Current Transformers

A current transformer (CT) is used to scale high AC currents down to a lower, manageable level for measurement and monitoring. By reducing primary current into a proportional secondary current, these devices allow technicians and energy managers in Hong Kong’s infrastructure and manufacturing sectors to monitor heavy loads safely.

RS provides an extensive range of highly accurate instruments, including the core balance current transformer and split-core models. Our selection is sourced from leading manufacturer brands like Siemens, Murata Power Solutions, LEM, and RS PRO to suit diverse industrial applications.

How Current Transformers Work

A CT transformer operates on the principle of electromagnetic induction, functioning differently from standard voltage transformers. When an alternating current flows through the primary winding, a magnetic flux is created within a laminated core.

This flux then induces an AC in the secondary winding that is proportional to the current in its primary.

The primary winding often consists of the main conductor itself passing through the core, while the secondary winding contains many coil turns that are wound atop a magnetic, low-loss material. Because the secondary winding supplies a reduced current to a load—such as an ammeter or a protective current transformer relay—the device is often referred to as a "series transformer."

This setup allows for high-current measurement without disconnecting the supply or load, provided the secondary circuit remains closed to prevent excessive voltage buildup.

Benefits of Current Transformers

Implementing a current transformer within an electrical system provides several technical advantages for both safety and data integrity. These benefits make them a standard component in modern power distribution networks:

• Safety and Isolation: By electrically isolating the measuring instruments from the high-voltage primary circuit, CTs protect personnel and sensitive equipment from dangerous surges. This isolation is a requirement for safe hands-on maintenance and troubleshooting.
• Accurate Measurement: High turn ratios allow for the precise scaling of large currents, ensuring that even minor fluctuations in a high-power grid are captured by standard metering devices.
• Versatility: Available in various formats, such as split-core for retrofitting or toroidal for permanent installation, these units can be integrated into almost any switchgear or panel board configuration.

Different Types of current transformer (CT meter)

Selecting the right CT transformer depends on the physical installation requirements and the level of precision needed for the application. While all models operate on induction, the physical construction of the core and windings varies. As a leading supplier in Hong Kong, RS provides three primary configurations to suit different mounting scenarios:

Toroidal Current Transformer

In the shape of a doughnut, these transformers are designed without having a primary winding as part of their construction. Rather, a toroidal transformer has a window or hole through which the current flowing in the network passes. These are frequently used as a core balance current transformer to detect leakage currents by passing all phase conductors through a single centre.

Wound Current Transformer

A wound current transformer features a primary winding that is physically connected line-to-line with the conductor bringing the measured current through the circuit. These are typically used for lower current ranges where a higher turn ratio is needed to produce a measurable secondary AC output. Because the primary is integrated into the unit, they offer high accuracy for specialised protective current transformer applications.

Bar-type Current Transformer

This type of transformer uses the cable, or busbar, of the main circuit as its primary winding, effectively one turn of the transformer. In most cases, they are bolted directly to current-carrying devices and are fully insulated from high operating voltages. For easier installation in existing systems, a split-core design is often used, allowing the core to open and snap around the busbar without disconnecting the circuit.

Industrial Applications of Current Transformers

Current transformers are indispensable for measurement and monitoring across a diverse range of industries in Hong Kong. By enabling the safe monitoring of high-voltage AC lines, these instruments facilitate precision and efficiency in the following applications:

Generating Stations

Generating stations use a high-capacity CT transformer to monitor the massive output from turbines and alternators. These devices provide the primary data required for grid synchronisation and load balancing. Because of the extreme currents involved, these units are often custom-built by a specialised manufacturer to withstand significant thermal stress.

Electrical Substations

In substations, a protective current transformer is used to drive protective relays. If a fault occurs, the CT senses the surge and triggers the relay to isolate the circuit, preventing widespread damage to the local power grid.

Commercial Electric

Large commercial buildings use a core balance current transformer to monitor energy consumption for billing and safety. These are vital for detecting earth leakage in complex wiring systems, ensuring that multi-storey structures remain compliant with local safety standards.

Power Distribution

Power distribution networks rely on current transformers to step down transmission-level currents for standard metering equipment. This allows utility companies to track energy flow across different districts without exposing sensitive instruments to high voltages.

Electrical Panels

Inside industrial control panels, a split-core current transformer is often the preferred choice for retrofitting power monitoring tools. These units integrate seamlessly with ammeters and energy meters to provide real-time data on machine performance and circuit health.

How to Choose the Right Current Transformer

Selecting the correct CT transformer is essential for achieving accurate results and maintaining system safety. Consider these factors when you compare the price and specifications of different models:

• Current Ratio: Match the ratio to your expected primary current and the input requirements of your meter. For example, a 100:5 ratio ensures that a 100A primary flow is safely converted to a 5A secondary signal for standard measuring equipment.
• Accuracy Class and Burden: Determine the required precision for your application. Billing-grade meters require a higher accuracy class, while the "burden" (the total impedance of the secondary circuit) must not exceed the transformer's rated capacity to prevent measurement errors.
• Installation Considerations: Evaluate whether you need a split-core or solid-core design. A split-core model allows for quick installation around existing cables without interrupting the power supply, whereas a solid-core or wound current transformer is often better for new builds where maximum accuracy is preferred.

Choose RS Hong Kong for Industrial-Grade Current Transformers

RS Hong Kong is a trusted supplier and distributor of current transformer solutions and power monitoring equipment. We offer a wide range of CT transformer models, including split-core and wound varieties sourced from reputable brands like LEM, Socomec, and RS PRO, ensuring reliable options for every measurement task.

Beyond the standard current transformer, we also provide a wide array of other process control instruments for various other applications, such as temperature transmitters, energy meters, and safety IO modules, catering to all your automation and monitoring requirements. Whether you require a core balance current transformer for earth leakage detection or a protective current transformer for grid safety, our competitive price points and extensive stock ensure you find the right component for your budget.

Explore our user-friendly online site for the full range of products and order today for fast delivery. With our flexible shipping options, you can receive your products precisely when needed to keep your projects on schedule. For comprehensive details on delivery times and fees, please consult our Delivery page.