Three phase CT questions

[Bigrhamr]

I'm looking at some installation standards from my power co for 400 amp three phase. It shows their primary voltage lines going to their transformer which is on the ground, then to My CT enclosure which contains their current transformers. I understand the transformer reduces voltage down to my 120/208 but what are the current transformers in the enclosure? I'll have an onsite meet with power co engineer soon and it would nice If I could sound like I had some faint clue about this stuff. If my explanation isn't clear the diagram which is a 5 meg file is here:

http://www.norlight.org/servicestandards.pdf

scroll down to S-10

Looking at it closer it appears the current transformers are for over 400 amp so maybe I won't be dealing with that part since I'm doing 400.

Thanks for any help.

 

[Stuart in MN]

The current transformers inside the CT cabinet get connected to the utility meter. They're little loop things, each of the power conductors passes through one. The utility provides them for installation by the electrician. Usually, CT enclosures are required for 400 amp services and larger. By the way, the CT cabinet needs to be one that's approved by your local utility; they may be able to provide you with a list of companies that build them.

Edit: I see where they say on their diagram that particular install is for "over 400 amp"; I said earlier that CT enclosures are usually required at 400 amps, but every electric utility has their own rules and regulations. The local engineer will have to tell you exactly what they want.

 

[walrus]

They use the CT to measure your incoming power. Its a coil of wire with a know amount of wraps. your use of current will induce a current on the coil which can be measured and by knowing the turns will able them to know exactly how many Khw you are using.

 

[nadogail]

The first two responders got it right

 

[Bigrhamr]

Thanks guys,
thats one question out of the way, I'm sure I'll have a bunch more before this is all finished.

 

[porcupine73]

If by chance the incoming leads pass through the CT's but you aren't going to use the CT's, make sure the CT's are shorted.

 

[RPH]

Don't forget there is a burden resistor in parallel with the windings.