When is it Time to Replace an Old Transformer Bushing?

It is hard to decide when to throw away an old bushing and buy a new one. You want to save money, so you might keep using the old one as long as it looks okay. But waiting until it explodes is a very expensive mistake. Here are the signs that your bushing is reaching its end.

Look for many small cracks 

A few tiny scratches on the surface are normal over time. But if you see a "spider web" of many small cracks, that is a bad sign. In the industry, we call this "crazing." These cracks catch water and dirt. Once moisture gets deep into those cracks, you cannot clean it out. If the surface looks dull and rough instead of shiny, it is probably time to look for a replacement.

The oil is changing color 

If your bushing has an oil level gauge, look at the color of the oil. It should be clear or a light yellow. If it starts to look dark brown or cloudy, something is wrong inside. This usually means the internal paper insulation is burning up or breaking down. You can’t "fix" the oil inside a sealed bushing easily. When the oil goes dark, the bushing is telling you it is about to fail.

White powder or salt on the metal 

Check the parts where the porcelain meets the metal hardware. If you see white powder or green rust, it means there is corrosion. This often happens in places near the ocean or near chemical plants. Corrosion eats away at the seals. Once the seal is gone, air gets inside. If you see "pitting" (small holes) in the metal, you shouldn't trust that bushing for much longer.

The "hot spot" won't go away 

If you use an infrared camera and see that the top of the bushing is much hotter than the rest, you have a problem. Sometimes it’s just a loose bolt on the outside, which you can tighten. But if you tighten everything and it stays hot, the connection inside is failing. Continuous heat will eventually crack the insulator. It is better to replace it during a planned shutdown than to have it fail in the middle of the night.

It has been there for 20+ years 

Even if a bushing looks perfect, age matters. The glue and rubber parts inside get brittle over twenty or thirty years. Most experts agree that once a bushing hits a certain age, the risk of a sudden failure goes up fast. If your transformer is critical for your business, replacing a 25-year-old bushing is much cheaper than cleaning up the mess after a fire.