Current Transformers

Current transformers (CT) are devices that are used to transform the current from a high current into a lower current so that the power can then be measured and controlled. The transformers consist of a core, a primary and a secondary winding. When an alternating current flows through the primary winding, a magnetic flux is created which then induces an alternating current in the secondary winding. This alternating current is proportional to the current in its primary. Current transformers are also known as a "step-down transformers". Once the current has been reduced it can be safely monitored and measured using an ammeter.


Current transformers can perform circuit control and perform roles for safety protection and current limiting.


Current transformers are found in:


  • Generating stations,

  • Electrical substations

  • Commercial electric power distribution.

Types of Current Transformers


Toroidal Current Transformer - These transformers are shaped like a doughnut they do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the toroidal transformer. Toroidal transformers offer high efficiency with minimal excess noise and little excess leakage inductance.


Wound Current Transformer - The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turns ratio of the transformer.


Bar-type Current Transformer – This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device.


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Description Price Current Ratio Inner Diameter Transformer Type Overall Height Overall Width Overall Depth Input Current Output Current Minimum Temperature Maximum Temperature Dimensions For Use With Mounting Type Series
RS Stock No. 510-9152
Mfr. Part No.7KT1201
BrandSiemens
$292.21
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- 13mm Straight Through - - - - - - - - - DIN Rail -
RS Stock No. 123-9539
Mfr. Part No.7KT1200
BrandSiemens
$262.53
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60:5 13mm Straight Through - - - 60A 5 A - - 105 x 71 x 90 mm - Panel Mount 7KT12
RS Stock No. 123-9517
Mfr. Part No.4NC5222-0CE20
BrandSiemens
$90.11
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200:1 28mm Window - - - 200A 1 A -10°C 55°C 80 x 64 x 85.5 mm Measuring Devices Panel Mount 4NC
RS Stock No. 772-1212
Mfr. Part No.5SV8700-0KK
BrandSiemens
$174.72
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- 20mm Toroidal Ring 34mm - - - - -10°C +50°C - - - -
RS Stock No. 772-1221
Mfr. Part No.5SV8702-0KK
BrandSiemens
$251.65
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- 35mm Toroidal Ring 79mm 100mm 58.8mm - - -10°C +50°C - - - -
RS Stock No. 175-6166
Mfr. Part No.4NC5224-0CE21
BrandSiemens
$119.03
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300:1 - Window 67.6mm 60.6mm 78.4mm 300A 1 A -30°C +55°C 60.6 x 78.4 x 67.6 mm Sentron Power Monitoring Devices Chassis Mount Sentron
RS Stock No. 175-6224
Mfr. Part No.4NC5930-5LT21
BrandSiemens
$8.58
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- - - - - - - - - - - 4NC53 Current Transformers H-Rail Mount Sentron
RS Stock No. 175-6124
Mfr. Part No.3UL2304-1A
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$307.17
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- - Toroidal Ring 64mm 124.5mm 130mm 30 mA → 40 A 225 A -25°C +60°C 124.5 x 130 x 64 mm Fault Current Monitoring Chassis Mount -
RS Stock No. 175-6154
Mfr. Part No.4NC5123-2FC21
BrandSiemens
$119.24
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250:5 - Window 65.6mm 60mm 75.3mm 250A 5 A -30°C +55°C 60 x 75.3 x 65.6 mm Sentron Power Monitoring Devices Chassis Mount Sentron
RS Stock No. 175-6167
Mfr. Part No.4NC5224-0DE21
BrandSiemens
$128.23
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300:1 - Window 67.6mm 60.6mm 78.4mm 300A 1 A -30°C +55°C 60.6 x 78.4 x 67.6 mm Sentron Power Monitoring Devices Chassis Mount Sentron
RS Stock No. 123-9526
Mfr. Part No.4NC5325-2CE20
BrandSiemens
$116.92
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400:5 36mm Window - - - 400A 5 A -10°C 55°C 82.5 x 80 x 98 mm Measuring Devices Panel Mount 4NC
RS Stock No. 175-6225
Mfr. Part No.4NC5940-5LT21
BrandSiemens
$8.58
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- - - - - - - - - - - 4NC54 Current Transformers H-Rail Mount Sentron
RS Stock No. 175-6176
Mfr. Part No.4NC5231-2FE21
BrandSiemens
$100.00
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800:5 - Window 62.2mm 59.5mm 75.3mm 800A 5 A -30°C +55°C 59.5 x 75.3 x 62.2 mm Sentron Power Monitoring Devices Chassis Mount Sentron
RS Stock No. 175-6180
Mfr. Part No.4NC5325-2CE21
BrandSiemens
$126.26
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400:5 - Window 84.1mm 82.5mm 98mm 400A 5 A -30°C +55°C 82.5 x 98 x 84.1 mm Sentron Power Monitoring Devices Chassis Mount Sentron
RS Stock No. 175-6155
Mfr. Part No.4NC5124-2FE21
BrandSiemens
$119.24
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300:5 - Window 65.6mm 60mm 75.3mm 300A 5 A -30°C +55°C 60 x 75.3 x 65.6 mm Sentron Power Monitoring Devices Chassis Mount Sentron
RS Stock No. 175-6143
Mfr. Part No.4NC5121-2DC21
BrandSiemens
$124.73
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150:1 - Window 65.6mm 60mm 75.3mm 150A 1 A -30°C +55°C 60 x 75.3 x 65.6 mm Sentron Power Monitoring Devices Chassis Mount Sentron
RS Stock No. 123-9540
Mfr. Part No.7KT1202
BrandSiemens
$245.30
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150:5 13mm Straight Through - - - 150A 5 A - - 105 x 71 x 90 mm - Panel Mount 7KT12
RS Stock No. 175-6214
Mfr. Part No.4NC5438-2CJ21
BrandSiemens
$197.83
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2000:5 - Window 84.1mm 112mm 127.5mm 2kA 5 A -30°C +55°C 112 x 127.5 x 84.1 mm Sentron Power Monitoring Devices Chassis Mount Sentron
RS Stock No. 175-6208
Mfr. Part No.4NC5435-0DH21
BrandSiemens
$252.87
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1500:1 - Window 84.1mm 112mm 127.5mm 1.5kA 1 A -30°C +55°C 112 x 127.5 x 84.1 mm Sentron Power Monitoring Devices Chassis Mount Sentron
RS Stock No. 175-6187
Mfr. Part No.4NC5327-0DE21
BrandSiemens
$156.74
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600:1 - Window 84.1mm 82.5mm 98mm 600A 1 A -30°C +55°C 82.5 x 98 x 84.1 mm Sentron Power Monitoring Devices Chassis Mount Sentron