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Bilge Rat
motors and controls.........
9,394 Posts
There are basically 3 types of transformers in medium-voltage switchgear.

1) CTs. Current transformers. These units always have a ratio, some are multi-ratio. It'll be something like 600:5. They cannot be used for anything except metering and instrumentation. If they become disconnected with current flowing, VERY high voltage will appear across the disconnected leads. And the CT will be immediately destroyed. The voltage across a CT is very low, usually less than one volt.

2) PTs. These are potential transformers. They have an exact voltage ratio. Their purpose is to reduce the voltage on the bus to something more useful. They too have a ratio. Something like 14,400:120. They supply metering and instrumentation, and their current is really low. Like milliamps. They will supply a load, but they will not be as accurate as is usually needed.

3) Power transformers. These are the more familiar types. They feed actual loads, and are usually connected to some sort of distribution system, like a panel.

Usually, the CTs are buried in the buswork somewhere and you can't see them. The PTs and power transformers are often contained in some sort of draw-out assembly or a tip out sort of thing. They usually have medium-voltage fuses, most of these are contained in the draw-out, and often the fuse clips are mounted directly to the transformers.
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