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More than lead and elbows
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Discussion Starter · #1 ·
First post, long time lurker, a wealth of knowledge here. Anyway, another guy at work recently finished a project where the engineers had him feed a panel with a potential transformer (could have been a CT, can't remember). The gist of it is, 15kv cable, a PT, and a 100A panel with a few single phase 15A loads. Has anyone ever heard of this? Is it legal? I've heard of the guys using springs near transmission lines on the reservations, but never seen anything like this?
 

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First post, long time lurker, a wealth of knowledge here. Anyway, another guy at work recently finished a project where the engineers had him feed a panel with a potential transformer (could have been a CT, can't remember). The gist of it is, 15kv cable, a PT, and a 100A panel with a few single phase 15A loads. Has anyone ever heard of this? Is it legal? I've heard of the guys using springs near transmission lines on the reservations, but never seen anything like this?
First time I've heard of it.:)

Welcome aboard...:thumbsup:
 

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Bilge Rat
motors and controls.........
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9,398 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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Electron Factory.Worker
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351 Posts
Do the look something like this?






PT's aren't made to power any significant load so they can easily be overloaded. As long as there is proper protection I don't see why it would be a problem. Pocos will often use then on poles to power reclosers and capacitor banks. The main thing about PTs that differentiates them from power transformers is their precise winding ratios so that they accurately reflect the primary voltage.
 

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More than lead and elbows
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Discussion Starter · #6 ·
Wow thanks guys, there is honestly so much information on this site it's baffling! I didn't see the actual job, but my understanding it was a PT, potential transformer for voltage measurement... And to be honest, I'm not sure if it was a 5kv or 15kv cable, I'll ask him again. I learned something new though, thank you guys!
 

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Most PTs I have seen are rated at a "burden" (maximum load without affecting accuracy) of between 25 and 100 watts, not VA, but watts. I have never seen one rated 500VA, but that's just me, I'm not really that deep into the utility side of things. But certainly, someone feeding a "100A panel with a few single phase 15A loads" was overstepping the boundaries of sound practice in my opinion. PTs are NOT made for that.
 
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