A building's defense = base defense + the points of the fuses placed. The Thief places decoders until it reaches zero. Alarms don't add defense, they only alert you.
Security System. The panel on the wall of every building. In its slots you place fuses (raise defense) and alarms (alert you). The attacker places Decoders on it to bring defense down to zero.
Electronic Fuse: buy $20,000 · research $10,000. Fuse: +1 defense (confirmed in the game data).
Electronic Fuse: buy $100,000 · research $150,000. Fuse: +3 defense (data from players). Can be obtained through barter.
Electronic Fuse (III): research $500,000. Fuse: +5 defense (data from players).
Alarm System: buy $30,000 · research $15,000. An alarm for the Security System: alerts all players at the location when someone starts breaking in.
Alarm System: buy $60,000 · research $25,000. An alarm for the Security System: alerts the owner when someone starts breaking in.
Alarm System: buy $200,000 · research $50,000. An alarm for the Security System: alerts the owner and the whole clan when someone starts breaking in.
Decoder. A single-use breaching tool: −1 defense on a building's security system. It doesn't open safes.
Decoder (II). −3 defense (data from players). 50 pieces = Eviction Right (barter).
Decoder. −5 defense (data from players). Loot only, according to robrunwiki.
Master Key: buy $55,000 · research $50,000. The key that opens safes and security cabinets in someone else's base (in 1 second).