides on it. Draw the HV loop from memory - pack, main positive and negative contactors, precharge resistor and contactor, DC-link caps, inverter, motor, plus DC-DC converter, OBC, and the BMS/IMD watching it all. Then overlay HVIL and the MSD and say out loud what breaks the loop and what opens the contactors. If you can't sketch it on a shop towel, you're not ready for the diagnostic items.
2. Days 4-6: Memorize the numbers cold - they are free points and they don't argue. 60 V DC / 30 V AC hazard threshold; Class 0 = 1000 V AC / 1500 V DC, red, 6-month retest; CAT III 1000 V meter and leads; 500 ohms/V DC (400 V pack = 200 kilohms) and 100 ohms/V AC; 500 V DC megohmmeter; 0.1 ohm bonding; 1.2 cal/cm2 arc flash boundary; 4/8/25/40 cal PPE cats; 50 ft quarantine. Write them from a blank page daily until you can do it in under three minutes.
3. Days 7-9: Drill procedure sequences in order, not as trivia. De-energize, verify zero energy with live-dead-live, isolation test, LOTO. Then work EXCEPT and Technician A/B sets specifically - for every A/B item, decide each statement true or false separately before you look at the choices. That one habit is worth more on this test than any other single tactic.
4. Days 10-12: Full-length timed practice, then review only the misses, and write the one-sentence reason each wrong answer was wrong. Read one OEM Emergency Response Guide end to end and the ASTM D120 glove-care rules; the damaged-vehicle and PPE-care questions come almost verbatim from that territory.

Sample XEV questions

Straight from the bank — answers highlighted, with the explanation underneath.