HT yard CT confusion

100-50/5 is given for the installation engineer to decide what ratio he require for protecting the system depending on the output load of the breaker to be protected. As per the design of the system requirement the ratio is decided . Once it is set ,the ratio selected should be written clearly on the panel for the information of the operating and maintenance staff.

CTs will have primary and secondary.General conventional say 100/5 means 100 amps primary 5 amps secondary.this will have secondary terminals s1 and s2 for secondary and terminal P1 and P2 for primary.

In your case you have mentioned 100-50/5, this means dual ratio CT ,which means there will be tappings on the secondary side and terminals will be marked as S1,S2and S3and primary terminals will be P1 and P2.

This dual ratio CTs are incorporated in a system where in there will be increase in load at a later stage or for expansion,.In your case may be initialy you will have 50 Amps primary current and later you will increase to 100 amps.

In this instance you may have to connect the ammeter to s1 and s2 and short terminals s2 and s3 for ratio of 50/5.

for ratio of 100/5, you have to connect ammeter to the terminals s1 and s3.

Since you have mentioned 6 terminals I presume the Cts are dual core,dual ratio.Dual core means one core will be used for instruments and other core will be used for protection.You should not mix the cores,the class rating for instruments may be class1 and protection core may me 5p10.

I suggest send the drawings to us for study and advise.

thankyou sir,

what ever u sent to me i got it.how it works.but i have one doubt.CT has two core protections.core protection one & core protection two.that i didn't get two core protection for what.please reply me sir.thanks

It is usual practice to have 2 core CTs, one for instruments circuit and other core for protection circuit.In certain cases both the cores will be used for protection circuit with different class, may be one core for IDMT relay and other one Differential protection relay.I can not comment without looking at the complete drawings of the Substation.

Are you same person posted the query or different person?

Sorry I made a mistake, there is no need to short the terminals s2 and s3 when you connect the instrument /relay between s1 and s2.

I deeply regret for the error.

Sir,

I have one doubt in this context. PLease tell me how to interpret the CT class "5P10" or whatever?