SUC diagram

Graphical representation of the system under study: equipment positioned in their Purdue levels, networks, then the layout of zones and conduits. Synchronisation with ZCR 3 is automatic in both directions.

What it is really for

This is not an illustrative sketch: it is the completeness-control tool for the study.

• Every asset in the scope must appear on it as equipment.
• Any link that crosses a zone boundary must be a conduit. Cross-checking with the access points captured in ZCR 1 lets you verify none was forgotten.
• Each zone displays its aggregated maximum initial risk and a suggested SL-T.

The "◉ ZCR2 risk" display

Enable it to have each piece of equipment carry a coloured badge corresponding to the asset's maximum initial risk. A dashed circle flags an asset without any ZCR 2 row: the analysis is not complete as long as one remains.

Splitting into zones — the order that works

1. First apply the separations required by the standard (ZCR 3.3–3.6): IT/enterprise separated from OT, SIS in its own zone, wireless equipment kept apart, temporary connections (maintenance laptops, USB) kept apart, remote/external access isolated. Zone types (BPCS, SIS, Wireless, Temp, External, IT, DMZ…) exist to carry these separations.
2. Then refine with the heat map — group assets with similar requirements.

Why homogeneity matters

Two neighbouring pieces of equipment with very different initial risks should not share a zone: it would inherit the most demanding SL-T (extra cost), or the critical asset would be under-protected.