VAMP 265 - Page 161

Technical description
7 Applications
7.4 Current Transformer Selection
VM265.EN011
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161
Marking:
Accuracy limit factor is written after the accuracy class.
E.g. 10 VA 5P10, 15 VA 10P10, 30 VA 5P20.
The actual accuracy limit factor k
A
depends on the actual
burden.
Equation 7.4.1-3.
Ai
Ni
ALFA
SS
SS
kk



k
ALF
=
Accuracy limit factor at rated current and rated burden
S
i
=
Internal secondary burden. (Winding resistance R
CT
in Figure
7.4.1-1)
S
N
=
Rated burden
S
A
=
Actual burden including wiring and the load.
Figure 7.4.1-2 This figure of Equation 7.4.1-3 shows that it is essential to
know the winding resistance R
CT
of the CT if the load is much less than the
nominal. A 10 VA 5P10 CT with 25% load gives actual ALF values from
15..30 when the winding resistance varies from 0.5  to 0.05 .