Equipment can be fed from more than one place. Opening the supply you know about leaves the ones you do not.
Generators, UPS units, solar and control-circuit feeds all energise equipment that looks isolated. One open switch and one label is how people get hurt believing they did everything right.
Follow it in the right order. Sources first, then isolation, then verification. Getting that backwards -- isolating what you found and then testing -- produces a zero reading at the point you tested and tells you nothing about a second feed arriving somewhere else in the equipment.
Check yourself: The normal supply is removed. Is the equipment dead?
Answer: Not necessarily. A UPS can maintain power after the normal supply is removed.
A UPS exists to keep equipment running when the normal supply goes. Removing that supply is the condition it was installed to respond to, so it is the moment it is most likely to be feeding the equipment.
No card. No account needed to start. Score by topic against the 70% mark.
Written from the owner's poster for this lesson. Nothing added beyond what the poster states.
Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.
Electrician licensing exam prep: practice questions, timed exam simulations, and step-by-step help finding every answer in the NEC.