What happens if only tertiary explosion protection is applied?

Primary: prevention
Secondary: protection
Tertiary: follow up action
Interesting question. If the 2 (primary & secondary) do not work, the tertiary has to.
If this is the case, why not only go for the tertiary?!
What do we know about tertiary explsion protection?
This relates to stopping an explosion from spreading and limiting its effects. Here explosion protection is a combination of technical solutions and legal provisions.

Original question not yet modified: Shall we only go for tertiary Ex?
Comments welcome.
Ps.: if primary/secondary works well no need the tertiary… some say…
Keep up good work.