Current transformers (CTs) measure alternating current safely by scaling high currents down to levels instruments and relays can handle. They are essential in power metering, protective relaying, energy monitoring, and industrial automation – and choosing the right one depends on accuracy class, burden, frequency, and installation type. This guide explains how current transformers work, the main types available, and how to select one for your OEM or ODM project.
What Is a Current Transformer?
A current transformer is an instrument transformer that produces a reduced, proportional current in its secondary winding when the primary conductor carries alternating current. Unlike a voltage transformer, the CT is connected in series with the load, and its primary is often a single pass of the conductor through the core aperture. The secondary is normally terminated with a low burden such as an ammeter, energy meter, or relay. Standard secondary outputs are 5 A, 1 A, or a small voltage signal in the case of Rogowski and sensing CTs.
How Current Transformers Work
Current transformers operate on the same electromagnetic induction principle as power transformers, but with a different winding ratio philosophy. The primary current magnetizes the core, and the secondary winding develops a current inversely proportional to the turns ratio: I secondary = I primary / N. The core operates at a low flux level by design, so the primary current is reproduced accurately across a wide range – from light load to full-scale current. If the secondary circuit is opened while the primary carries current, the core saturates and dangerous voltage spikes can appear; CT secondaries should always be shorted when not connected to a load.
The accuracy of reproduction depends on the burden impedance connected to the secondary and the core material. Low-burden, high-permeability cores such as nanocrystalline and high-grade silicon steel maintain ratio accuracy from 0.1% to 1% depending on the accuracy class.
Types of Current Transformers
Wound-type CT: The primary winding is wound on the core along with the secondary; used for low primary currents where multiple turns are practical.
Window (bar-type) CT: The primary conductor passes through a window in the core; the simplest and most common type for medium and high currents.
Split-core CT: The core can be opened and clamped around an existing conductor without disconnecting it; ideal for retrofit metering and monitoring installations.
Zero-sequence CT: Encloses all three phase conductors to detect residual (earth fault) current in ground-fault protection systems.
Miniature PCB CT: Compact through-hole designs for PCB mounting in power supplies, inverters, and motor drives, usually providing a voltage output proportional to current.
Rogowski coil: An air-core coil that outputs a voltage proportional to the rate of change of current; flexible and used for large or awkward conductors.
Key Parameters for Selecting a Current Transformer
Primary current rating: Choose a rating comfortably above the maximum continuous load, typically 120-150% of nominal.
Secondary output: Standard 5 A or 1 A for metering and protection; mA or mV output for PCB sensing CTs.
Accuracy class: Metering CTs commonly use class 0.5 or 0.5S; protection CTs use classes 5P10, 5P20, or 10P10, defining the composite error at rated accuracy-limit current.
Burden (VA): The maximum load the CT can drive while maintaining accuracy, e.g., 2.5 VA or 5 VA at the rated secondary current.
Frequency: Power-frequency CTs are rated at 50/60 Hz; high-frequency sensing CTs operate from kHz to MHz for switching applications.
Insulation and safety: Confirm the insulation class, voltage rating, and creepage distances required by your application and standard.
Installation type: Through-hole, split-core, or winding terminals – match the CT to your busbar, cable, or PCB layout.
How to Choose a Current Transformer
Define the maximum primary current, the secondary device burden, the required accuracy class, and the operating frequency. Select the installation type that fits your conductor – split-core for retrofit, window for new busbars, PCB-mount for electronic sensing. Verify the accuracy at your operating point, and derate for high ambient temperature if needed. For OEM quantities, provide your mechanical envelope and termination preference; Shaanxi SiRui Technology Co., Ltd supplies custom current transformers – nanocrystalline, silicon steel, and ferrite core designs – manufactured under ISO 9001:2015 quality management with RoHS and REACH compliant materials.
Applications of Current Transformers
Energy metering and smart meters, protective relaying and earth-fault protection, power quality monitoring, motor protection, industrial automation and drives, UPS and inverter current sensing, EV charging monitoring, renewable energy systems, and building energy management. Miniature and split-core CTs are widely used in retrofits where conductors cannot be disconnected.
FAQ
What is the difference between a current transformer and a voltage transformer?
A current transformer is connected in series and scales current down to a standard secondary output; a voltage transformer (VT) is connected in parallel and scales voltage down. CTs must never be left with an open secondary while energized.
Can I use a current transformer to measure DC?
Conventional CTs only measure alternating current because they rely on changing flux. For DC measurement, use a Hall-effect current sensor, shunt resistor, or fluxgate sensor.
What does accuracy class 0.5 mean?
It means the ratio and phase error remain within 0.5% and 30 minutes of phase angle across the specified measuring range – suitable for revenue-grade metering applications.
Do you manufacture custom current transformers?
Yes. Shaanxi SiRui Technology Co., Ltd designs and manufactures wound, window, split-core, zero-sequence, and PCB-mount current transformers – from core and winding design to prototypes and volume production.
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