Glossary · Fault behavior and system states
Safe default
Also known as: safe default value
In functional safety, a safe default is a predefined value or behavior selected to reduce risk when information is missing, invalid or uncertain.
- Functional safety
- Data and interfaces
In one sentence
A safe default is the value or behavior a system uses when data is missing or invalid; it is chosen from the risk assessment, not for convenience.
Example
If the speed limit from the fleet manager is missing, an AGV applies its lowest permitted speed instead of its last received value.
How it applies
- Functional safety: What is safe depends on context. Zero speed is safe for a conveyor, but de-energizing a vertical axis or a vacuum gripper can drop the load. Derive each default from the risk assessment, not from the zero value of the data type.
- Integrated systems: Define defaults per data item in the interface specification, together with the conditions that trigger them (missing, invalid, stale or bad quality) and how long they may apply.
- Technical documentation: Operators need to know when a machine runs on defaults, for example through an HMI message, and what that means for operation.
Safe default vs. safe state
A safe default is a value or behavior substituted for missing or doubtful information. A safe state is the condition a machine is brought into to avoid a hazardous situation. A safe default can be one way of reaching or keeping that state, but it is not the state itself.