Three tests
- Effective — the barrier must be able to stop that pathway on its own. "It helps a bit" is not a barrier.
- Independent — the same failure, the same person or the same supply must not take two barriers down together.
- Auditable — its working must be demonstrable through a record, a test or an observation.
Boxes that typically fail
| Written as | Fails on |
|---|---|
| "Work carefully" | Auditability — it cannot be measured |
| "There is a procedure" | Effectiveness — a document alone stops no pathway |
| Two interlocks fed from one panel | Independence — one supply takes both down |
| "An experienced operator" | Auditability — person dependent, no record |
| Annual training | Effectiveness — it raises awareness, it does not stop the pathway |
Weak controls need not be deleted
Controls that fail the test are often useful; they are simply not barriers. Leaving them with a separate mark is more honest than deleting them: "we do something here, but there is nothing to rely on".
What the test really produces
In practice the test reduces the barrier count; a third of the boxes on a crowded diagram fall away. That is a gain, not a loss: what remains is what can actually be relied on, and maintenance, testing and budget go there.