As positive or negative electric current is passed through an FTL drive core, it acquires a static electrical charge. Drives can be operated an average of 50 hours before they reach charge saturation. This changes proportionally to the magnitude of mass reduction; a heavier or faster ship reaches saturation more quickly.
If the charge is allowed to build, the core will discharge into the hull of a ship. All ungrounded crew members are fried to a crisp, all electronic system are burned out, and metal bulkheads may be melted and fused together.
The safest way to discharge a core is to land on a planet and establish a connection to the ground, like a lightning rod. Larger vessels like dreadnoughts cannot land and must discharge into a planetary magnetic field1.
As the hull discharges, sheets of lightning jump away into the field, creating beautiful auroral displays on the planet. The ship must retract its sensors and weapons while dumping charge to prevent damage, leaving it blind and helpless. Discharging at a moon with a weak magnetic field can take days. Discharging into the powerful field of a gas giant may require less than an hour. Deep space facilities such as the Citadel often have special discharge facilities for visiting ships.