Is this right?

turn ratio = (Npri/Nsec) = (Vpri/Vsec) = (Isec/Ipri)

shouldn't Ipri in the example be Ipri = (Irelay)/(CT ratio)??

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CTs take a relativey high current and reduce it to a readable/measureable quantity.  Conversely, if CTs are not properly shunted when not in use, it then looks like an open step-up XFMR (could be on the order of 10s - 100s of thousands of volts, depending on the system being measured).  

Hopefully this steers you in the right direction; if not, I'll divulge further.

applepieordie said:

turn ratio = (Npri/Nsec) = (Vpri/Vsec) = (Isec/Ipri)

shouldn't Ipri in the example be Ipri = (Irelay)/(CT ratio)??

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Your doubt is appreciated and the given Ipri= I relay*CT Ratio is also correct. The CT ratio is Ipri/Isec and not Isec/I pri. That is a convention we use. Do not use the Xformer formula here. You can use it in PT but not in case of a CT. It is a good idea to solve the CT questions by unitary method. The secondary current of CT is relay current and primary current of CT is line current. So line current/Relay current=CT ratio

You can use the XFMR formula for CTs, but you have to think about it first. This is how I do it:

The transformer formula is Np * Ip = Ns * Is

If you plug in the ct ratio numerator for Np and denominator for Ns you are going to have a bad time!

Realize that Np = 1, because the primary conductor passes through the CT one time. Then plug in the CT ratio for Ns.

cos90 said:

You can use the XFMR formula for CTs, but you have to think about it first. This is how I do it:

The transformer formula is Np * Ip = Ns * Is

If you plug in the ct ratio numerator for Np and denominator for Ns you are going to have a bad time!

Realize that Np = 1, because the primary conductor passes through the CT one time. Then plug in the CT ratio for Ns.

Click to expand...

I agree. Let me put it this way, if it helps better for benefit of everyone here.  CT is indeed a transformer and can be understood on the same principles. V1/V2=N1/N2=I2/I1, provided you put the things in that order. Actually in Conventional Transformers and PTs we are conventionally given the ratios of voltages, which are same as ratios of turns, but current ratio behaves the other way and CT ratio is a current Ratio. That creates the confusion. Bit tricky but easy.

The confusion stems from candidates misinterpreting what is primary and secondary on a CT.  

Cos90 is correct that Np=1 in most circumstances, but understand this is not how CTs are rated/defined.  Conventionally, CT ratios are along the lines of 400:5, 600:5, 800:5, and 1200:5, just to name a few of the most common.  What this actually means from the standpoint of a relay - when 400 amps are present on the line, the burden will see a full deflection of 5 amps.

I see. I misinterpreted the CT ratio as the transformer turn ratio but the CT ratio is the ratio (Ipri/Isec) which makes a lot more sense.

Thanks for the explanations.