How to Perform Vacuum Degree Test for Vacuum Interrupter in the Field

Introduction

The vacuum interrupter is the core component of a vacuum circuit breaker. Its ability to interrupt current safely depends entirely on maintaining a high vacuum level inside the sealed envelope. Over time, due to aging, mechanical stress, or manufacturing defects, the vacuum degree may deteriorate.

A vacuum degree test is therefore a critical field test used to verify whether a vacuum interrupter is still suitable for service. Unlike factory tests, field testing must be non-destructive, portable, and safe, while still providing reliable judgment.

This article explains why vacuum degree testing is needed, which test methods are suitable for field use, and how to perform them step by step.

Why Vacuum Degree Testing Is Necessary

Loss of vacuum inside a vacuum interrupter can lead to:

• Reduced dielectric strength

• Increased risk of internal flashover

• Poor breaking performance

• Accelerated contact erosion

• Catastrophic breaker failure

Because the interrupter is a sealed device, vacuum loss cannot be seen directly. Electrical testing is the only practical field method to evaluate its condition.

What “Vacuum Degree” Means in Practice

In engineering terms, vacuum degree refers to the internal pressure level inside the interrupter, typically:

• Healthy vacuum: better than 10⁻⁴ Pa

• Marginal vacuum: 10⁻² to 10⁻¹ Pa

• Failed vacuum: above 1 Pa

In the field, we do not measure pressure directly. Instead, we infer vacuum quality by electrical withstand performance.

Common Field Test Methods

Several methods exist, but only some are suitable for field use.

Methods Used in the Field

• Power frequency withstand (AC hipot) test

• DC vacuum degree test (recommended)

• Vacuum interrupter tester (specialized equipment)

Methods NOT Used in the Field

• Helium leak detection

• Residual gas analysis

• Internal pressure sensors

These are factory or laboratory methods only.

Most Recommended Field Method: DC Vacuum Degree Test

Why DC Testing Is Preferred

DC vacuum testing is widely accepted because:

• Equipment is portable

• Test is non-destructive

• Clear pass/fail criteria

• No need to disassemble interrupter

It is the most common field method for MV vacuum interrupters.

Required Test Equipment

To perform a DC vacuum degree test, you need:

• DC high-voltage vacuum interrupter tester

• Insulated test leads

• Grounding cables

• Insulating gloves and PPE

• Test record sheet

Most testers are designed specifically for 12 kV, 24 kV, and 36 kV vacuum interrupters.

Safety Precautions Before Testing

Before starting any test:

• Isolate the breaker completely from the system

• Lock out and tag out all power sources

• Discharge capacitors

• Ensure the breaker is in the open position

• Confirm grounding of non-tested phases

Safety is critical because high DC voltages are involved.

Step-by-Step DC Vacuum Degree Test Procedure

Step One: Breaker Preparation

• Remove the vacuum circuit breaker from service or rack it out

• Open the breaker mechanically

• Clean the surface of the vacuum interrupter

• Visually inspect for cracks or contamination

Do not test a physically damaged interrupter.

Step Two: Test Connection

• Connect the positive HV output to one contact terminal of the vacuum interrupter

• Connect the negative output to the other terminal

• Ground the interrupter housing and surrounding metal parts

Only one interrupter is tested at a time.

Step Three: Apply Test Voltage

Apply DC voltage slowly and smoothly, typically at:

• 12 kV interrupter → test voltage around 20–30 kV DC

• 24 kV interrupter → test voltage around 40–60 kV DC

• 36 kV interrupter → test voltage around 70–80 kV DC

Always follow manufacturer or tester recommendations.

Step Four: Hold Time Observation

• Maintain the test voltage for one minute

• Observe leakage current and stability

• Listen for abnormal noise or discharge

A healthy interrupter will show stable, very low leakage current.

Step Five: Result Judgment

• Pass: No breakdown, no sudden current rise

• Fail: Flashover, breakdown, or unstable current

If breakdown occurs, the vacuum degree is insufficient.

Interpretation of Test Results

Good Vacuum Condition

• No breakdown

• Smooth voltage rise

• Stable current

• No audible discharge

The interrupter can remain in service.

Degraded Vacuum Condition

• Partial discharge noise

• Fluctuating current

• Lower than expected withstand voltage

The interrupter should be monitored or replaced.

Failed Vacuum Condition

• Immediate breakdown

• Arc at low voltage

• Sudden current surge

The interrupter must be replaced immediately.

AC Power Frequency Test as an Alternative

Some utilities still use AC withstand testing.

Limitations of AC Testing

• Requires large test transformers

• Risk of excessive stress on interrupter

• Less sensitive to early vacuum degradation

AC testing is acceptable but less preferred than DC testing.

Specialized Vacuum Interrupter Testers

Modern testers integrate:

• Automated voltage ramp

• Leakage current measurement

• Pass/fail indication

These testers simplify field work and reduce operator error.

Test Frequency Recommendations

Typical testing intervals:

• Commissioning: mandatory

• After major fault interruption

• Periodic maintenance: every 3–5 years

• After long storage

Utilities may specify different intervals based on experience.

Common Field Testing Mistakes

• Testing with breaker closed

• Applying voltage too fast

• Inadequate grounding

• Ignoring surface contamination

• Using incorrect voltage level

Avoiding these mistakes improves test accuracy and safety.

Standards and References

Vacuum degree testing practices are referenced in:

• IEC 62271-100

• IEC 62271-1

• Utility maintenance guidelines

Exact test voltages may vary by manufacturer.

When Testing Is Not Enough

Vacuum degree testing verifies dielectric integrity, but it does not assess:

• Contact erosion

• Mechanical wear

• Operating mechanism condition

It should be combined with mechanical and timing tests.

Conclusion

Performing a vacuum degree test for a vacuum interrupter in the field is a critical maintenance and diagnostic activity that ensures safe and reliable breaker operation. Among available methods, DC vacuum testing using a dedicated interrupter tester is the most practical, accurate, and widely accepted approach.

By following correct procedures, observing safety precautions, and interpreting results properly, field engineers can confidently determine whether a vacuum interrupter remains fit for service or requires replacement—helping prevent unexpected failures and extend switchgear service life.