The cable which supplies the Evangelion Units with external electrical power. Once severed, they rely on a rather limited internal battery supply. Since such massive constructs consume vast amounts of electricity, current battery technology allows for only one to five minutes of operation under such conditions.
Because of this, the internal battery is typically used only as an emergency backup until an external power supply can be reattached via an Umbilical Cable. These cables can be found scattered around New Tokyo-3 inside specially designated Armament Buildings. The term 'umbilical cable' refers to the 'umbilical cord', which supplies an unborn child with nourishment while in the womb.
Based on Neon Genesis Evangelion Platinum: 06 DVD Booklet
- The Leash Symbolism
The symbolism behind an Umbilical Cable is evident. Not only does it supply power to the Evangelion Units, but also acts as a leash to restrain them. Once Evangelion Unit-01 obtains an Sē Engine from Zeruel, the Fourteenth Angel, it no longer requires an Umbilical Cable, and thus becomes feared by both NERV and SEELE. As a result it is immediately placed into stasis. This is a not-so-subtle coming of age of both Evangelion Unit-01 and it's pilot Shinji Ikari. By obtaining the means to function without an Umbilical Cable, Shinji Ikari and Evangelion Unit-01 effectively grow beyond NERV's or SEELE's control. As stated by SEELE, an Evangelion Unit with an infinite supply of energy is a God, one whose creation they never authorized.