0.00 V Between Rails With a Live Pack Terminal
A 2020 PHEV with a 355 V pack is in for HV battery removal. The technician removes the service disconnect, turns off the 12 V system, and waits 10 minutes. At the inverter terminals he measures 0.00 V between B+ and B-, and 0.00 V from each terminal to chassis. He concludes the system is de-energized and starts disconnecting the pack cables. A second technician notes that the service disconnect is on the negative side of the pack and that the positive contactor is upstream of the disconnect. The pack is isolated from chassis. The inverter DC-link capacitors are 1000 microfarad.
1. Explain how a 0.00 V reading between B+ and B- can coexist with a live conductor, and what additional measurements are required before touching the pack cables.
Read the worked answer
A 0.00 V reading between B+ and B- proves only that the potential difference across the DC-link capacitor is zero. It does not prove that either conductor is at earth potential. With the negative side opened, B- is floating; a high-impedance meter can pull the floating rail toward B+ and display 0 V. Meanwhile the pack positive conductor upstream of the open contactor can still be at pack potential. A reading from a floating rail to chassis can also be misleading if the chassis is not at a known reference.
Before touching the pack cables:
- Prove the meter on a known live source, then measure, then re-prove the meter.
- Measure B+ to B-, B+ to a verified chassis ground, B- to the same ground, and the pack terminals themselves.
- Identify all HV sources and backfeeds, and follow the OEM test points. Treat the battery terminals as energized at all times; a battery cannot be de-energized, only isolated.
2. The technician's meter is CAT III 600 V. Is it suitable for this 355 V pack? What is the live-dead-live check?
Read the worked answer
CAT III 600 V is adequate for a 355 V fixed bus only if the meter's DC working-voltage rating is at least 600 V DC; confirm the marking rather than assuming the AC rating applies. The leads and fuse must also be rated for the voltage.
Live-dead-live:
- Test the meter and leads on a known live source or proving unit.
- Test the conductor or terminal that is supposed to be de-energized.
- Return to the known live source and confirm the meter still reads correctly.
This proves the meter, leads, and fuse were working before and after the absence check. A non-contact voltage tester must not be the sole proof of absence.
3. The pack is removed. Before reinstalling, what checks prevent a repeat of the mistake?
Read the worked answer
Review the OEM disable procedure and identify exactly which pole the service disconnect opens and which conductors remain live. Update the job aid to require verification at the pack terminals, not only at the inverter. Require lockout/tagout, a proven meter, and a second-person check for HV work. Cover and label exposed terminals, use insulated tools and the correct gloves, and wait for capacitor discharge before opening covers. On reassembly, inspect contactors for welding, verify isolation resistance and capacitor discharge, and confirm that the 12 V system cannot re-enable HV contactors unexpectedly.