Prove dead: live, dead, live
Using a multimeter in 3D to prove a panel dead: test on a live source, test the panel, test the live source again.
Try this in the lab: Commission the transfer-pump starterAbout the Panel Lab
Transcript
The probes touch the isolator terminals. The meter first reads 415 V on a live source, then 0 V on the isolated panel, then 415 V again on the live source.