A z-9 electron pack was an explosive device. Around half a dozen were kept on board Storm Mine 4. The Fourth Doctor explained that, by passing "a positive charge through the metal plate, you'd be able to magnetise them and have anti-robot bombs". This was carried out by Lish Toos and Kiy Uvanov, who utilised the bombs to destroy V5. (TV: The Robots of Death)
Tardis