The ISA/IEC 62443 standards explain that continuous operation is often required in IT systems (such as data centers and online services), but for IACS environments, scheduled operation (for example, planned maintenance windows) is typically sufficient. IACS systems may accept limited downtime for maintenance, but require high availability during production runs.
[Reference: ISA/IEC 62443-1-1:2007, Section 4.4.4 (“Availability in IACS and IT contexts”); Table 7 (Comparison of IT and IACS requirements)., , , , ]
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