When a high-voltage cable is subjected to an impulse test, the first question is usually straightforward: Can the cable withstand the specified voltage?
For a cable test engineer, however, the actual test is rarely that simple.
A test voltage may reach several hundred kilovolts or even higher, but the peak voltage alone does not determine how the cable insulation is stressed. The shape of the impulse waveform, the rise time, the decay time, the test circuit, and even the cable termination can all influence the result.
This becomes particularly important when testing high-voltage and extra-high-voltage cables, where the cable itself has a relatively large capacitance and the complete test circuit behaves very differently from a simple laboratory load.
That is why, in impulse testing, generating the required voltage is only part of the job. Generating the right waveform at the test object is what really matters.
1. A Cable Does Not Experience “Voltage” Alone
The standard lightning impulse waveform commonly used for high-voltage insulation testing is 1.2/50 μs.
The first number describes the approximate front time of the impulse, while the second represents the time for the voltage to decay to half of its peak value. IEC 60060-1 provides the framework for impulse voltage tests and waveform evaluation.
It is tempting to think of a 1.2/50 μs impulse simply as a voltage pulse with a certain peak value.
From the cable engineer's point of view, this is not enough.
During the first microseconds of the impulse, the voltage distribution along the cable insulation is changing rapidly. The electrical stress is affected by the geometry of the conductor, insulation system, screen, termination and connected test circuit.
A cable that withstands a slowly rising voltage does not necessarily behave in exactly the same way when the same voltage is applied with a very steep front.
This is one reason impulse testing is used in addition to power-frequency withstand testing.
The purpose is not simply to apply a higher voltage. It is to reproduce a transient electrical stress that is much closer to what insulation may experience when exposed to lightning-related overvoltages or other fast transient events.
2. Why the 1.2/50 μs Waveform Is Important
For cable manufacturers, the waveform is part of the test requirement rather than a decorative parameter.
A typical lightning impulse has two visually obvious characteristics:
- a relatively steep rising front;
- a much slower decay toward the tail.
The steep front determines how quickly the electric field is established across the insulation system. The tail determines how long the insulation remains under significant electrical stress.
This distinction matters because cable insulation is a distributed system.
Unlike a small electrical component, a long high-voltage cable contains substantial capacitance and inductance. The impulse therefore does not simply appear simultaneously at every point along the cable.
The test circuit and the cable form an electrical network.
As a result, the waveform measured at the impulse generator is not necessarily identical to the waveform actually applied to the cable termination.
For a test engineer, this is where many apparently simple impulse tests become more complicated.
3. Cable Capacitance Changes the Test Circuit
A high-voltage cable can present a significant capacitive load to the impulse generator.
When the impulse generator discharges into the test object, the cable capacitance becomes part of the discharge circuit. The generator, connecting conductors, measuring divider, termination and cable under test all influence the resulting waveform.
This means that changing the cable length can change the electrical behavior of the test circuit.
For example, a test setup developed around a relatively short cable sample may produce an acceptable waveform under one configuration. When the cable length is increased, the capacitance of the test object increases, and the impulse circuit may require adjustment.
The engineer may then observe:
- a change in front time;
- increased oscillation;
- overshoot;
- a longer or shorter tail;
- a reduction in peak voltage;
- waveform distortion around the peak.
This is why impulse generators are normally designed with adjustable circuit parameters rather than being treated simply as high-voltage power supplies.
In practical impulse testing systems, the charging voltage, generator stages and circuit components are selected according to the required test voltage, energy and waveform. Marx-type impulse generators are widely used for this purpose.
4. The Cable Termination Is Part of the Test
There is another point that is sometimes underestimated: the termination is not just a mechanical connection.
At high voltage and during a fast transient, the termination becomes an important part of the electrical system.
Poor field grading, unsuitable geometry, excessive stray capacitance or an inappropriate connection arrangement can produce local electric-field concentrations.
In a production or laboratory environment, this can create a difficult situation.
