General Requirements
Formula symbols/terms used (in accordance with IEC 60044-6, as defined)
K
ssc
= rated symmetrical short-circuit current factor
(example: CT 5P20 → K
SSC
= 20)
K'
ssc
= effective symmetrical short-circuit current factor
K
td
= rated transient dimensioning factor
Ι
scc max (ext.
fault)
= maximum symmetrical through flowing fault current
Ι
pn
= CT rated primary current
Ι
sn
= CT rated secondary current
R
ct
= secondary winding d.c. resistance at 75
o
C
(or other specified temperature)
R
b
= rated resistive burden in Ω
R'
b
= R
lead
+ R
relay
= connected resistive burden in Ω
T
p
= primary time constant (net time constant)
U
knee
= knee-point voltage in V (r.m.s.)
R
relay
= relay burden in Ω
with:
Ι
= single conductor length from CT to relay in m
ρ
= specified resistance = 0.0175 Ω mm
2
/m copper wires) at 20
o
C
(or other specified temperature)
A
conductor cross-section in mm
2
The transient rated dimensioning factor K
td
depends on the device version and the primary time constant T
p
.
For the devices 7UT6x with a required saturation-free time of only
1
/
4
period, the influence of T
p
is negligible.
For CT's that are defined by the rated symmetrical short-circuit current factor K
ssc
and the rated burden R
b
(e.g.
5P, 10P), the effective K’
ssc
can be calculated by the formula:
[wdl-anf-kssc, 1, en_GB]
The minimum required K’
ssc
can be calculated by the formula:
[wdl-anf-kssc-ex-fehler, 1, en_GB]
Condition: K’
ssc
(required) ≤ K'
ssc
(r.m.s.)
Current Transformer in accordance with BS 3938/IEC 60044-1 (2000)
The values of an IEC class P transformer can be converted to the values for an IEC class PX (BS class X) trans-
former by using the following formula:
D.1
Current Transformer Requirements
D.1 General Requirements
524 SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
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