Final answer:
Household fire alarm systems, as a part of residential electrical installations, should utilize a three-wire system for safety, including a circuit breaker or fuse for thermal overload prevention, and grounding of the neutral wire and appliance casing for shock prevention.
Step-by-step explanation:
An AC primary power source for all household fire alarm systems should include several safety features to ensure both thermal and shock hazard prevention.
The modern standard for residential and industrial wiring is a three-wire system. This system, as outlined by the provided figures and safety guidelines, includes critical components such as a circuit breaker or fuse to protect against thermal overload, grounding of neutral wires to prevent potential differences that could lead to shocks, and a protective case around devices to prevent direct contact with electrical components.
The significant safety features of the three-wire system are the grounding of the neutral wire and the case of the appliance to the earth, which helps ensure that both are at zero volts relatively and offer an alternative current return path. The circuit breaker or fuse is incorporated in series with the live/hot wire to protect the system and the users from the hazards associated with excessive current flow.