current transformer sizing

[jeff s]

Hello, im interested in your thoughts or "rule of thumb" on current transformer sizing. Ive had been assigned a project with ct sizes specified they seem oversize to me?

 

[Gil47]

All depends on the burden.
Is the burden likely to increase.
Some are nearly impossible to replace at a later date.
Don't make them to small.

Now a bit of information from you will get improvements on my answers

 

[jeff s]

these are going on machine main breakers, one application is a 700 amp breaker and they spec 600 amp ct's but each leg only see's around 150 amps on average. does this decrease resolution?

 

[leitmotif]

Jeff
IF my installation I would size CTs for the main breaker ie 700 amp.
Generally the secondary current is 5 amp at full load ie 700, therefore you have a 700:5 CT.
To make the math a little easier assume 750:5 CT -- at 150 amp it would "deliver" 1 amp.

Resolution is a little hard to understand and commonly confused with accuracy. Compare to a target ie bullseye. Resolution means you hit the same spot every time EVEN if it is the wrong spot. Accuracy means you hit the bullseye - when you aim for it of course. So with good accuracy and good resolution in theory (disregard wind and barrell heating etc etc) all bullets will go thru the same hole in the target.

SO with good resolution and rated at say 1% the 750:5 at 750 amp
output is +/- 0.01 amp.
With good accuracy then output is 1.00 +/- 0.01 amp.
If accuracy is off by 10% high output is 1.1 +/- 0.01 amp.

I think I have it right -- anyone care to correct me if wrong?
Dan Bentler

 

[Andybr]

CT's are truly analogue devices so resolution is not an issue (though it may be with the connected instrument/relay). The actual ratio of the transformer is not the only thing to consider however. If the CT is being used for protection rather than metering it must be able to measure fault currents considerably in excess of the rated current of the circuit. The "class" of CT used is therefore extremely important and this should match the application. CT's designed for protection will normally have a "P" added on to the class designation and an indication of the maximum fault current that the transformer will measure without saturating. 20P would indicate that the CT will handle currents up to 20 times the nominal range. I am sure that any protection engineers reading this will be able to add much more detail but the bottom line is that the CT must match the job you are asking it to do.

Andybr

 

[Mickey]

Jeff
IF my installation I would size CTs for the main breaker ie 700 amp.
Generally the secondary current is 5 amp at full load ie 700, therefore you have a 700:5 CT.
To make the math a little easier assume 750:5 CT -- at 150 amp it would "deliver" 1 amp.

Resolution is a little hard to understand and commonly confused with accuracy. Compare to a target ie bullseye. Resolution means you hit the same spot every time EVEN if it is the wrong spot. Accuracy means you hit the bullseye - when you aim for it of course. So with good accuracy and good resolution in theory (disregard wind and barrell heating etc etc) all bullets will go thru the same hole in the target.

SO with good resolution and rated at say 1% the 750:5 at 750 amp
output is +/- 0.01 amp.
With good accuracy then output is 1.00 +/- 0.01 amp.
If accuracy is off by 10% high output is 1.1 +/- 0.01 amp.

I think I have it right -- anyone care to correct me if wrong?
Dan Bentler

I disagree with your definition of resolution. What you call resolution is actually "precision or repeatability". ( see
link below for a better explanation.

http://www-group.slac.stanford.edu/met/GeneralMet/StatDefs.html

Resolution is a measure of detail or sharpness ( how much does the input have to change to change the output)

 

[Lancie1]

....rated at say 1% the 750:5 at 750 amp
output is [1]+/- 0.01 amp.
With good accuracy then output is 1.00 +/- 0.01 amp.
If accuracy is off by 10% high output is 1.1 +/- 0.01 amp.

Dan, I am sure you know this: that if the current through the CT is 750 Amperes, then the CT output current should be 5 Amperes +/- error, not 1 Amp.

 

[leitmotif]

Dan, I am sure you know this: that if the current through the CT is 750 Amperes, then the CT output current should be 5 Amperes +/- error, not 1 Amp.

Well I told you to find my mistakes -- thank you.
change current to 150 amps please. I really need to be more careful writing these at midnight after work.

Dan

 

[leitmotif]

I disagree with your definition of resolution. What you call resolution is actually "precision or repeatability". ( see
link below for a better explanation.

http://www-group.slac.stanford.edu/met/GeneralMet/StatDefs.html

Resolution is a measure of detail or sharpness ( how much does the input have to change to change the output)

See previous comment aboiut writing after work at midnite
I said they are easy to confuse and went ahead and didt it anyway.
I agree with you.
Dan