How to Accurately Select the Insulation Level of a Neutral Grounding Resistor?

Writer： admin Time：2026-09-10 14:32:29 Browse：23℃

NGR Insulation Levels: Standards, Selection and Testing

Neutral grounding resistors (NGRs) are widely used in transformer and generator grounding systems to limit ground-fault current and reduce damage to windings, cores, cables, and other electrical equipment.

The insulation level of an NGR is an important part of its electrical design. It determines the required withstand voltage, insulation coordination, electrical clearance, creepage distance, and dielectric test requirements.

For international projects, NGR insulation should not be selected simply according to the system line voltage. The NGR rated voltage, applicable standards, system configuration, altitude, environmental conditions, and project requirements must all be considered.

1. What Is the Insulation Level of an NGR?

The insulation level of an NGR defines the voltage stresses that its insulation system is designed to withstand without electrical breakdown.

Typical insulation-related requirements include:

• Power-frequency withstand voltage

• Lightning impulse withstand voltage

• Insulation resistance

• Electrical clearance

• Creepage distance

• Insulation coordination

• Environmental and altitude requirements

For an NGR directly connected between a transformer or generator neutral point and earth, the voltage across the resistor during a ground fault is primarily related to the system phase-to-neutral voltage.

Therefore, the NGR insulation rating should not automatically be equated with the system line-to-line voltage.

2. Which Standards Apply to NGR Insulation?

The following international standards may be relevant to NGR insulation design and testing:

Standard	Main Application
IEC 60076-25:2023	Dry-type neutral grounding resistors
IEC 60071-1:2019	Insulation coordination
IEC 60071-2	Application guidance for insulation coordination
IEC 60664-1	Insulation coordination for low-voltage equipment
IEC 60076-3	Transformer insulation and dielectric tests
IEEE C57.32 / IEEE C57.32a	Neutral grounding devices and related requirements where specified

For dry-type NGRs, IEC 60076-25:2023 is the most directly applicable IEC product standard.

The purchaser's specification should also be reviewed because additional or project-specific tests may be required.

3. How Is the NGR Rated Voltage Calculated?

For an NGR directly connected between the system neutral and earth, IEC 60076-25:2023 uses the phase-to-neutral voltage as the basis for determining the NGR rated voltage.

The basic relationship is:Ur = U / √3

Where:

• Ur = NGR rated voltage

• U = system nominal line-to-line voltage

For example, for a 13.8 kV system:

Ur = 13.8 kV / √3

Ur = 7.97 kV

The calculated NGR rated voltage is then used to select the applicable standard insulation level.

IEC 60076-25:2023 specifies that the selected Um should be equal to or higher than the calculated NGR rated voltage:

Um ≥ Ur

Therefore, for a 13.8 kV system:

Ur = 7.97 kV

The next applicable standard value is:

Um = 12 kV

Accordingly, the corresponding insulation values are:

• Power-frequency withstand voltage: 28 kV RMS

• Lightning impulse withstand voltage: 75 kV peak

• Minimum air clearance: 120 mm

This is an important distinction when specifying NGR insulation for medium-voltage systems.

4. NGR Insulation Level Selection Table

The following table applies the IEC 60076-25:2023 selection principle:

Ur = U / √3

and then selects the next standard Um satisfying:

Um ≥ Ur

For systems below 1,000 V, IEC 60076-25:2023 refers to IEC 60664-1 for the applicable insulation coordination approach. Therefore, LV systems should not automatically be assigned the medium-voltage insulation values from the IEC 60076-25 table.

