# Instrument & measurement: PT & CT - Question



## bethy (Feb 1, 2011)

If we have a 3 phase AC, unbalance system (or delta-Y system or Y-delta), how many PT &amp; CT do we need to measure the load? Should it be (3) CT &amp; (3) PT??

If there is a neutual in the system, do we need (1) more CT &amp; (1) more PT?

In the text book didn't say much about this topic. Any link or advise for Instrument &amp; Measurement section??

Thanks,

Beth


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## cableguy (Feb 1, 2011)

This boils down to Blondel's Theorum. If you google on that term, you'll find a number of articles / papers written about multi wattmeter (CT &amp; PT) methods for measuring load.

http://en.wikipedia.org/wiki/Blondel%27s_theorem


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## bethy (Feb 1, 2011)

Thanks a lot! Cableguy, by any chance do you know any link/source to find some sample for "Relays, switches &amp; PLCs". I got some from google, but not sure that is a best one.

Thanks,

Beth


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## cableguy (Feb 2, 2011)

This is a good reference:

http://www.gedigitalenergy.com/multilin/no...rtsci/index.htm

If you look around the Internet and use your Google-Fu, you can find Blackburn's Protective Relaying: Principles and Applications in PDF format. Note that some of it is incorrect (device numbers in chapter 1 are wrong in the Third Edition), but it does have examples of CT connections and calculations in there... like calculating based on delta-wye transformers with wye-delta CT connections, etc. I would not buy the book though, as it's not THAT great of a resource for the exam, unless you find it for $20 or less.

The switches / plc's you need to focus on is ladder logic. I don't know of sources (maybe a PLC vendor like Allen-Bradley or someone has something on their web site), but you need to understand how to decode ladder logic with normally open, normally closed, open/close/seal-in coils, that kind of thing. It's really not too hard, nothing terribly complicated.


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## bethy (Feb 2, 2011)

Thank you!


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