An insulation-coordination report is valuable only if procurement retains both conclusions and assumptions. Keep the selected equipment highest voltage, transformer terminal withstand levels, arrester data/location, grounding states, line/cable/GIS models, studied overvoltage types, key worst cases and report revision. Also define what future changes—such as arrester relocation, cable replacement or grounding modification—require the study to be revisited.
Keep a terminal-by-terminal insulation schedule
Identify each winding line terminal and neutral with its equipment-voltage basis and required dielectric duties. Distinguish lightning impulse, switching impulse where applicable, power-frequency tests and any installation-specific assessment. State whether voltage values are peak or RMS and the relevant reference to earth or another phase. The purchase order and manufacturer's approved drawings should point to the same schedule, so a general transformer insulation label cannot obscure a terminal-specific requirement.
Retain the evidence behind the selected margin
Save the source and grounding cases, line/cable/GIS model data, arrester characteristics and physical locations, and the limiting calculated waveforms or voltage-duration envelopes. Identify measured parameters, supplier calculations and assumptions separately. The highest peak is not necessarily the governing case for every device: duration, front steepness and arrester energy can govern different checks. Preserve the report and model revisions so the selection can be reproduced rather than relying on a summary slide stating that coordination passed.
Close the report with a controlled-change boundary
Define which changes require review: transformer replacement, altered grounding, arrester relocation, cable-route changes, GIS modification or a new source configuration. Retain unresolved assumptions and the party responsible for confirming them before equipment release. After construction, reconcile the approved arrangement with the as-built installation. The resulting dossier supports procurement and later maintenance; it is not a blanket guarantee that any future network configuration remains within the original insulation assessment.
Technical reference framework
[1] IEC 60071-1:2019 — Insulation co-ordination, Definitions, principles and rules.
[2] IEC 60071-2:2023 — Insulation co-ordination, Application guidelines.
[3] IEC 60076-3:2013 + AMD1:2018 — Insulation levels, dielectric tests and external clearances.
[4] IEEE C57.98-2026 — Transformer impulse tests.