Selection of CT, PT?

Is it for metering or protection ?

Ummmm... "ratio" means you divide one side by the other. So if you have an 11kV line, and you want to transform that down to a safe level to work with in your metering by using a PT, say, 110V, then you divide the primary (11,000) by the desired secondary (110) and come up with a 100:1 ratio. But fiorst you have to decide what your secondary potential needs to be. Here in the US we use 120V for secondaries of PTs, but with you having 11kV I know you are not in the US.

The same basic principal hold true for CTs as well, only with current. The one difference is however, you want your normal full load operating current to fall as close to the middle of a metering range as possible. That's because at the top end of a CTs operating ratio, the accuracy drops off rapidly, therefore you want to be operating in the 'sweet spot: of the middle of the range. So for instance if you are metering the primary side of that transformer, the full load current will be 200,000/11,000= 18A So you would want your CT ratio to be roughly for double that so that at full load, the 18A is in the middle of the range. So a 36:1 ratio would be right, except that CTs are usually based on a 5A secondary, so you would express it as a 180:5 ratio. Now again, CTs usually come in standard sizes, so the closest standard size will be 150:5 or 200:5, I would go with the 200:5 ratio.

Additionally, you want to make sure your CT is designed for 11 kV or higher. Some CT's, even though the hole will fit over the wire, do not have the insulation resistance to handle high voltages (or higher medium voltages).

dear please give me the available ratio of CT in market

Dear Sir,

In India, generally PT ratio is specified by 11000/sqrt.3 : 110/sqrt.3, could you please through some light on why primary and secondary voltage level are divided by sqrt. 3?

are we making both primary and secondary winding of PT for Phase-Neutral measurement, if so, how we are using same PT with help of 7 position selector switch to measure P-P and P-N voltage?

the ct shall have a ratio of 10/5 A.its very simple divide the the capacity/demand by sq.root of 3 and system voltage.

to select a ct u need more information:-

1)class of accuracy

2)type of meter

3)burden to be connected to the ct.

4)fault level etc.,