Why You Can't Remove Siemens GIS Bushing Cover for PD Testing

As the title suggests, when performing live partial discharge (PD) testing on Siemens GIS using the UHF method—specifically by accessing the signal through the metal flange of the bushing insulator—you must not directly remove the metal cover on the bushing insulator.

Why?

You won’t realize the danger until you try. Once removed, the GIS will leak SF₆ gas while energized! Enough talk—let’s go straight to the diagrams.

As shown in Figure 1, the small aluminum cover inside the red box is typically the one users intend to remove. Removing it allows electromagnetic waves from partial discharge to escape, enabling detection with offline PD equipment. This method is commonly used on many GIS brands. But why does removing it on Siemens equipment cause gas leakage?

Siemens bushing insulators are designed with two sealing rings. As labeled in Figure 2:

• No. 01: The first seal, located on the epoxy resin casting of the bushing insulator.

• No. 02: The second seal, located on the aluminum alloy metal flange.

The small aluminum cover you intend to remove is mounted on this metal flange. If these two seals were independent and not interconnected, removing the small cover (Figure 1) would pose no risk—no gas leakage would occur.

However, in Siemens design, there is a small notch in the lower-left area of Figure 2 that connects the gas chambers of the two sealing rings. For a clearer view, see the enlarged Figure 3.

Due to this small notch (Figure 3), the GIS gas sealing relies not only on the second seal (No. 02) on the metal flange but also on the small aluminum cover itself. Underneath that small cover is high-pressure SF₆ gas—remove it, and you’ll get a shocking surprise.

In contrast, for single-phase bushing insulators like the one shown in Figure 4, the two seals are not interconnected. The internal high-pressure SF₆ gas is primarily sealed by the first seal (No. 01) on the epoxy bushing. Therefore, removing the small aluminum cover as shown in Figure 5 is safe—no gas leakage will occur.

Conclusion:
Before removing any small cover on a bushing insulator for live (offline-type) partial discharge testing on GIS from any manufacturer, always consult the manufacturer to confirm whether the cover can be safely removed—especially for Siemens equipment, where improper removal can lead to dangerous SF₆ gas leakage under live conditions.