Rockwell Automation 865 Differential Protection Relay User Manual
Page 149
Applications
8-7
865-UM001A-EN-P – July 2009
Accuracy limit factor k
ALF
The ratio of the accuracy limit current to the rated primary current.
Equation
8.6
N
AL
ALF
I
I
K
=
The standard accuracy limit factors are 5, 10, 15, 20 and 30.
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
8.7
A
i
N
i
ALF
A
S
S
S
S
k
k
+
+
=
k
ALF
=
Accuracy limit factor at rated current and rated burden
S
i
=
Internal secondary burden. (Winding resistance R
CT
in
Figure 8.3)
S
N
=
Rated burden
S
A
=
Actual burden including wiring and the load.
Figure 8.4 – Effect of CT Winding Resistance
This figure of Equation 8.7 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 Ω to 0.05 Ω.