The center diagram shows a section of collector plate damaged by corrosion. This is caused by too much ambient air entering the precipitator box at the electrode supports, at a leaky hatch, or through the casing itself. Sharp edges at corrosion holes draw sparks from the high tension electrodes even if everything else is aligned correctly.
The diagram at the left shows patches welded over corrosion holes. They don’t help–sharp edges are still exposed to the electrodes. There is a correct way to make the repair, but it requires some thought. Each transformer typically serves a field of 80 or so rows of electrodes. A single close clearance causes the entire field to reduce power. Often it’s better to strategically remove electrodes than to accept spark-over. Many other factors contribute to close clearances. Wire electrodes can oscillate or “whip” due to inadequate wire weights or weights hung-up on the lower frame. The high tension frame may be misaligned due to careless construction or structural distortion. The diagram at the right shows a typical “post” style anti-sway insulator. This design is notorious for allowing the entire lower electrode frame to sway like a child’s swing.