System Voltage U	NGR Rated Voltage Ur	Selected Um	Power-Frequency Withstand Ud	Lightning Impulse Withstand Up	Minimum Air Clearance
0.380 kV	0.219 kV	LV	See IEC 60664-1	See IEC 60664-1	Project dependent
0.400 kV	0.231 kV	LV	See IEC 60664-1	See IEC 60664-1	Project dependent
0.415 kV	0.240 kV	LV	See IEC 60664-1	See IEC 60664-1	Project dependent
0.480 kV	0.277 kV	LV	See IEC 60664-1	See IEC 60664-1	Project dependent
0.600 kV	0.346 kV	LV	See IEC 60664-1	See IEC 60664-1	Project dependent
0.690 kV	0.398 kV	LV	See IEC 60664-1	See IEC 60664-1	Project dependent
1.200 kV	0.693 kV	1.0 kV	2.2 kV RMS	12 kV peak	14 mm
2.400 kV	1.386 kV	3.6 kV	10 kV RMS	40 kV peak	60 mm
3.300 kV	1.905 kV	3.6 kV	10 kV RMS	40 kV peak	60 mm
3.600 kV	2.078 kV	3.6 kV	10 kV RMS	40 kV peak	60 mm
4.160 kV	2.402 kV	3.6 kV	10 kV RMS	40 kV peak	60 mm
6.300 kV	3.637 kV	7.2 kV	20 kV RMS	60 kV peak	90 mm
6.600 kV	3.811 kV	7.2 kV	20 kV RMS	60 kV peak	90 mm
7.200 kV	4.157 kV	7.2 kV	20 kV RMS	60 kV peak	90 mm
10.000 kV	5.774 kV	7.2 kV	20 kV RMS	60 kV peak	90 mm
11.000 kV	6.351 kV	7.2 kV	20 kV RMS	60 kV peak	90 mm
13.800 kV	7.967 kV	12 kV	28 kV RMS	75 kV peak	120 mm
14.400 kV	8.314 kV	12 kV	28 kV RMS	75 kV peak	120 mm
14.760 kV	8.521 kV	12 kV	28 kV RMS	75 kV peak	120 mm
15.000 kV	8.660 kV	12 kV	28 kV RMS	75 kV peak	120 mm
20.000 kV	11.547 kV	12 kV	28 kV RMS	75 kV peak	120 mm
22.000 kV	12.702 kV	17.5 kV	38 kV RMS	95 kV peak	160 mm
27.600 kV	15.935 kV	17.5 kV	38 kV RMS	95 kV peak	160 mm
34.500 kV	19.919 kV	24 kV	50 kV RMS	125 kV peak	220 mm
44.000 kV	25.403 kV	36 kV	70 kV RMS	170 kV peak	320 mm
66.000 kV	38.105 kV	52 kV	95 kV RMS	250 kV peak	480 mm

These values provide a useful reference for NGR insulation selection. The final design should always be verified against the applicable standard and project specification.

5. Standard NGR Insulation Levels

IEC 60076-25:2023 provides standardized insulation levels for NGRs.The principal values include:

Um	Power-Frequency Withstand Ud	Lightning Impulse Withstand Up	Minimum Air Clearance
1.0 kV	2.2 kV RMS	12 kV peak	14 mm
3.6 kV	10 kV RMS	40 kV peak	60 mm
7.2 kV	20 kV RMS	60 kV peak	90 mm
12 kV	28 kV RMS	75 kV peak	120 mm
17.5 kV	38 kV RMS	95 kV peak	160 mm
24 kV	50 kV RMS	125 kV peak	220 mm
36 kV	70 kV RMS	170 kV peak	320 mm
52 kV	95 kV RMS	250 kV peak	480 mm
72.5 kV	140 kV RMS	325 kV peak	630 mm
100 kV	185 kV RMS	450 kV peak	900 mm
123 kV	230 kV RMS	550 kV peak	1100 mm
145 kV	275 kV RMS	650 kV peak	1300 mm
170 kV	325 kV RMS	750 kV peak	1500 mm
245 kV	460 kV RMS	1050 kV peak	2100 mm

One important point is that Um in IEC 60076-25:2023 has an NGR-specific definition. It should not automatically be interpreted in exactly the same way as Um in other high-voltage equipment standards.

6. Example: 13.8 kV, 8 Ω, 1000 A NGR

Consider a neutral grounding resistor with the following specification:

Parameter	Value
System voltage	13.8 kV
NGR current	1,000 A
NGR resistance	8 Ω
Fault duration	15 s

First calculate the NGR rated voltage:

Ur = U / √3

Ur = 13.8 kV / √3

Ur = 7.97 kV

The selected standard insulation level must satisfy:

Um ≥ Ur

Therefore:

Um = 12 kV

The corresponding IEC 60076-25:2023 insulation values are:

Parameter	Value
Selected Um	12 kV
Power-frequency withstand Ud	12 kV RMS
Lightning impulse withstand Up	75 kV peak
Minimum air clearance	120 mm

The required resistance can be calculated from the phase-to-neutral voltage and the specified fault current:

R = Ur / I

Therefore:

R = 7.97 kV / 1,000 A

R ≈ 7.97 Ω

If the specified NGR resistance is 8 Ω, the actual fault current is approximately:

I = Ur / R

I = 7.97 kV / 8 Ω

I ≈ 996 A

This is consistent with a nominal 1,000 A NGR, subject to the specified resistance tolerance and actual system conditions.

7. Insulation Resistance Test

Insulation resistance measurement is part of the routine test program for NGRs covered by IEC 60076-25:2023.

The test verifies the insulation condition between electrically live components and grounded components.

Typical measurement points may include:

• NGR terminals to enclosure

• Live resistor elements to earth

• Insulated supports to grounded components

• Neutral terminals to the enclosure

The actual test voltage and acceptance criterion should be specified according to the applicable standard and project requirements.

