Five types
| Type | How it works | Example |
|---|---|---|
| Passive hardware | Works because it exists; it never acts | Bund, wall, seal, separation distance |
| Continuous hardware | Runs without interruption | Ventilation, continuous cooling |
| Active hardware | Detects and intervenes by itself | Sensor plus automatic interlock |
| Active human | A person detects, decides and acts | Patrol check, manual shutdown |
| Active hardware plus human | Hardware detects, a person acts | Alarm sounds, operator closes the valve |
The order is a preference order
Moving down the list, the barrier depends on more and more conditions: power, calibration, somebody being there, and that person deciding correctly. Where two options address the same risk, the one higher on the list is preferred.
Three steps in any human barrier
Every barrier involving an active human must have all three steps in place:
- Detect — can the person become aware? Is the alarm audible, is the indicator visible?
- Decide — is the action defined in advance, and is the person authorised?
- Act — is it physically possible, is there time, is the tool within reach?
An alarm that sounds while nobody has written down what to do is a missing middle step — the most common gap found on site.
The mixing rule
Three barriers of the same type on one pathway are weaker than two of different types: the same type shares the same weakness. A power failure takes three active hardware barriers down at once and does not touch a passive bund.