The Four Tiers

Industry practice converges on a four-tier pyramid, in which each tier is more frequent and further from harm than the one above it.

Tier	What it counts
Tier 1	Loss of primary containment of greater consequence — above defined quantity thresholds, or causing injury, fire or evacuation
Tier 2	Loss of primary containment of lesser consequence — smaller releases above a lower threshold
Tier 3	Challenges to the safety system: demands on protective devices, excursions outside safe operating limits, primary containment inspection failures
Tier 4	Management system performance: overdue safety-critical maintenance, training completion, procedure currency, audit findings, MOC compliance

Reading the pyramid

Tiers 1 and 2 are lagging: they count failures that have already occurred. Tiers 3 and 4 are leading: they describe conditions that make tier 1 and 2 events more likely. An organisation with low tier 1 counts and deteriorating tier 3 and 4 measures is not performing well — it is accumulating risk that has not yet been realised.

Reporting tier 1 alone gives senior management a number that is almost always zero, which tells them nothing until the year it is not.