CT Sizing

[BECREZ]

We have several 6.9kV - 480V (delta/wye) transformers. These transformer neutral bushing CTs are sized at 2000/5 to which we tie to a 51G relay. However, I need to understand the basis for sizing such a CT as the potential phase to ground current (in the event of a ground fault) would far exceed the rating of the CT or am I missing something?

 

[dpc]

The same is true of the phase CTs as well. The CT 2000/5 rating is a continuous rating. The short time rating will be much higher. The faults will generally be short - a second or less.

If you size the CT for the maximum fault current, it will not be sensitive to low level currents.

 

[racobb]

Should be large transformers to have 2000/5 Ct's on the neutral. What size are they?

Alan

 

[Kiribanda]

BECREZ,

To determine whether the NCT is oversized one should know the maximum allowable ground fault current at 480 V level.

What is your calculated or allowable ground fault current at 480 V level?