Insulation coordination for MV/HV — Um, BIL and the protective margin (IEC 60071-1)

The [creepage/clearance insulation coordination guide per IEC 60664-1](/guides/cable/kruipweg-luchtweg-isolatiecoordinatie-iec-60664-1) covers how low-voltage equipment is coordinated via physical distance tables (creepage distance, clearance), derived from the overvoltage category and pollution degree of the application. For medium-voltage and high-voltage equipment, insulation coordination works fundamentally differently: not primarily via distance tables, but via standardized withstand-voltage levels, defined in IEC 60071-1.

The three voltage levels being coordinated

Um — the highest voltage for equipment: the maximum system voltage the equipment must continuously withstand, usually somewhat higher than the nominal operating voltage of the network it is used in.
Rated short-duration power-frequency withstand voltage (Ud) — represents temporary power-frequency overvoltages, such as those occurring during an earth fault in the network.
Rated lightning impulse withstand voltage (BIL — basic insulation level, also denoted Up) — represents very fast transient overvoltages from lightning strikes or switching operations, tested with a standardized 1.2/50 µs impulse waveform.

For higher voltage levels, a separate rated switching impulse withstand voltage (Us) is sometimes added, tested with a slower 250/2500 µs waveform, for voltage levels where switching transients tend to dominate over lightning transients.

Why a standardized series instead of a per-project calculated margin

Rather than each project calculating its own insulation strength requirement from scratch, IEC 60071-1 defines standardized insulation levels — combinations of Ud and BIL — per Um voltage class. This ensures equipment interoperability between manufacturers and projects, and statistically corresponds to a defined protective margin relative to a surge arrester's protection level.

The relationship with the surge arrester

The MV surge arrester guide covers the residual voltage (Ures) that an arrester lets through during a surge. For working insulation coordination, this residual voltage must stay below the selected BIL of the protected equipment with an adequate protective margin — otherwise the arrester's protection on paper is not sufficient in practice. This margin must also account for:

the inductance of the arrester's own connecting leads — the voltage drop across those leads during the very fast current rise of a lightning surge adds to the residual voltage actually seen by the protected equipment;
the distance between the arrester and the protected equipment — traveling-wave reflections along the connection can raise the actually observed voltage at equipment further from the arrester above the arrester's own rated residual voltage.

Practical relevance

When specifying MV/HV switchgear, transformers, or cable terminations, an insulation coordination study must confirm that the equipment's selected Ud/BIL provides sufficient margin against both the actual network overvoltage stress and the protection level (including connecting-lead inductance and distance) of the applied surge arrester — not merely matching Um to the nominal system voltage.

Common mistakes

Only matching Um to the nominal system voltage, without verifying whether the BIL margin against the arrester's actual protection level is sufficient.
Ignoring the inductance of the arrester's connecting leads when calculating the effective protective margin — an overly long or winding connecting lead can make the actual residual voltage at the equipment significantly higher than the arrester's rated residual voltage.
Applying the low-voltage creepage/clearance coordination logic (IEC 60664-1) directly to MV/HV equipment, which uses a fundamentally different, withstand-voltage-test-based coordination method.
Neglecting the switching impulse withstand voltage at higher system voltage levels, where switching transients rather than lightning can be the dominant stress.