Which circuit class is defined as a fail-safe circuit?

In fire alarm systems, a fail-safe circuit is one that continues to function properly even if a single component or wire fails. Class D circuits are defined as fail-safe because they provide a continuous signal path that ensures system operation despite potential faults. In a Class D configuration, the circuitry is typically designed to maintain its loop integrity, meaning that even if one part of the circuit fails, the signals can still traverse through the remaining components or rerouted paths. This can be particularly important in fire alarm applications, where consistent functionality can be critical for safety.

Class A circuits, while also designed for redundancy, may not always guarantee the same level of fail-safe operation in every situation, as they rely on dual paths to mitigate the effects of a failure. Understanding these distinctions is vital for ensuring reliable operation in fire alarm systems.