Signs

The “SF6 in oil – Bushing seal defects” does not show any characteristic signs; however, through visual inspection, it is possible to:

• assess the general condition of the accessory (corrosion) and the presence of leaks,
• check the dilation of the bellows (oil/SF6), an indicator of the amount of oil present in the insulator;
• check in particular the general conditions of the capacitive jack (if any), thanks to which it is possible to perform appropriate electrical tests (capacity, dissipation factor).

Despite the presence of oil in the casing in SF6 being more critical, is much more convenient to take an aliquot of oil (from the oil compartment of the isolator) and analyse it to assess the presence of SF6. This indicator is sufficient to declare criticality because a sealing defect between the insulator and the outer casing would produce both oil in the SF6 and SF6 in the oil at the same time.

Representative sampling of an oil-SF6 insulator is delicate and complex. It is necessary to employ appropriate operating procedures (in accordance with the specifications of the insulator manufacturer), special sampling devices and specifically-trained staff. 
The difficulty lies in the presence of a very low volume of oil in the insulator compartment (oil), so it is necessary to sample while compensating with the same quantity of oil (pretreated). The operation must be carried out with extreme care to avoid the smallest infiltration of air bubbles.