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The ECH program is designed to help defend the ship it is positioned on from hostile alien takeover. Between immunity to weapons fire, an unlimited arsenal, and the ability to transfer to different sections of the ship almost instantly, this is surely a foe to reckon with.
To prevent easily dispensing with the ECH by deactivation, it is automatically activated every time a weapon goes off on board the ship (outside of the holodeck or holosuite) and can only be deactivated by command authorization from the commanding officer, first officer, cheif medical officer, or chief engineer.
The ECH system functions most effectively in a ship or starbase where every room is equipped with holo-emitters.
~Jeremy Lennert, Simon Dark of the 7th Fleet