Lightning Impulse (LI) testing is one of the most important dielectric tests used in transformer manufacturing. It ensures a transformer can survive the types of electrical stresses created by lightning strikes on overhead power lines. Even if a substation has surge protection, a transformer may still experience fast and intense voltage surges. For this reason, every medium- and high-voltage transformer must pass a strict set of impulse tests before it is shipped to a customer.

IEC 60076-3 defines exactly how a Lightning Impulse test must be performed, what voltage levels must be used, how many shots must be applied, what conditions must be met, and how to judge if a transformer has passed or failed.

In this new article, we break down how IEC 60076 defines and performs LI tests, including:

What Is a Lightning Impulse Test?

A lightning impulse is a very fast, high-energy voltage spike that travels through a network when lightning strikes an overhead transmission line. This surge can reach the transformer terminals and place extreme stress on the winding insulation.

To simulate this real-life lightning event, transformers are tested using a standardized 1.2/50 microsecond impulse waveform:

• 1.2 μs = time to rise to peak
• 50 μs = time to fall to half of its peak value

IEC 60076-3 requires that transformers are tested with this wave to verify that the winding insulation, inter-turn insulation, phase-to-phase insulation, and insulation to the tank can all withstand lightning stresses.

In short:

The LI test proves the transformer will survive real lightning that enters the substation.

When Is the Lightning Impulse Test Required?

IEC 60076-3 categorizes transformers according to Um, the “highest voltage for equipment.”

For Um ≤ 72.5 kV

LI is a type test (performed on one design, not every unit).

For 72.5 kV < Um ≤ 170 kV

LI is a routine test, meaning every transformer must pass it.

For Um > 170 kV

LI is not used — instead, a more severe test called LIC (Chopped Wave Lightning Impulse) is required.

Purpose of the LI Test

IEC states that the LI test verifies the transformer’s ability to withstand:

• Very fast voltage rise
• High-frequency stresses
• Non-uniform voltage distribution inside windings
• Peak overvoltages that occur in milliseconds

Lightning impulses stress the transformer differently than AC tests or switching impulse tests because lightning surges are fast-front transients.

Preparing the Transformer for the LI Test

Before applying impulses, IEC 60076-3 requires:

1. The transformer must be fully assembled

All internal insulation components must be in place.

2. All non-tested terminals must be grounded

This includes:

• Other phases
• Neutral terminal
• Windings not under test

This prevents unwanted transferred overvoltages.

3. Tap changer must be on principal tap

Unless another tap position is more severe, the principal tap is used.

4. Test equipment is calibrated

The impulse generator must produce a correct 1.2/50 µs wave shape according to IEC 60060-1.

Determining the Test Voltage

The peak test voltage depends entirely on Um, the highest system voltage. For example:

Um (kV)	LI (kV)	LIC (kV)	SI (kV)	Applied Voltage / LTAC (kV)
<1.1	–	–	–	3
3.6	20	22	–	10
	40	44	–	10
7.2	60	66	–	20
	75ᵃ	83ᵃ	–	20
12	75	83	–	28
	95	105	–	28
	110ᵃ	121ᵃ	–	34ᵃ
17.5	95	105	–	38
	125ᵃ	138ᵃ	–	38
24	125	138	–	50
	145	160	–	50
	150ᵃ	165ᵃ	–	50
36	170	187	–	70
	200ᵃ	220ᵃ	–	70
52	250	275	–	95
72.5	325	358	–	140
	350ᵃ	385ᵃ	–	140
100	450	495	375ᵃ	185
123	550	605	460ᵃ	230
145	550	605	460ᵃ	230
	650	715	540ᵃ	275
170	650	715	540ᵃ	275
	750	825	620ᵃ	325
245	850	935	700ᵃ	360
	950	1 045	750ᵃ	395
	1 050	1 155	850ᵃ	460
300	950	1 045	750	395
362	1 050	1 155	850	460
420	1 175	1 290	950	510
	1 300	1 430	1 050	570
	1 425	1 570	1 175ᵃ	630
550	1 300	1 430	1 050	570
	1 425	1 570	1 175	630
	1 550	1 705	1 300ᵃ	680
	1 675ᵃ	1 845ᵃ	1 390ᵃ	–
800	1 800	1 980	1 425	–
	1 950	2 145	1 550	–
	2 050ᵃ	2 255ᵃ	1 700ᵃ	–
	2 100	2 310	1 675ᵃ	–
1 100	1 950	2 145	1 425	–
	2 250	2 475	1 800	–
1 200	2 250	2 475	1 800	–

To use IEC 60076-3’s insulation levels, you begin by identifying the Um (Highest Voltage for Equipment) of each transformer winding. Once Um is known, you go to the corresponding row in the Table, which lists all required dielectric test voltages: LI (Lightning Impulse), LIC (Chopped Wave), SI (Switching Impulse), and AC (Applied Voltage). The test voltage must not be lower than the minimum values defined by IEC. All values in the same row must be used together because IEC bases insulation coordination on complete test sets, not individual numbers. The correct row in the Table tells you exactly how high each test voltage must be during factory testing.

Which of these tests you must apply depends on the winding’s voltage class. For Um ≤ 72.5 kV, LI is used as a type test; for 72.5–170 kV, LI becomes a routine test; and for Um > 170 kV, LIC replaces LI and SI becomes mandatory. Each winding on a multi-voltage transformer uses its own row in the Table—for example, a 145 kV winding uses higher impulse values than a 36 kV winding. In this way, the Table provides a complete and standardized method to assign the correct dielectric strength to every winding, ensuring the transformer is tested safely and consistently according to international practice.

