How do you calculate the load on a CT?

[bdn2004]

Attached is a portion of a schematic for an existing 4160V motor. We are changing out the 4160V gear. The new gear will come with an electronic relay to provide the 50/51 functions and it has the ammeter built in. (Schweitzer SEL710). Presently this circuit also feeds a remote Power Factor Meter and remote ammeter as shown at the top of the sketch. We are going to re-feed this the same way.

How can I assure myself that the correct CT is being ordered? What is the calculation?

 

[davidbeach]

Look for any of the following technical papers on the SEL website.

Selecting CTs to Optimize Relay Performance

Standard CT Accuracy Ratings in Metal-Clad Switchgear

The Impact Of High Fault Current and CT Rating Limits on Overcurrent Protection

 

[bdn2004]

Thanks for the answer but is there a short version of this?
Do you go through this, or can you just visually inspect based on your experience and see that it is ok?

We hired an engineering firm to specify the medium voltage MCC.
And I'm wondering if the CT's they specified are correct. My real question is becoming...if you didn't have the power study information could you assume that since the old system used a certain CT, if you re-used the same one spec, would that be ok?

Obviously the goal is to avoid saturation. Actually what happens if the CT saturates? It gives false readings?

Reading through the first document, I'm seeing where it says I must know a few things:

a. CT burden: how would I know that for these existing meters?
b. Maximum fault current (can get from power study)
c. System X/R (can get from power study)
d. CT ohms/turn resistance values
e. Wire resistance in ohms per 1000'

Equation (5)

To avoid saturation I have to be 20>or equal to = Absolute value
[1+ X/R] * (Fault current in per unit of CT rating) * (CT burden in per unit of standard burden)