Understanding the Time Current Method of Measuring VI Contact Wear

Vacuum technology is renowned for delivering exceptionally low arc energy during switching events. As a result, true contact wear in vacuum interrupters is extremely small. They are typically less than one millimetre over the entire operating life, even when operating at full short circuit ratings. However, this minimal physical wear presents a challenge: traditional measurement techniques do not reliably show the actual condition of the contacts.

Why Length Based Wear Measurement Is Unreliable

Conventional approaches often rely on mechanically measuring the physical length of a vacuum interrupter to infer contact erosion. But as industry research shows, this method is inaccurate and potentially misleading. After current interruption, the contact surfaces develop microscopic “hills and valleys” as molten metal briefly forms and re solidifies within the vacuum chamber. These tiny structural variations can cause the total measured length of the interrupter to increase or decrease unpredictably, providing no consistent correlation to actual wear.

Furthermore, the behaviour of the arc itself varies slightly with each operation due to natural material and thermal dynamics. These non systematic variations mean that length based measurements cannot serve as a dependable indicator of the interrupter’s long term condition. This position is supported by independent technical analysis from industry experts, who have concluded that length measurement may lead to wrong indications and cannot be considered a sufficient way to judge contact wear.

The Time Current Method: A More Accurate Indicator of Wear

To address these limitations, NOJA Power utilises the Time Current Method, a condition based approach that monitors the cumulative time current integral of all switching operations throughout the VI’s service life.

Because contact wear correlates directly to the energy of the arc, and arc energy is proportional to current magnitude and duration, this method provides a precise, physics based measurement of actual wear.

This approach offers several advantages:

• Accurate lifetime tracking — The time current integral reflects the real stress placed on the contacts.
• Automatic monitoring — Modern relays equipped with breaker condition monitoring record cumulative switching energy without manual intervention.
• Predictive maintenance — By basing assessments on actual operating duty rather than mechanical approximation, asset managers can plan maintenance with greater confidence.

Technical authorities recognise this as the most reliable method for monitoring VI contact wear and recommend it as the preferred diagnostic technique.

Supporting Field Maintenance

While the Time Current Method delivers the most accurate picture of contact wear, NOJA Power also recommends conventional contact resistance tests and AC withstand tests as part of routine field maintenance. These tests validate the ongoing integrity of the interrupter assembly and ensure safe, reliable performance across the equipment’s service life.

Commitment to Accurate Condition Monitoring

NOJA Power Group Managing Director, Neil O’Sullivan, commented:

“Our customers depend on the reliability of their medium voltage assets. By adopting the Time Current Method, we ensure they benefit from the most accurate and technically reliable measure of vacuum interrupter health. This enables smarter maintenance, greater asset confidence, and a clearer understanding of switching duty over time.”

NOJA Power remains committed to advancing the reliability, safety, and clarity of diagnostic tools used across the global power distribution industry.