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Siemens LV 10.1 · 2011
General criteria for the selection of current transformers for measurement purposes
4NC53 current transformers
Standards
EN 60044-1
Window-type current transformers
The conductor to be measured (busbar or cable) is passed through the window opening and constitutes the
primary circuit of the window-type current transformer.
Pin-wound transformers: An economical solution especially for small primary currents of 5 ... 75 A is achieved
when the conductor to be measured is pin-wound several times.
Rated primary current
I
pn
Current transformers can be continuously loaded with 1.3 times the rated primary current (
I
pn
).
Rated secondary current
I
sn
1 A
Particularly suitable for longer measuring leads. Cable losses of only 4 % in contrast to 5 A current transformers.
5 A
5 A current transformers generate 25 times the power losses on measuring leads as compared with 1 A current
transformers. These stray losses result in higher power in the case of long cables. Only recommended for use
with short measuring leads.
Accuracy class
Class 1
Operation measurement, internal metering
Current error
±
1 % at 1 x
I
pn
and 1.2 x
I
pn
Class 3
Coarse measurement
Current error
±
3 % at 0.5 x
I
pn
and 1.2 x
I
pn
Rated power
P
n
The rated power of transformers is specified in VA. The actual load rating should be similar to the rated power;
a lower actual load rating (underburden) increases the overcurrent factor and measuring devices may be dam-
aged in case of a short-circuit, a higher actual load rating (overburden) has a negative effect on the accuracy.
With a frequency of 60 Hz the rated power increases to 1.2 times. With 16
2
/
3
Hz the output power decreases to
1
/
3
of the rated power.
Maximum voltage for equipment
U
m
This is the rms value of the maximum voltage between the conductors of a system. For this voltage the insula-
tion must be rated at normal operating conditions.
4NC5 current transformers are suitable for 720 V.
Overcurrent limiting factor FS
The overcurrent limiting factor is expressed using the characters FS and a factor, e.g. FS5 or FS10.
When a short-circuit current flows through the primary winding of a current transformer, the load on the measur-
ing devices connected to the current transformer is the lower the smaller the overcurrent limiting factor is.
Rated short-time thermal current
I
th
The rated short-time thermal current
I
th
is the rms value of the primary current with a duration of one second,
whose heat effect the current transformer can resist without being damaged in the event of a short-circuited
secondary winding.
Rated impulse current
I
dyn
The rated impulse current
I
dyn
is the highest instantaneous value of the current after a short-circuit whose force
the current transformer can resist without being damaged.
The rated impulse current is specified as peak value.