It is therefore not technically appropriate to state that every NGR must use a 2,500 V insulation resistance tester and achieve a universal minimum value such as 100 MΩ.

The correct acceptance criterion depends on the equipment design, applicable standard, manufacturer specification, and purchaser requirements.

8. Power-Frequency Withstand Voltage Test

The power-frequency withstand test verifies that the NGR insulation can withstand the specified AC test voltage without breakdown or flashover.

For example, if:

Um = 12 kV

the corresponding power-frequency withstand voltage is:

Ud = 28 kV RMS

The test is normally applied between the appropriate insulated electrical parts and earth or enclosure according to the standard test arrangement.

The test evaluates the integrity of:

• Insulated neutral terminals

• Terminal bushings

• Insulated supports

• Internal insulation

• Barriers

• Electrical connections

• Insulation between energized components and the enclosure

The exact test duration and acceptance requirements should follow the applicable standard.

9. Lightning Impulse Withstand Test

Lightning impulse testing verifies the ability of the NGR insulation system to withstand transient overvoltages.

For:

Um = 12 kV

the corresponding lightning impulse withstand level is:

Up = 75 kV peak

However, lightning impulse testing should not be described as a mandatory routine test for every NGR.

Under IEC 60076-25:2023, lightning impulse testing is classified as a special test.

It may be required when specified by:

• The purchaser

• The project technical specification

• The insulation coordination study

• The system voltage level

• The installation environment

• The applicable utility standard

10. Electrical Clearance and Creepage Distance

Electrical clearance is the shortest distance through air between conductive parts at different electrical potentials.

Creepage distance is the shortest distance along the surface of an insulating material.

Both parameters are important in NGR design.

For example, when:

Um = 12 kV

IEC 60076-25:2023 specifies a minimum air clearance of:

120 mm

However, the final design may require a larger clearance depending on:

• Installation altitude

• Pollution level

• Indoor or outdoor installation

• Humidity

• Insulating material

• Surface contamination

• Transient overvoltage

• Project requirements

Therefore, the standard minimum clearance should not automatically be treated as the final physical dimension of every NGR enclosure.

11. Altitude and Environmental Conditions

Altitude affects the dielectric strength of air.

As altitude increases, the insulation strength of air decreases. Consequently, equipment installed at high altitude may require increased electrical clearance or other insulation coordination measures.

For installations above 1,000 m, the insulation requirements should be evaluated using the applicable guidance in IEC 60071-2.

An NGR installed at a high-altitude power plant may therefore require:

• Increased air clearance

• Increased creepage distance

• Higher insulation level

• Modified enclosure construction

• Additional environmental protection

The installation altitude should therefore be clearly stated in the NGR technical specification.

12. NGR Insulation Tests and Test Classification

IEC 60076-25:2023 separates NGR tests into routine tests, type tests, and special tests.

Test	Classification
Visual and dimensional inspection	Routine Test
Rated resistance measurement	Routine Test
Power-frequency withstand voltage test	Routine Test
Insulation resistance measurement	Routine Test
Temperature-rise test	Type Test
Inductance measurement	Special Test
Lightning impulse test	Special Test
Mechanical design verification	Special Test
Degree-of-protection test	Special Test

This classification is important when preparing a factory test plan.

For example, lightning impulse testing and IP degree-of-protection testing should not automatically be presented as mandatory type tests for every NGR.

They are special tests under IEC 60076-25:2023 and should be included when required by the project or purchaser.

13. Temperature Rise and NGR Insulation Design

The insulation system of an NGR is closely related to its thermal design.

During a ground fault, the resistor elements can reach a high temperature depending on the fault current and duration.

IEC 60076-25:2023 specifies maximum temperature-rise values for resistive elements according to the operating duty:

NGR Duty	Maximum Temperature Rise of Resistive Elements
Rated time ≤ 60 s	760 K
Extended time < 10 min	610 K
Continuous duty	385 K

Temperature rise and absolute temperature are different concepts.

For example:

ΔT = 760 K

means a temperature rise of 760 K above the reference temperature. It does not mean that the resistor element is operating at exactly 760 °C.

The actual element temperature depends on the initial temperature and operating conditions.

NGR thermal design should consider:

• Resistor element material

• Resistance value

• Fault current

• Fault duration

• Thermal capacity

• Ventilation

• Enclosure construction

• Insulating materials

• Terminal temperature

• Adjacent component temperature

Electrical insulation and thermal performance should therefore be evaluated together.

