The first bell above the bell with the corona could not support even 5 kV and cracked when the test transformer reached that voltage.
Detection of a shorted bell in a porcelain insulator
by using the DayCorŽ camera
".. In a 230 kV substation a buzzing/arcing sound was heard from one of the insulators during wet and foggy conditions. By using the DayCorŽ camera the bell that caused the noise was easily identified. The insulator was removed and the bells were tested for their ability to support voltage. Each bell was energized using a test transformer. The first bell above the bell with the corona could not support even 5 kV and cracked when the test transformer reached that voltage.
A similar observation of corona on the first sound bell after a string of shorted bells was reported by EPRI.
Donald L. Ruff
Senior Structural Engineer
BPA - Bonneville Power Administration
USA
Porcelain insulator, 12 bells 40 years old.
Note: the UV signal is from the space between the cap and the bell of the second Bell.
The bad insulator: It is punctured and arcs at about 20 kV. Notice that cracks radiate from the cap at about the 2:30 and 8:30 positions
