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High Voltage Oil Circuit Breaker – A look inside the tank

Image of Inside Oil Circuit Breaker Tank

Have you ever wondered what the inside of an old high voltage oil circuit breaker looks like? Maybe you have driven by a utility substation and saw three large round tanks hanging on a frame with high voltage wires coming into tall insulators on the top of each tank. Well, these actually are oil filled circuit breakers. The oil is a good electrical insulator, so it is used to fill the tank and protect all the energized electrical equipment inside the tank. In years past little rectangular tank oil circuit breakers where even used to protect 240 volt circuits. That was 75 years ago. But there are 5000 volt, 15,000, 25,000, 35,000 and 69,000 volt and much higher voltage oil circuit breakers still in service. Some Utilities have high voltage units still operating. In recent decades and years, high voltage oil circuit breakers have been replaced with vacuum circuit breakers and SF6 filled circuit breakers. But here is a picture of the inside of a 35,000 volt oil circuit breaker. Not a lot there, mostly oil, which has been removed in this image. The whole tank is full of oil when the unit is in service. Oil circuit breakers, or OCBs, as they are known in the trade, are just big tanks full of oil with a bit of mechanical equipment inside to interrupt the current during an overload or electrical fault. The oil does deteriorate over the years and it gets dark if the OCB has to frequently interrupt a lot of current. With new oil, you can see right through it, to the bottom of the tank. But, on some old units, the oil can get so dark you can’t see anything in the oil. Part of the maintenance services on OCBs is replacement of the oil when it deteriorates. It all sounds very crude for equipment operating at thousands of volts and hundreds of amps, but OCBs have been used reliably for many decades. General Electric oil circuit breakers or Westinghouse oil circuit breakers, they all work about the same. MIDWEST thought you might like to actually see the inside of a high voltage oil circuit breaker.

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