Lightning impulse and switching impulse represent different overvoltage waveforms and system mechanisms. Their relevance depends on the highest voltage for equipment and the applicable standard, and the required withstand values are not interchangeable. A specification should identify the exact terminal, test type and standard basis rather than using “impulse level” as one undifferentiated number.
Similar peak voltage does not imply similar insulation stress
Lightning and switching impulses have different time characteristics and can distribute voltage differently across a winding and external insulation. A peak value without its waveform and test definition is therefore incomplete. The applicability of particular duties depends on the equipment voltage class and governing standard. Avoid selecting a switching-impulse requirement merely by copying the lightning-impulse number or assuming that passing one test automatically demonstrates the other duty.
Assign each requirement to a terminal and connection
A multiwinding transformer can have different insulation schedules on different windings and at their neutrals. Identify the tested terminal, the treatment of other terminals and any transferred-stress concern relevant to the design. Procurement should retain the full dielectric schedule with the manufacturer's confirmed test arrangement. A general statement such as “impulse tested” does not reveal which winding, terminal or waveform was covered by the supplied evidence.
Keep service-waveform assessment alongside standard testing
Standard dielectric tests establish specified withstand evidence under defined conditions. The installation may also introduce unusual cable reflections, vacuum-breaker transients or GIS-related fast fronts that need an interface study. Those studies should inform the agreed design and protective measures without claiming that every service waveform is identical to a standard impulse. Retain both the test requirements and the installation-specific coordination conclusions in the purchase record.
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.