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Technical
Section 11
Dimensions
Section 12
7
CT,
Ip/5A ratio (cont.)
Current
transformers
CT selection - Electrical aspect Ip/5 A
b
b
We recommend that you choose the ratio immediately higher than the maximum
measured current (In).
Example:
In = 1103 A; ratio chosen = 1250/5.
b
b
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 Ameasurement,
for example, will be more accurate with a 100/5 CT than with a 40/5 CT.
b
b
Specific case of the motor starter:
to measure motor starter current, you must choose a CT with primary current
Ip = Id/2 (Id = motor starting current).
Validation of measurement solution according
accuracy class
It consists in controlling the right adaptation of the CT on the assucary class aspect.
The accuracy class is specified in the project. The total dissipated power of the
measurement circuit (meter + cables) should not be superior to the specified limit of
the CT. This limit is for different standard classes. If necessary, the choice of the
cable section, the CT or meter should be modify to fit the requirement.
Copper cable
cross-section
(mm
2
)
Power per doubled
meter at 20 °C
(VA)
Schneider Electric
device
Consumption of
the current input
(VA)
Ammeter
72 x 72 / 96 x 96
1.1
Analogue ammeter
1.1
Digital ammeter
0.3
PM700, PM800
0.15
PM3000
0.3
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 %.
Application example
Project specification:
200 A
, in
Ø27
mm cable, accuracy class 1.
Our choice is
METSECT5MA020
.
For this CT selected on the chart (next page), the max acceptable power is
7 VA
(for
"Accuracy class 1" which is specified in the project).
Internal
profile
type
Cables
(mm)
Bars
(mm)
Rating
Ip/5 A
(A)
Cat. no.
Accuracy class
0.5 1
3
Max. power (VA)
MA
Ø27
10 x 32
15 x 25
150
METSECT5MA015
3
4
-
200
METSECT5MA020
4
7
-
250
METSECT5MA025
6
8
-
300
METSECT5MA030
8
10 -
400
METSECT5MA040
10 12 -
Control of the conformity of the measurement chain:
b
b
PM3000 multi-meter: 0.3 VA.
b
b
4 meters of 2.5 mm
2
, doubled wires: 0.41 x 4 = 1.64 VA.
Total:
0.3 + 1.64 = 1.94 VA (< 7 VA)
Conclusion:
this CT is well adapted as the accuracy class will be even better than 1.