why the ct has turns ratio 1 or 5 in the secondary side irrespective of the primary side???how they determine the windings in the secondary side ?

North American rating is 5A, European (IEC) rating is 1A.

These are standards for the instrumentation and protection fed from the CT.

@Mr. Sadananda, Can you be more clear in your question ?

There was no CT with a turns ratio of "1". If it exists, it is an isolation device.

canu please give me no. of windings of secondary with respect to secondary

Echoing Raymond Lee, looks like you are referring to the secondary current rating, not a "turns ratio." Although the turns ratio is tied directly to the primary/secondary current ratings. 1A and 5A are standard secondary ratings which correspond to the standard ratings of devices that use the CT current (meters and protective relays). Note that these are continuous current ratings. The meter or relay may also have a overload capacity or short-time current limit as well.
The design of a CT depends on the accuracy required, which can be different for CTs applied for metering purposes as opposed to those applied for relaying purposes. Particularly for relaying purposes, the design will need to be able to provide secondary current up to a certain level without going into saturation for a given burden in accordance with the accuracy classes stipulated in the standards.
For detailed information on applying CTs for both metering and relaying purposes, I recommend the IEEE Recommended Practice, IEEE Std 3004.1. It covers applications based on both IEEE and IEC standards.

I fully agree with Mr. Gary Fox, 5A and 1A are just the standard rating of metering, relaying and protection instruments in the market. The winding on the secondary will always be dependent on the primary on how much they need in order to bring down the current to 5A or 1A considering other factors like burdens and saturation point.

In china ,we use both 1A and 5A. just as Enrique said,it depend on accurancy and burdens.for instance ,we always use 1A at the substation up 220kV due to long distance cable.

Hi,I think it also depend on economics, 1A type ct is more expensive than 5A type ,but secondary cable for 5A is more expensive than 1A. so in substation design, we should find out the better choice on technology and economics

Hi,I think it also depends on the economics. 1A type CT is more expensive than 5A type ,but the secondary cable for 5A is more expensive than 1A.