Hardware failures and override commands can place abnormal stress on the total impulse propulsion system (IPS), requiring various degrees of engine shutdown. System sensors, operation software, and human action work in concert to deactivate impulse propulsion system components under conditions such as excessive thermal loads, thrust imbalances between groups of individual engines, and a variety of other problems.

            The most common internal causes for low-level emergency shutdown in Starfleet experience include fuel flow constriction, out-of-phase initiator firings, exhaust vane misalignment, and plasma turbulence within the accelerator stage. Some external causes for shutdown include asteroidal material, survivable combat phaser fire, stellar thermal energy effects, and crossing warp field interaction from other spacecraft.

            Emergency shutdown computer routines generally involve a gradual valving off of the deuterium fuel flow and safing of the fusion initiator power regulators, simultaneously decoupling the accelerator by bleeding residual energy into space or into the ship’s power network. As these procedures are completed, the driver coil assembly (DCA) coils are safed by interrupting the normal coil pulse order, effectively setting them to a neutral power condition, and allowing the field to collapse. If the shutdown is in an isolated engine, the power load distribution is reconfigured at the first indication of trouble.

            Variations on these procedures are stored within the main computer and IPS command coordinators. Crew monitoring of a shutdown is a Starfleet requirement, although many scenarios have seen engines being safed before reliable human reactions could be incorporated. Voluntary shutdown procedures are dependable and accepted by the main computer in 42% of the recorded incidents.