Which of the following is a 'fail-safe' circuit?

• A. Class A
• B. Class B
• C. Class C
• D. Class D

Answer: (D)

Fail-safe means the system will alert or indicate an alarm if something goes wrong in the circuit, so occupants aren’t left in a silent fault. In this material, the circuit type described as fail-safe is Class D. It’s designed so that a fault in the wiring or a device failure does not leave the system quiet; the alarm condition is still reached, or the fault is clearly indicated, helping ensure life safety. The other circuit types are treated as not inherently fail-safe because certain faults could occur without triggering an alarm, which is why Class D is identified as the fail-safe option.