Vacuum interrupter testing - AC v DC

2 November 2007

Electrical Tester

The vacuum interrupters in vacuum breakers do not last forever. Leakage starts after years or decades and the interrupters fill with air, making the breaker unreliable. In most cases the leakage process is rapid once it has started. In addition to leakage, dirt on the poles and on the exterior surface of the interrupter can make it unsafe during operation. Also, the mechanics of the breaker can become misaligned so that the distance between the poles no longer is adequate.

Both AC and DC vacuum interrupter testers can be used to test the integrity of vacuum breakers, including the Programma Vidar Vacuum Interrupter Tester which uses high DC voltage.

Why?
Because the electrical resistance of the vacuum in a breaker is identical in behaviour for AC and DC. The main difference in using DC vs AC is that AC also is sensitive to the capacitance of the breaker. The DC (resistive) current component is 100 to 1000 times lower in magnitude than the AC (capacitive) current component, depending on the individual interrupter capacitance and, therefore, difficult to distinguish when testing using AC. As a result, AC requires heavier equipment for testing compared to the lighter 6 kg Vidar unit.

When the Vidar Vacuum Interrupter Tester was introduced, a rigorous study was undertaken at a major vacuum interrupter manufacturer. The study concluded that the Vidar unit was suitable for testing all potential sources of dielectric malfunction of a vacuum breaker in the field and that using AC did not add any additional information.

At the national Institute of Radiation Protection in Stockholm, Sweden, a Vidar unit, together with a Siemens N 677 vacuum breaker were analysed for x-ray radiation. The outcome proved safe for anyone operating with either the Vidar or the vacuum breaker.

Finally, both the DC and the AC test methods are detailed in standards. The DC method is described in the ANSI/IEEE 37.20.2-1987 standard.

Vidar Vacuum Interrupter Tester