10. Aliens Of London/World War Three
In episodes four and five of series one, companion Rose Tyler (Bille Piper) and the Doctor (Christopher Eccleston) go back to London so that Rose can visit her mother and boyfriend, only to discover that Rose has been gone for 12 months, and not 12 hours as the Doctor has promised. An alien ship crashes, and draws national panic while an alien family known as the Slitheen who have been infiltrating the British government for years begin preparations for a takeover that will enable them to start a nuclear war. The war will leave the planet barren, and the Slitheen can then sell it to the highest bidder in need of a radiation source. At the end of Aliens in London, the Doctor is part of a panel of alien experts lead by two Slitheen in human guise. The Slitheen use the specially modified ID badges to electrocute the panel's experts.
When He Should Have Regenerated: At the beginning of World War Three, the Doctor takes the ID badge he is being electrocuted with and puts it onto a Slitheen, which in turn electrocutes them all, including the one terrorizing Rose's mother Jackie in her flat. All of the humans being electrocuted died, but the Doctor survives, saying only that it is "deadly to humans, maybe." The Doctor has previously shown a resistance to electricity in the original series, but never has he stated that Time Lords are immune to it. In addition, it is shown with Matt Smith's Eleventh Doctor that he is indeed susceptible to electricity, when a massive shock stops one of his two hearts. This sustained exposure to electricity would have definitely damaged, if not stopped, both of his hearts if both instances are regarded as canon. Also, the badge mysteriously stops electrocuting him once it is ripped off his neck, despite being held in his hand and visually produing electricity. While it may not have been shocking him with as much oomph given it's new location, it still would have been dealing damage until he let it go completely.