Material selection for GIS spring fingers

Three materials are used today for the spring fingers (canted coil spring contacts) fitted in GIS switchgear: chromium zirconium copper (ASTM C18150), beryllium cobalt copper (ASTM C17500) and beryllium copper (ASTM C17200).

Conductivity versus strength

Of the three materials, conductivity is highest for chromium zirconium copper and lowest for beryllium copper, while strength follows the reverse order: chromium zirconium copper is the weakest and beryllium copper the strongest. In general, the higher the conductivity, the lower the strength. That raises a question: can a spring finger with high conductivity but low strength and small contact pressure still carry more current than a beryllium cobalt copper finger? The literature reports that the allowable current density of beryllium cobalt copper is twice that of beryllium copper.

Temperature-rise tests

In a series of GIS temperature-rise tests, spring fingers of beryllium cobalt copper and chromium zirconium copper of the same size were installed in the same position and compared over a test current range of 2000 A to 4400 A. Under the same current, the contact-surface temperature rise of the chromium zirconium copper finger was 4 °C to 10 °C lower than that of the beryllium cobalt copper finger, and the difference became more obvious as the test current increased.

Conclusion

The tests confirmed that although the chromium zirconium copper spring has low contact strength and small contact pressure, its conductivity is still significantly better than that of the lower-conductivity beryllium cobalt copper material.