Effect of CT Burden on its Accuracy and ISF

Burden of CT is defined as the volt ampere loading which is permissible without errors exceeding the limits for an assigned accuracy class. It is expressed as volt ampere (VA). In simple words, it is the load connected to CT secondary. CT burdens are specified as 10 VA, 15 VA, 20 VA, 30 VA. You will rarely find the CT Burden of 5 VA, 2.5 VA or lower.

The burden of CT is basically decided on the basis of rough calculation of burden of leads and meters connected to the CT secondary. The calculation result is then inflated to be on safer side. However, while selecting the burden, one should be very careful as over estimation (more than required) may lead to selection of higher burden value. The higher burden of CT may result in

  • Damage to the connected meters and
  • Introduction of measurement errors and hence inaccurate reporting of meter reading.

Let us now discuss the effect of higher value of CT Burden and try to understand the above two points.

Damage to connected Meters:

A higher value of burden basically extends the saturation characteristics of the CT core. During the condition of fault, the current increases to a higher value. If the Instrument Safety Factor (ISF) of CT is assumed to be 5 then CT metering core should saturate if fault current is more than 5 times the rated CT current. Thus, the CT secondary current is restricted and hence meter is not damaged.

But, due to overestimation of CT Burden, the saturation characteristics gets extended, this means the CT may not saturate under the scenario. This indirectly means that ISF is now more than 5. This may lead to flow of high current (more than the rated value of connected meter) and hence may lead to its damage.

Let us consider a case in which the actual CT burden requirement is 5 VA, however due to calculation error or overestimation, the selected CT have burden of 15 VA. Assume ISF requirement is 5. Assume CT Ratio is 500 / 1 A. This means, saturation current will be 5 A for ISF 5.

For the above assumed case, corresponding to 5 VA burden requirement, the saturation voltage is calculated as

Vs = [VA / (CT Saturation Current for ISF)] x ISF

     = (5/5) x 5 Volt

     = 5 Volt

Since, the VA burden is chosen 15 VA, therefore the saturation voltage for this VA

= (15/5) x 5

= 15 V

From the above calculation, it is clear that the saturation voltage has increased to 15 V while the actual requirement was 5 V.

Therefore, ISF for 15 VA burden

= (15/5) x 5

= 15

This means, the ISF has now became 15 against the required ISF of 5. This means, under fault condition when the current is more than 5 times of rated current, the CT will not saturate to limit the secondary current. This will damage the meter connected.

Measurement Error due to Inappropriate Burden Selection:

Higher burden selection against the required burden results in CT measurement inaccuracy. Generally, a CT measurement is accurate or the error is less when the actual CT burden is equal to the rated CT burden. Thus, if a CT designed for 15 VA burden is operated for 5 VA, there will be higher error in its measurement even when the CT is operating at its rated current.

Thus, it is very important to select a CT with appropriate burden. The burden of CT should not be much more than the required burden i.e. burden of connected meter plus burden of leads plus burden of CT secondary winding. While calculating CT burden, we must not overestimate it.

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