Terms related to CT/PT(Burden, Accuracy Class, Instrument Safety factor, knee point voltage etc.)

There are many terms we come across when we read about current transformer(CT) and Potential transformer(PT) or voltage transformer(VT) /capacitive voltage transformer( CVT). below are meaning of these terms.

CT/PT Burden :

The Load connected to secondary winding of a CT/PT is called burden. the following form the burden.

  • Resistance of the secondary winding
  • Resistance of the cable/wire connecting the meter/relay with the CT/PT.
  • The Burden of the relays/meters

Burden are usually specified in VA(Volt-Ampere) the typical values of burden for the CTs are 5, 7.5, 10, 15 and 30 VA and values for PTs are 25,50,75,100,150,200 and 500 VA.

Accuracy Class :

Accuracy class it denotes how accurately CT/PT does their job. as you know a CT is used to step down the current for measurement of the current and PT does the step down of the voltage to measure the voltage.

So during the work of CT/PT , some errors are introduced in the magnitude and phase angle of the current or voltage. the error in magnitude is called ratio error and error in phase angle is called phase angle error.

Accuracy class of the CT (used for measurement or tariff metering): 0.1, 0.2, 0.5, 1.0, 3.0, 5.0

Accuracy class of the CT(used for protection): 5P, 10P, 20P

Accuracy class of the PT (used for measurement or tariff metering): 0.1, 0.2, 0.5, 1.0, 3.0

Accuracy class of the PT(used for protection): 3.0, 5.0

Rated Voltage Factor :

This term is associated with only PT. it actually denotes how much excess a PT can sustain the over voltage. a voltage factor of 1.5 for 30 seconds ( means PT can withstand 50 % excess voltage for 30 seconds during over voltage).

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