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 Priced to Clear
RS Stock No. 510-9152
Mfr. Part No.7KT1201
BrandSiemens
£161.66
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- 13mm Straight Through - - - - - - - - - DIN Rail -
RS Stock No. 772-1212
Mfr. Part No.5SV8700-0KK
BrandSiemens
£84.84
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- 20mm Toroidal Ring 34mm - - - - -10°C +50°C - - - -
RS Stock No. 772-1221
Mfr. Part No.5SV8702-0KK
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£100.34
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- 35mm Toroidal Ring 79mm 100mm 58.8mm - - -10°C +50°C - - - -
RS Stock No. 123-9539
Mfr. Part No.7KT1200
BrandSiemens
£174.26
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60:5 13mm Straight Through - - - 60A 5 A - - 105 x 71 x 90 mm - Panel Mount -
RS Stock No. 123-9540
Mfr. Part No.7KT1202
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£162.80
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150:5 13mm Straight Through - - - 150A 5 A - - 105 x 71 x 90 mm - Panel Mount -
RS Stock No. 123-9519
Mfr. Part No.4NC5223-0CE20
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£52.37
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250:1 28mm Window - - - 250A 1 A -10°C 55°C 80 x 64 x 85.5 mm Measuring Devices Panel Mount -
RS Stock No. 123-9532
Mfr. Part No.4NC5328-2CE20
BrandSiemens
£62.77
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750:5 36mm Window - - - 750A 5 A -10°C 55°C 82.5 x 80 x 98 mm Measuring Devices Panel Mount -
RS Stock No. 123-9526
Mfr. Part No.4NC5325-2CE20
BrandSiemens
£62.77
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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 -
RS Stock No. 123-9505
Mfr. Part No.4NC5113-0BC20
BrandSiemens
£68.04
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60:1 17.5mm Window - - - 60A 1 A -10°C 55°C 74.5 x 48 x 80 mm Measuring Devices Panel Mount -
RS Stock No. 123-9523
Mfr. Part No.4NC5225-0CE20
BrandSiemens
£59.14
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400:1 28mm Window - - - 400A 1 A -10°C 55°C 80 x 64 x 85.5 mm Measuring Devices Panel Mount -
RS Stock No. 123-9507
Mfr. Part No.4NC5115-0BC20
BrandSiemens
£60.26
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75:1 17.5mm Window - - - 75A 1 A -10°C 55°C 74.5 x 48 x 80 mm Measuring Devices Panel Mount -
RS Stock No. 123-9537
Mfr. Part No.4NC5434-0CH20
BrandSiemens
£101.44
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1500:1 45mm Window - - - 1.5kA 1 A -10°C 55°C 112 x 80 x 117.5 mm Measuring Devices Panel Mount -
RS Stock No. 175-6183
Mfr. Part No.4NC5326-0DE21
BrandSiemens
£64.91
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Units
500:1 - Window 84.1mm 82.5mm 98mm 500A 1 A -30°C +55°C 82.5 x 98 x 84.1 mm Sentron Power Monitoring Devices Chassis Mount -
RS Stock No. 175-6173
Mfr. Part No.4NC5225-2DE21
BrandSiemens
£51.65
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Units
400:5 - Window 67.6mm 60.6mm 78.4mm 400A 5 A -30°C +55°C 60.6 x 78.4 x 67.6 mm Sentron Power Monitoring Devices Chassis Mount -
RS Stock No. 175-6162
Mfr. Part No.4NC5223-0CE21
BrandSiemens
£49.30
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250:1 - Window 67.6mm 60.6mm 78.4mm 250A 1 A -30°C +55°C 60.6 x 78.4 x 67.6 mm Sentron Power Monitoring Devices Chassis Mount -
RS Stock No. 175-6184
Mfr. Part No.4NC5326-2CE21
BrandSiemens
£48.74
Each
Units
500:5 - Window 84.1mm 82.5mm 98mm 500A 5 A -30°C +55°C 82.5 x 98 x 84.1 mm Sentron Power Monitoring Devices Chassis Mount -
RS Stock No. 175-6198
Mfr. Part No.4NC5431-2DH21
BrandSiemens
£77.66
Each
Units
800:5 - Window 84.1mm 112mm 127.5mm 800A 5 A -30°C +55°C 112 x 127.5 x 84.1 mm Sentron Power Monitoring Devices Chassis Mount -
RS Stock No. 175-6174
Mfr. Part No.4NC5227-2FE21
BrandSiemens
£48.82
Each
Units
600:5 - Window 62.2mm 59.5mm 75.3mm 600A 5 A -30°C +55°C 59.5 x 75.3 x 62.2 mm Sentron Power Monitoring Devices Chassis Mount -
RS Stock No. 175-6163
Mfr. Part No.4NC5223-0DE21
BrandSiemens
£49.47
Each
Units
250:1 - Window 67.6mm 60.6mm 78.4mm 250A 1 A -30°C +55°C 60.6 x 78.4 x 67.6 mm Sentron Power Monitoring Devices Chassis Mount -
RS Stock No. 175-6144
Mfr. Part No.4NC5121-2FA21
BrandSiemens
£52.37
Each
Units
150:5 - Window 65.6mm 60mm 75.3mm 150A 5 A -30°C +55°C 60 x 75.3 x 65.6 mm Sentron Power Monitoring Devices Chassis Mount -