CT BURDEN

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Hi! I have a doubt about C.T.burden rating. This should be more than the circuit burden?

Or some where I heard that the circuit burden at trip current value (I trip * I trip * circuit resistance) should be 25 to 100 % of C.T. burden rating.

Where will I get the specifications for this?

Look at the specifications for the CT you are using. Do not operate beyond specified limits. Select right type of CT for your requirement.

without getting into the design of current transformers right now i will try to explain what you need to know.

a CT is constructed with an accuracy class as designated by the standards.

there exists a relationship between load - primary current as a percentage of full load rated primary current). at very low loads and loads very close to full load the accuracy of any CT drops substantially. the characteristic is defined by a curve that is nearly flat in the 20% to 85% load region.

correspondingly there exists a relationship between percentage burden imposed and the accuracy. if you are using a CT with a burden load of only 10% then you are using the CT in a very low accuracy band. try to select a CT after ascertaining your instrument or relay loads such that the CT operates in the 40% to 80% band. this will give best results.

since you mention 1.5VA as your instrument load then you should use a 2VA CT or a 3VA CT.

I have a different problem. I have a 2000/5A metering CT rated for 15VA 0.5 FS5, but burden tests reveal that then total connected burden is 30VA, twice the rated burden. What effect will this have on the metering accurccy. Is there a formula that can determine the losses and therefore what % will the 'read' value on meter be compared to real current value