The test engineer may see an abnormal waveform or even a breakdown, but the actual problem may not be the cable insulation itself.
It could originate from:
- the cable termination;
- the connection between the impulse generator and the test object;
- an unsuitable grounding arrangement;
- excessive stray inductance;
- insufficient clearance;
- measuring-system response;
- or an interaction between the cable capacitance and the impulse circuit.
For this reason, a meaningful cable impulse test requires the entire test arrangement to be considered, rather than looking only at the nameplate voltage of the impulse generator.
5. Why Oscillation and Overshoot Should Not Be Ignored
One of the most useful things an experienced test engineer looks at is the actual recorded waveform.
A nominally correct test voltage does not automatically mean that the impulse is satisfactory.
Unwanted oscillations or overshoot may appear when the generator, cable and measurement circuit interact with each other.
The problem is closely related to the high-frequency nature of the impulse.
Every physical conductor has some inductance, while every part of the test arrangement has some capacitance. During a fast voltage transition, these parasitic parameters become significant.
The result can be an RLC-type transient response.
This is why the design of an impulse test system cannot be separated completely from:
- generator circuit parameters;
- connection geometry;
- cable capacitance;
- measuring divider characteristics;
- grounding arrangement;
- and the physical layout of the test bay.
Modern impulse test systems therefore use dedicated impulse voltage dividers and measurement systems to verify the actual waveform. IEC 60060-2 specifically addresses measurement requirements for high-voltage impulse measurements, including the performance of impulse voltage measuring systems.
6. The Measurement System Can Affect the Result
Another practical mistake is to regard the voltage divider as just a measuring accessory.
At impulse voltage levels, measurement is part of the test system.
The divider must reproduce the fast-changing impulse accurately enough for the engineer to evaluate the waveform. Its response characteristics, connection arrangement and physical location can affect the recorded result.
For this reason, high-voltage impulse systems normally use dedicated impulse dividers rather than conventional voltage measurement equipment. Manufacturers of complete impulse test systems specify impulse dividers and measurement systems as integral parts of the test arrangement.
For a cable manufacturer, this distinction is important.
If the recorded waveform is inaccurate, the engineer may end up troubleshooting the cable when the real problem is in the measurement chain.
In other words:
A reliable impulse test is not simply a high-voltage source plus a cable. It is a complete measurement system.
7. What Should a Cable Test Engineer Look at?
When commissioning or evaluating an impulse test system for high-voltage cables, the peak voltage is only one of several parameters worth checking.
A practical evaluation should normally consider at least:
Test voltage
Can the generator reach the required test voltage with sufficient margin under the actual cable load?
Waveform
Can the system generate and maintain the specified impulse waveform at the test object?
Front time
Is the rising portion of the impulse within the required tolerance?
Tail time
Does the decay portion meet the applicable test requirement?
Oscillation and overshoot
Is the waveform sufficiently clean for reliable evaluation?
Energy capability
Can the generator deliver the required impulse energy into the cable's capacitive load?
Measurement accuracy
Can the divider and measurement system reproduce the actual impulse waveform accurately?
Test repeatability
Can the same test conditions be reproduced from one test to another?
The last point is particularly important for cable manufacturers.
A test system used for product development may tolerate a certain amount of engineering adjustment. A production qualification system cannot rely on trial and error every time a new cable specification is tested.
8. Why Cable Testing Is Different From Testing a Small Insulation Sample
This is probably the most important difference from a general discussion of impulse testing.
When testing a small insulation specimen, the test object may have relatively low capacitance and the physical connection is comparatively simple.
A long high-voltage cable is different.
The cable itself becomes a significant electrical component of the impulse circuit.
Its length, conductor configuration, insulation system and termination influence the test behavior. The longer the cable and the higher the voltage level, the more important the interaction between the test object and the generator becomes.
This is also why cable impulse testing equipment cannot always be selected simply by looking at the maximum voltage rating.
Two systems with the same nominal voltage may behave differently when connected to a large-capacitance cable.
