Dual ratio Current Transformer thermal rating 2

[electrical429]

When you have a dual ratio CT, say 1000/500/1, and the thermal rating of CT is 100%. Does this mean that on the 1000/1 ratio you will 1000 A thermal rating and on the 500/1 ratio you will have 500 A thermal rating. Or is thermal rating independent of the ratio you choose and it will be 1000 A in this case for both ratios?

 

[cranky108]

Sort of depends on the TRF of the CT.

 

[RRaghunath]

I guess a 1000/500/1A CT is a bar primary CT. So, the taps are on secondary and thermal rating is for the secondary winding.

 

[scottf]

You apply the rating factor to both ratios....really meaning that you apply it to the rated secondary current.

So, for your example of a 500/1000:1A ratio with an RF=1.0, that RF of 1.0 applies when operated at the 1000:1 ratio and when operated at the 500:1A ratio.

For MV and HV CTs where the primary conductor tends to be the limiting winding for RF, it's common to see the RF expressed at each ratio...for example 500/1000:1A, RF 2.0/1.0. That would mean that you can operate at 1000A continuous current on either ratio.