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over a period of time. A sharp dip in trending of insulation resistance values, point towards insulation
degradation and further investigation is warranted to diagnose the problem.
3.2.5 Burden Test
Burden can be defined as the total impedance in ohms on the CT output terminals. Each CT has a
secondary burden when connected in a relay or metering circuit. The total burden is a combination of
impedance offered by watt-hour meter coils, relay current coils, contact resistance, terminal blocks,
wire resistance and test switches used in the secondary loop.
Burden test is performed to verify that CT under test is capable of supplying a known current into a
known burden while maintaining its stated accuracy. A burden test is typically performed at full rated
secondary current value. Burdens are typically expressed in VA.
CT’s can be classified into two groups.
1) Metering CT’s
2) Relaying CT’s
Metering CT’s are typically specified as
0.2 B 0.5
The last number specifies the Burden in ohms. For a CT with secondary current of 5 A the VA burden
rating can be calculated as
VA= Voltage * Current = (Current)
2
* Burden = (5)
2
* 0.5 =12.5 VA
Relaying CT’s are typically specified as
10 C 400
The last number specifies the max. Secondary voltage at 20 times the rated secondary current without
exceeding the 10 % ratio error. For a CT with secondary current rated at 5 A, 20 times rated current
secondary current would give a burden of 4 ohms.
Burden= 400/ (20*5) = 4 ohms
Burden in VA can be specified as
VA = Voltage * Current = (Current)
2
* Burden = (5)
2
* 4 =100 VA
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