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Technical terms of current transformers
Term
Primary rated current I
symbol I
)
N
Secondary rated current I
Rated power S
r
Rated frequency f
R
Accuracy class
Rated insulation level U
Overcurrent limiting factor (FS)
Rated continuous thermal current I
Rated short-time thermal current I
Rated peak current I
dyn
"Open voltage" of
current transformers
Grounding of secondary terminals
SCT6xxx
Explanation
(alternative
Value of the rated current on the primary side
pr
Value of the rated current on the secondary side.
sr
Value of apparent power (in [VA]) that the transformer can deliver
to the secondary circuit at secondary rated current and rated load
Value of the rated frequency.
Indication that the measurement deviations are within specified
limits under prescribed conditions of use.
Highest voltage; RMS value of the highest conductor-to-conductor
m
voltage for which a measuring transformer is rated with respect to
its insulation
The value of the rated insulation level is given in three values:
1. maximum value of the conductor-to-conductor voltage for which
the insulation of the transformers is designed;
2. value of the rated short-duration power frequency withstand volt-
age (50 Hz, 1 min), with which the insulation safety of the de-
vices is tested
3. value of the surge voltage level (this specification is mostly un-
occupied here, since according to IEC 61869/1 a specification is
only required for transformers with a conductor-to-conductor
voltage of > 1.2 kV)
Ratio of rated limiting current to primary rated current.
Value of the continuous current in the primary winding at which the
cth
overtemperature does not exceed the value specified in the
standard, with the secondary winding loaded with the rated load.
Value of short-time current for a limited time in the primary winding
th
at which the overtemperature does not exceed the value specified
in the standard, with the secondary winding loaded with the rated
load.
Maximum value of the primary current whose electromagnetic force
effect does not cause electrical and mechanical damage to the
current transformer with short-circuited secondary winding.
Current transformers which are not directly connected to a load
must be short-circuited on the secondary side for safety reasons!
A current transformer operated open on the secondary side induces
very high peak voltage values at its secondary terminals. The
magnitudes of these voltages can reach values of up to several
kilovolts, depending on the dimensioning of the current transformer,
and thus represent a danger to persons and the functional safety of
the transformer. For safety reasons, open operation should
generally be avoided in order to prevent magnetization of the core
iron during secondary open operation.
According to DIN VDE 0141 (01/2000) paragraph 5.3.4, current and
voltage transformers for nominal voltages from U
grounded on the secondary side. For low voltages (U
grounding is not required if the transformer housings do not have
any metal surfaces with large contact areas.
Version: 1.3
Product overview
= 3.6 kV must be
m
≤ 1.2 kV),
m
11
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