For the cable manufacturer, the better question is therefore not:
“How many kilovolts can the generator produce?”
but:
“Can the complete system produce the required impulse at the actual cable test load, with a controlled and measurable waveform?”
That is a much more useful engineering question.
9. Selecting an Impulse Voltage Generator for Cable Testing
For high-voltage cable testing, the impulse generator should be considered as part of the complete test system.
The selection normally starts with the required test voltage and applicable standard, but several other parameters must then be considered.
The cable capacitance and expected test configuration should be evaluated early in the design stage.
The generator may need sufficient charging voltage and energy capability, while the impulse circuit must also provide the required waveform characteristics.
The measuring divider, test leads, grounding arrangement and termination should be designed together with the generator rather than added later.
This approach becomes increasingly important as cable voltage levels and sample lengths increase.
For this reason, manufacturers of impulse test systems commonly configure the generator according to the intended application, including the required charging voltage, number of stages and impulse energy.
10. A Practical Engineering View
For cable manufacturers, impulse testing is ultimately about confidence.
The objective is not to produce an impressive voltage number on the equipment specification sheet.
The objective is to create a repeatable electrical stress, measure it correctly, and determine whether the cable insulation system can withstand that stress without failure.
That requires the generator, cable, termination, divider, measurement system and test environment to work as one system.
A well-designed impulse test setup should therefore answer three questions clearly:
Did we reach the required voltage?
Did we generate the correct impulse waveform at the test object?
Can we prove what actually happened from a reliable measurement record?
If all three answers are yes, the impulse test becomes much more than a high-voltage stress test. It becomes a reliable engineering tool for verifying the insulation performance of high-voltage cable products.
Technical note
Why Impulse Waveform Matters in High-Voltage Cable Testing
When a high-voltage cable is subjected to an impulse test, the first question is usually straightforward: Can the cable withstand the specified voltage?
For a cable test engineer, however, the actual test is rarely that simple.
A test voltage may reach several hundred kilovolts or even higher, but the peak voltage alone does not determine how the cable insulation is stressed. The shape of the impulse waveform, the rise time, the decay time, the test circuit, and even the cable termination can all influence the result.
This becomes particularly important when testing high-voltage and extra-high-voltage cables, where the cable itself has a relatively large capacitance and the complete test circuit behaves very differently from a simple laboratory load.
That is why, in impulse testing, generating the required voltage is only part of the job. Generating the right waveform at the test object is what really matters.
1. A Cable Does Not Experience “Voltage” Alone
The standard lightning impulse waveform commonly used for high-voltage insulation testing is 1.2/50 μs.
The first number describes the approximate front time of the impulse, while the second represents the time for the voltage to decay to half of its peak value. IEC 60060-1 provides the framework for impulse voltage tests and waveform evaluation.
It is tempting to think of a 1.2/50 μs impulse simply as a voltage pulse with a certain peak value.
From the cable engineer's point of view, this is not enough.
During the first microseconds of the impulse, the voltage distribution along the cable insulation is changing rapidly. The electrical stress is affected by the geometry of the conductor, insulation system, screen, termination and connected test circuit.
A cable that withstands a slowly rising voltage does not necessarily behave in exactly the same way when the same voltage is applied with a very steep front.
This is one reason impulse testing is used in addition to power-frequency withstand testing.
The purpose is not simply to apply a higher voltage. It is to reproduce a transient electrical stress that is much closer to what insulation may experience when exposed to lightning-related overvoltages or other fast transient events.
2. Why the 1.2/50 μs Waveform Is Important
For cable manufacturers, the waveform is part of the test requirement rather than a decorative parameter.
A typical lightning impulse has two visually obvious characteristics:
- a relatively steep rising front;
- a much slower decay toward the tail.
The steep front determines how quickly the electric field is established across the insulation system. The tail determines how long the insulation remains under significant electrical stress.
This distinction matters because cable insulation is a distributed system.
Unlike a small electrical component, a long high-voltage cable contains substantial capacitance and inductance. The impulse therefore does not simply appear simultaneously at every point along the cable.