14. What About 380 V to 690 V NGRs?

Low-voltage NGRs are used in applications such as:

• Generator auxiliary systems

• Industrial power systems

• Mining power systems

• Motor systems

• Plant auxiliary transformers

• Low-voltage generator systems

Common system voltages include:

System Voltage	Equivalent in kV
380 V	0.380 kV
400 V	0.400 kV
415 V	0.415 kV
480 V	0.480 kV
600 V	0.600 kV
690 V	0.690 kV

These systems are below 1,000 V.

Therefore, the insulation coordination approach should consider IEC 60664-1 and the actual low-voltage equipment design rather than directly applying the medium-voltage insulation table from IEC 60076-25:2023.

For example, for a 400 V system:

Ur = 400 V / √3

Ur ≈ 231 V

This calculation does not mean that the NGR should automatically be assigned the 1.0 kV insulation level from the IEC 60076-25 table.

The final insulation design depends on factors such as:

• Overvoltage category

• Pollution degree

• Clearance

• Creepage distance

• Insulation material

• Working voltage

• Transient overvoltage

• Equipment construction

• Applicable project requirements

15. Practical NGR Insulation Selection Procedure

A practical engineering procedure can be summarized as follows.

Step 1: Determine the system voltage

Identify the nominal line-to-line system voltage.

Example:

U = 13.8 kV

Step 2: Calculate the NGR rated voltage

For a directly connected neutral grounding resistor:

Ur = U / √3

Therefore:

Ur = 13.8 / √3 = 7.97 kV

Step 3: Identify the applicable standard

For a dry-type NGR, check IEC 60076-25:2023.

For systems below 1,000 V, also consider IEC 60664-1 and the applicable low-voltage requirements.

Step 4: Select the standard Um

Select the next standard value satisfying:

Um ≥ Ur

For a 13.8 kV system:

Ur = 7.97 kV

Therefore:

Um = 12 kV

Step 5: Determine the withstand levels

For Um = 12 kV:

Ud = 28 kV RMS

Up = 75 kV peak

Step 6: Check electrical clearance

For Um = 12 kV:

Minimum air clearance = 120 mm

Then determine whether additional clearance or creepage distance is required.

Step 7: Review environmental conditions

Check:

• Installation altitude

• Indoor or outdoor installation

• Pollution level

• Ambient temperature

• Humidity

• Seismic requirements where applicable

Step 8: Define the factory test program

Separate the tests into:

• Routine Tests

• Type Tests

• Special Tests

This approach provides a clearer and more defensible technical specification for an NGR.

16. Common Mistakes in NGR Insulation Selection

Mistake 1: Using the system line voltage directly

An incorrect approach would be:

13.8 kV system → 13.8 kV NGR rated voltage

For a directly connected NGR, the initial calculation is:

Ur = 13.8 / √3 = 7.97 kV

The appropriate standard Um is then selected based on:

Um ≥ Ur

which gives:

Um = 12 kV

Mistake 2: Treating lightning impulse testing as a routine test

Lightning impulse testing is a special test under IEC 60076-25:2023.

It should be specified when required by the project or insulation coordination requirements.

Mistake 3: Applying one insulation resistance limit to every NGR

A universal requirement such as:

Insulation resistance ≥ 100 MΩ

should not be presented as an IEC 60076-25 requirement for every NGR.

The test voltage and acceptance criterion should be established according to the applicable standard and project specification.

Mistake 4: Ignoring altitude

Air insulation strength decreases with increasing altitude.

High-altitude installations may therefore require additional insulation coordination measures.

Mistake 5: Confusing resistance tolerance with temperature-related resistance change

The rated resistance tolerance and the resistance change caused by heating are different technical requirements.

For example, an NGR may have a specified rated resistance tolerance:

R ± 10%

while its resistance variation during the temperature-rise test is evaluated separately.

These two requirements should not be treated as the same parameter.

17. Conclusion

NGR insulation selection should be based on the actual neutral-to-earth voltage, the applicable product standard, insulation coordination, and installation conditions.

For a directly connected NGR covered by IEC 60076-25:2023, the basic relationship is:

Ur = U / √3

The calculated Ur is then used to select the next applicable standard value satisfying:

Um ≥ Ur

For example, for a 13.8 kV system:

Ur = 13.8 / √3 = 7.97 kV

Therefore:

Um = 12 kV

The corresponding insulation levels are:

Ud = 28 kV RMS

Up = 75 kV peak

Minimum air clearance = 120 mm

For low-voltage systems such as 380 V, 400 V, 415 V, 480 V, 600 V, and 690 V, the insulation coordination approach should consider IEC 60664-1 and the actual equipment design rather than directly applying the medium-voltage NGR insulation table.

A properly designed NGR should integrate electrical, thermal, mechanical, environmental, and insulation requirements. Selecting the insulation level from the system voltage alone is not sufficient for a reliable international NGR specification.