Definition(s)
Down Time
Time interval during which an item is in a down state. NOTE The down time includes all the delays between the item failure and the restoration of its service. Down time can be either planned or unplanned.
Source: API STD 2000, Venting Atmospheric and Low-pressure Storage Tanks, Sixth Edition, November 2009. Global Standards
Downtime
Time interval during which an item is in a non-working state. NOTE The downtime includes all the delays between the item failure and the restoration of its service. Downtime can be either planned or unplanned.
Source: ISO 20815:2008, Petroleum, petrochemical and natural gas industries – Production assurance and reliability management. Global Standards