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CT current transformers
Current
transformers
Dimensions
Section 10
Choosing a current transformer
Choice of a CT depends on 2 criteria:
The Ip/5 A ratio
b
b
The installation type
b
b
The Ip/5 A ratio
We recommend that you choose the ratio immediately higher than the maximum
measured current (In).
Example: In = 1103 A; ratio chosen = 1250/5.
For small ratings from 40/5 to 75/5 and for an application with digital devices, we
recommend that you choose a higher rating, for example 100/5.
This is because small ratings are less accurate and the 40 A measurement, for
example, will be more accurate with a 100/5 CT than with a 40/5 CT.
The installation type
Choice of a CT model depends on the installation type:
Insulated cables
b
b
Mounting on bars
b
b
Important precaution
Never open the secondary circuit of a current transformer when the primary circuit is
energised.
Prior to working on the secondary circuit, the secondary terminals of the current
transformer must be short-circuited.
Determining the accuracy class of a CT
The accuracy class depends on the apparent power (VA) of the transformer and on
consumption of the complete measurement system.
The latter allows for consumption of all the devices and the connecting cables.
For a given accuracy class, consumption of the measurement system must not
exceed apparent power (VA) of the CT transformer.
Copper cable cross-section (mm
2
)
Power in VA per doubled meter at 20 °C
1
1
1.5
0.685
2.5
0.41
4
0.254
6
0.169
10
0.0975
16
0.062
For each temperature variation per 10 °C bracket, the power drawn up by the cables increases
by 4 %.
Merlin Gerin device
Consumption of the current input in VA
Ammeter 72 x 72 / 96 x 96
1.1
Analog ammeter
1.1
Digital ammeter
0.3
PM700, PM800, CM3000, CM4000
0.15
ME4zrt
0.05
PM9
0.55
Example: consumption of a measurement system at
20 °C
PM9
0.55 VA
4 meters of 2.5mm
2
doubled wires
+
16.4 VA
i.e. a measurement system consumption
=
2.19 VA
Based on the result, the CT accuracy class is determined (see previous page):
Class 3 for a 75/5 ratio CT
b
b
Class 1 for a 100/5 ratio CT
b
b
Class 0.5 for a 125/5 ratio CT.
b
b