The Standard LI Test Wave Shape

IEC defines strict waveform requirements:

• Front time (T1): 1.2 µs ± tolerance
• Time to half-value (T2): 50 µs ± tolerance
• No significant distortion
• No oscillation that masks true stresses

The impulse generator is adjusted until recordings match this specification.

Waveform Types Used in IEC Lightning Impulse Testing

To better understand how different impulse types stress the transformer, the diagram below compares the full wave, chopped wave, and front-of-wave waveforms.

The full wave (curve a) is the standard 1.2/50 µs lightning impulse used for most routine LI tests.

The chopped wave (curve b) is interrupted very early—usually within a few microseconds—creating a very steep rise. This greatly increases stress on the first turns of the winding. IEC requires this waveform for Um above 170 kV.

The front-of-wave (curve c) has an even faster rise time and simulates extremely steep lightning surges caused by very close or direct strikes.

The dashed volt-time insulation curve shows how insulation strength decreases as the impulse becomes faster. This explains why chopped and steep-front impulses are more severe than the standard LI wave.

The Lightning Impulse Test Sequence*

IEC requires impulses to be applied in a specific order to evaluate the transformer correctly:

1. One reduced-voltage impulse (about 50%–60% of LI level)

Used to check polarity, waveform, connections, and ensure no abnormal oscillations occur.

2. At least five full-voltage impulses

Applied at 100% of the required LI test voltage.
The transformer must withstand all impulses with no internal breakdown.

3. Optional overvoltage shots (typically 140–150% of LI)

Used only when waveform adjustment is difficult or when required by contract.
IEC allows these only under agreement.

The sequence ensures the insulation is tested under increasing levels of stress in a controlled and repeatable manner.

Testing Transformers With and Without Non-linear Elements

IEC distinguishes two cases:

A) Transformers WITHOUT Non-linear Elements

Most transformers fall into this category.
The LI test is straightforward:

• Standard connections
• Waveform measured at the line terminal
• No special arrangements

B) Transformers WITH Non-linear Elements

Some transformers include:

• Surge arresters inside the tank
• Protective spark gaps
• Specialty rectifier or converter equipment

IEC requires special testing precautions:

• Additional inductance may be added
• Elements must NOT trigger during test
• Energy must be controlled to avoid misrepresentation

Without these precautions, the waveform could be misleading.

What Is Considered a Passing LI Test? (Acceptance Criteria)

A transformer passes the test when:

No internal flashover occurs

This would show up as:

• Sudden collapse of voltage
• Distorted waveform
• Unnatural oscillation
• Drop in peak value

No external flashover

Unless specifically allowed by agreement.

The voltage wave shape remains stable

Recordings should be clean and consistent.

No abnormal sound or mechanical vibration

Internal discharge can often be heard.

IEC is very clear:

Any internal breakdown means the test FAILED.

How Failures Are Handled

If the transformer fails the LI test:

1. The cause must be investigated

The manufacturer must determine whether:

• A test setup issue caused the failure
• The transformer insulation failed internally

2. Re-tests are allowed only under strict rules

If an external flashover occurred (e.g., on a bushing),
a single repeat shot is allowed.

3. Internal insulation failure requires repair

After repair:

• The transformer must repeat the entire LI sequence

Lightning Impulse Test on the Neutral Terminal (LIN)

IEC also includes a special case:

If the neutral is not solidly earthed, or if non-uniform insulation is used, a Lightning Impulse Neutral (LIN) test may be required. It uses:

• The same waveform as LI
• Lower voltage level (specified by the purchaser)

This verifies that the neutral insulation can handle transferred overvoltages.

Simultaneous Lightning Impulses on Multiple Terminals (LIMT)

IEC 60076-3 includes these tests for special transformer types:

• Phase-shifting transformers
• Transformers with series windings
• Units used in HVDC or converter stations

Purpose:

• Simulate simultaneous surges on two or more terminals

This test is used only when requested.

Waveform Recording Requirements

Every impulse must be recorded using approved instruments. IEC requires that recordings capture:

• Peak voltage
• Front time
• Time to half-value
• Polarity
• Oscillations
• Any distortion

These recordings become part of the transformer’s quality documentation.

Why LI Testing Is So Important

Lightning can cause:

• Internal winding failures
• Inter-turn insulation breakdown
• Turn-to-tank faults
• Bushing failures
• Phase-to-phase flashovers

The LI test ensures the transformer has:

• Sufficient insulation strength
• Proper impulse voltage distribution
• Safe dielectric margins

This test is one of the most reliable indicators of a transformer’s long-term health.

A Simple Summary of the LI Test Procedure

Here’s the entire IEC process in easy steps:

1. Select test voltage from Table 2
2. Ground all non-tested terminals
3. Set tap changer to required position
4. Adjust impulse generator to create 1.2/50 μs wave
5. Apply 1 reduced-voltage shot
6. Apply at least 5 full-voltage shots
7. Record all waveforms
8. Verify no flashovers or waveform collapse
9. If failure occurs, follow Clause 15 procedures

Conclusion

Lightning Impulse testing is one of the most technologically important steps in transformer manufacturing. It verifies that a transformer will not fail under lightning stress—a condition that can occur multiple times over a transformer’s 30–50 year life expectancy.

IEC 60076-3 provides a precise, standardized way to:

• Apply the impulse
• Select the correct voltage
• Measure the response
• Interpret waveform changes
• Decide pass or fail

This ensures consistency worldwide and protects utilities, industries, and customers from unexpected transformer failures.