The test circuit and the cable form an electrical network.
As a result, the waveform measured at the impulse generator is not necessarily identical to the waveform actually applied to the cable termination.
For a test engineer, this is where many apparently simple impulse tests become more complicated.
3. Cable Capacitance Changes the Test Circuit
A high-voltage cable can present a significant capacitive load to the impulse generator.
When the impulse generator discharges into the test object, the cable capacitance becomes part of the discharge circuit. The generator, connecting conductors, measuring divider, termination and cable under test all influence the resulting waveform.
This means that changing the cable length can change the electrical behavior of the test circuit.
For example, a test setup developed around a relatively short cable sample may produce an acceptable waveform under one configuration. When the cable length is increased, the capacitance of the test object increases, and the impulse circuit may require adjustment.
The engineer may then observe:
- a change in front time;
- increased oscillation;
- overshoot;
- a longer or shorter tail;
- a reduction in peak voltage;
- waveform distortion around the peak.
This is why impulse generators are normally designed with adjustable circuit parameters rather than being treated simply as high-voltage power supplies.
In practical impulse testing systems, the charging voltage, generator stages and circuit components are selected according to the required test voltage, energy and waveform. Marx-type impulse generators are widely used for this purpose.
4. The Cable Termination Is Part of the Test
There is another point that is sometimes underestimated: the termination is not just a mechanical connection.
At high voltage and during a fast transient, the termination becomes an important part of the electrical system.
Poor field grading, unsuitable geometry, excessive stray capacitance or an inappropriate connection arrangement can produce local electric-field concentrations.
In a production or laboratory environment, this can create a difficult situation.
The test engineer may see an abnormal waveform or even a breakdown, but the actual problem may not be the cable insulation itself.
It could originate from:
- the cable termination;
- the connection between the impulse generator and the test object;
- an unsuitable grounding arrangement;
- excessive stray inductance;
- insufficient clearance;
- measuring-system response;
- or an interaction between the cable capacitance and the impulse circuit.
For this reason, a meaningful cable impulse test requires the entire test arrangement to be considered, rather than looking only at the nameplate voltage of the impulse generator.
5. Why Oscillation and Overshoot Should Not Be Ignored
One of the most useful things an experienced test engineer looks at is the actual recorded waveform.
A nominally correct test voltage does not automatically mean that the impulse is satisfactory.
Unwanted oscillations or overshoot may appear when the generator, cable and measurement circuit interact with each other.
The problem is closely related to the high-frequency nature of the impulse.
Every physical conductor has some inductance, while every part of the test arrangement has some capacitance. During a fast voltage transition, these parasitic parameters become significant.
The result can be an RLC-type transient response.
This is why the design of an impulse test system cannot be separated completely from:
- generator circuit parameters;
- connection geometry;
- cable capacitance;
- measuring divider characteristics;
- grounding arrangement;
- and the physical layout of the test bay.
Modern impulse test systems therefore use dedicated impulse voltage dividers and measurement systems to verify the actual waveform. IEC 60060-2 specifically addresses measurement requirements for high-voltage impulse measurements, including the performance of impulse voltage measuring systems.
6. The Measurement System Can Affect the Result
Another practical mistake is to regard the voltage divider as just a measuring accessory.
At impulse voltage levels, measurement is part of the test system.
The divider must reproduce the fast-changing impulse accurately enough for the engineer to evaluate the waveform. Its response characteristics, connection arrangement and physical location can affect the recorded result.
For this reason, high-voltage impulse systems normally use dedicated impulse dividers rather than conventional voltage measurement equipment. Manufacturers of complete impulse test systems specify impulse dividers and measurement systems as integral parts of the test arrangement.
For a cable manufacturer, this distinction is important.
If the recorded waveform is inaccurate, the engineer may end up troubleshooting the cable when the real problem is in the measurement chain.
In other words:
A reliable impulse test is not simply a high-voltage source plus a cable. It is a complete measurement system.
