In CTs, the secondary has very small impedance referred to as burden, so the CT practically operates on short circuit conditions. The burden for CT is the volt-ampere (VA) loading which is imposed on the secondary at rated current. The burden can also be expressed as the ratio between secondary voltage and secondary current.
A metering CT has lower VA capacity than a protection CT. A metering CT has to be accurate over its complete measuring range. Such a CT’s magnetising impedance at low current and hence low flux should be very high.
The magnetising impedance is not constant for a CT’s operating range due to the non-linear characteristics of the B-H curve. It cannot give linear response during large fault currents. For protection CT, linear response is expected for up to 20 times the rated current. It is also expected to give precise performance in the normal operating currents up to high fault level currents.
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