VAMP 265, Operation And Configuration Instructions

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7.4 Current Transformer Selection 7 Applications Technical description
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NOTE: All current based protection functions of VAMP relays, except arc
protection, thermal protection and 2nd harmonic blocking functions, are
using the fundamental frequency component of the measured current.
The IEC formula includes an RMS value of the current. That is why the
composite error defined by IEC 60044-1 is not ideal for VAMP relays.
However the difference is not big enough to prevent rough estimation.
Standard accuracy classes
At rated frequency and with rated burden connected, the
amplitude error, phase error and composite error of a CT shall
not exceed the values given in the following table.
Accuracy
class
Amplitude error
at rated primary
current
(%)
Phase displacement
at rated primary
current
(°)
Composite error

C
at rated
accuracy limit
primary current
(%)
5P 
1

1
5
10P
 3
- 10
Marking:
The accuracy class of a CT is written after the rated power.
E.g. 10 VA 5P10, 15 VA 10P10, 30 VA 5P20
Accuracy limit current I
AL
Current transformers for protection must retain a reasonable
accuracy up to the largest relevant fault current. Rated
accuracy limit current is the value of primary current up to
which the CT will comply with the requirements for composite
error

C
.
Accuracy limit factor k
ALF
The ratio of the accuracy limit current to the rated primary
current.
Equation 7.4.1-2
N
AL
ALF
I
I
K

The standard accuracy limit factors are 5, 10, 15, 20 and 30.