The relative number of failures referred to the related population. It comprises three phases (Figure A 26):
Early failures:
At the start of the operation of an installation (running-in period). Causes are design errors, installation errors, material errors.
Random failures:
On installations that have been run-in and that are correctly maintained the failures reach a minimum.
Wear-and-tear failures:
Occur after a specific operating time and as a rule are fatigue fractures or failures due to wear.
Figure A 26: Failure rate as a function of the operating time