7. What Should a Cable Test Engineer Look at?
When commissioning or evaluating an impulse test system for high-voltage cables, the peak voltage is only one of several parameters worth checking.
A practical evaluation should normally consider at least:
Test voltage
Can the generator reach the required test voltage with sufficient margin under the actual cable load?
Waveform
Can the system generate and maintain the specified impulse waveform at the test object?
Front time
Is the rising portion of the impulse within the required tolerance?
Tail time
Does the decay portion meet the applicable test requirement?
Oscillation and overshoot
Is the waveform sufficiently clean for reliable evaluation?
Energy capability
Can the generator deliver the required impulse energy into the cable's capacitive load?
Measurement accuracy
Can the divider and measurement system reproduce the actual impulse waveform accurately?
Test repeatability
Can the same test conditions be reproduced from one test to another?
The last point is particularly important for cable manufacturers.
A test system used for product development may tolerate a certain amount of engineering adjustment. A production qualification system cannot rely on trial and error every time a new cable specification is tested.
8. Why Cable Testing Is Different From Testing a Small Insulation Sample
This is probably the most important difference from a general discussion of impulse testing.
When testing a small insulation specimen, the test object may have relatively low capacitance and the physical connection is comparatively simple.
A long high-voltage cable is different.
The cable itself becomes a significant electrical component of the impulse circuit.
Its length, conductor configuration, insulation system and termination influence the test behavior. The longer the cable and the higher the voltage level, the more important the interaction between the test object and the generator becomes.
This is also why cable impulse testing equipment cannot always be selected simply by looking at the maximum voltage rating.
Two systems with the same nominal voltage may behave differently when connected to a large-capacitance cable.
For the cable manufacturer, the better question is therefore not:
“How many kilovolts can the generator produce?”
but:
“Can the complete system produce the required impulse at the actual cable test load, with a controlled and measurable waveform?”
That is a much more useful engineering question.
9. Selecting an Impulse Voltage Generator for Cable Testing
For high-voltage cable testing, the impulse generator should be considered as part of the complete test system.
The selection normally starts with the required test voltage and applicable standard, but several other parameters must then be considered.
The cable capacitance and expected test configuration should be evaluated early in the design stage.
The generator may need sufficient charging voltage and energy capability, while the impulse circuit must also provide the required waveform characteristics.
The measuring divider, test leads, grounding arrangement and termination should be designed together with the generator rather than added later.
This approach becomes increasingly important as cable voltage levels and sample lengths increase.
For this reason, manufacturers of impulse test systems commonly configure the generator according to the intended application, including the required charging voltage, number of stages and impulse energy.
10. A Practical Engineering View
For cable manufacturers, impulse testing is ultimately about confidence.
The objective is not to produce an impressive voltage number on the equipment specification sheet.
The objective is to create a repeatable electrical stress, measure it correctly, and determine whether the cable insulation system can withstand that stress without failure.
That requires the generator, cable, termination, divider, measurement system and test environment to work as one system.
A well-designed impulse test setup should therefore answer three questions clearly:
Did we reach the required voltage?
Did we generate the correct impulse waveform at the test object?
Can we prove what actually happened from a reliable measurement record?
If all three answers are yes, the impulse test becomes much more than a high-voltage stress test. It becomes a reliable engineering tool for verifying the insulation performance of high-voltage cable products.
Technical note
For high-voltage cable manufacturers, the relevant impulse-test requirements depend on the cable type, voltage class and applicable product standard. Standards commonly referenced in high-voltage cable testing include IEC 60840 and IEC 62067, while IEC 60060 provides the general framework for high-voltage test techniques and impulse measurement.
For high-voltage cable manufacturers, the relevant impulse-test requirements depend on the cable type, voltage class and applicable product standard. Standards commonly referenced in high-voltage cable testing include IEC 60840 and IEC 62067, while IEC 60060 provides the general framework for high-voltage test techniques and impulse measurement.
Related Equipment
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required test voltage, energy and test-object characteristics. please click :Impulse Voltage Generator